Session L1: Vortex Ratchet Effect

Experimental vortex ratchet in Nb films with magnetic and non-magnetic asymmetric potentials.

Electron beam lithography allows growing Nb film on arrays of periodic asymmetric potentials. Injecting an ac current in the sample yields a rectified vortex flow. The applied magnetic field and input current strength tune both the magnitude and polarity of the net vortex flow. We will address several points as the temperature and frequency dependence, magnetic pinning centers (Ni) vs. non-magnetic pinning centers (Cu) and the comparison of the behavior of this vortex ratchet system with different types of ratchet as for instance biological motors.   

Vortex Pinning by Symmetric Arrays of Magnetic Nanostructures

Defects present in a superconducting material can lead to a large variety of static and dynamic vortex phases. In particular, the interaction of a vortex lattice with regular arrays of pinning centers, such as holes or magnetic dots, gives rise to commensurability effects. These commensurability effects can be observed in the magnetoresistance and in critical current dependence with the applied magnetic field. In recent years, experimental results have shown that there is a dependence of the periodic pinning effect on the properties of the vortex lattice and also on the dots characteristics. However, neither the main pinning mechanisms by the magnetic dots nor the dependence on the geometry of the pinning arrays are well understood. To clarify the pinning mechanisms, we studied and compared periodic pinning effects in Nb films with rectangular dot arrays of Ni, Co, Fe and Ni covered with thin Ag layers of varying thicknesses, as well as the pinning effects in a Nb film deposited on a patterned substrate without any magnetic material. We will discuss the differences of pinning phenomena arising from magnetic and structural effects. To clarify the effects of the pinning geometry we studied the vortex-lattice dynamics in Nb films with rectangular arrays of Ni dots. We have performed magnetotransport experiments in which two in-plane orthogonal electrical currents are injected at the same time. This allows selecting the direction and intensity of the resultant driving current on the vortex motion. The background dissipation is angular dependent at low magnetic fields. Increasing the applied magnetic field smears out this angular dependence. The periodic pinning potential locks in the vortex motion along channeling directions. Because of this, the vortex-lattice motion maybe up to 85$^{\circ}$ off the driving force direction.   

Phase Diagram and Glassy Dynamics of Moving Vortex Lattices

Much progress was made in understanding the equilibrium properties of vortex systems including the discovery of a topologically ordered Bragg glass phase at low temperatures. By contrast little is known of the dynamics of vortex phases or of their fate once they are driven out of equilibrium and start moving. We describe results of time resolved transport measurements that probe the dynamics of vortex lattices and capture their evolution in response to an applied current pulse. The experiments lead to a dynamic phase diagram consisting of four regions defined by distinctly different response characteristics. In particular it contains a moving Bragg glass state which is the dynamic counterpart of the static Bragg glass. Once the driving force is turned off, the moving Bragg glass relaxes not to the initial stationary state, but to a new state whose properties strongly depend on the relaxation time. This state exhibits simple aging and memory of the direction of the previous moving state indicating that it is a true glass.   

Terahertz Generation \& Vortex Motion Control in Superconductors

A grand challenge is to controllably generate electromagnetic waves in layered superconducting compounds because of its Terahertz frequency range. We propose [1] four experimentally realizable devices for generating continuous and pulsed THz radiation in a controllable frequency range. We also describe [2-4] several novel devices for controlling the motion of vortices in superconductors, including a reversible rectifier made of a magnetic-superconducting hybrid structure [4]. Finally, we summarize a study [5] of the friction force felt by moving vortices. \\ 1) S. Savel'ev, V. Yampol'skii, A. Rakhmanov, F. Nori, Tunable Terahertz radiation from Josephson vortices, preprint \\ 2) S. Savel'ev and F. Nori, Experimentally realizable devices for controlling the motion of magnetic flux quanta, Nature Mat. 1, 179 (2002) \\ 3) S. Savel'ev, F. Marchesoni, F. Nori, Manipulating small particles, PRL 92, 160602 (2004); B. Zhu, F. Marchesoni, F. Nori, Controlling the motion of magnetic flux quanta, PRL 92, 180602 (2004) \\ 4) J.E. Villegas, et al., Reversible Rectifier that Controls the Motion of Magnetic Flux Quanta, Science 302, 1188 (2003) \\ 5) A. Maeda, et al., Nano-scale friction: kinetic friction of magnetic flux quanta and charge density waves, preprint   

Real-time Observation of Vortices in Superconductors by Lorentz Microscopy

The dynamics of individual quantized vortices in superconducting thin films became observable using coherent Lorentz microscopy with our field-emission transmission electron microscopes (1). The observation principle is based on the Aharonov-Bohm effect (2) Since a phase shift of 2$\pi $ is produced between two electron beams enclosing a magnetic flux of $h$/$e$, a vortex having magnetic flux of $h$/(2$e)$ is a phase object of $\pi $ for an illuminating electron beam, which cannot be observed by in--focus electron microscopy. However, the vortices are observable by holographic interference microscopy (3) and defocused Lorentz microscopy$^{ }$(4). Using Lorenz microscopy, various kinds of vortex motions in superconductors with pinning centers were observed. The vortex motions in niobium thin films were elastic, plastic and even rectified (5) depending on the sample temperature, and also on the distributions and strengths of the pinning centers. In high-$T_{c}$ superconductors, when the sample temperature decreased. The vortex motion changed from hopping to slow migration due to increasing pinning effect of atomic-size defects,which was so strong that even the pinning effect of columnar defects was hidden behind it. (1) A. Tonomura: ?Electron Holography? 2$^{nd}$ Edition, Springer, Heidelberg (1999) (2) M. Peshkin and A. Tonomura : ? The Aharonov-Bohm Effect? Lecture Notes in Physics,\textbf{ 340} (Springer-Verlag, Heidelberg, 1989). (3) J. E. Bonevich \textit{et al}. ? Electron holography observation of vortex lattices in a superconductor? Phys. Rev. Lett. \textbf{70} No. 19 (1993) p. 2952-2955. (4) K. Harada \textit{et al}. ? Real-time observation of vortex lattices in a superconductor by electron microscopy? Nature \textbf{360} ( 5 November 1992) p. 51-53. (5) Y. Togawa \textit{et al. } to be submitted to Phys. Rev. Lett.   

Session L2: Martensitic Phase Transformations Under Pressure

Synchrotron X-Ray and Magnetic Susceptibility Probes in Diamond-Anvil Cell

Multiple x-ray and allied techniques have been developed and integrated at synchrotron facilities focusing on a unified scientific goal -- exploring the rich behavior of materials under extreme pressures and temperatures. A plethora of synchrotron x-ray inelastic spectroscopic techniques has been introduced and applied, many of them for the first time, for high-pressure (HP) applications. These include \textit{HP x-ray emission spectroscope} which analyzes energies of the x-ray fluorescent photons with sub-eV energy resolution of the emission spectral lineshape to provide valuable information on the filled electronic states of the HP samples, \textit{HP x-ray inelastic near-edge spectroscopy} which opens a wide new field of HP chemical bonding studies of the light elements, \textit{HP electronic inelastic x-ray scattering spectroscopy }which provides unlimited access to high energy electronic phenomena, including electronic band structure, Fermi surface, excitons, plasmons, and their dispersions at HP, \textit{HP resonant inelastic x-ray scattering spectroscopy }which probes shallow core excitations and multiplet structures for highly correlated electronic systems as well as spin-resolved electronic structures for magnetic samples, and \textit{HP nuclear resonant x-ray spectroscopy }which reveals phonon densities of state and time-resolved M\"{o}ssbauer information. These new tools integrated with the existing magnetic susceptibility and electrical conductivity probes are unleashing the full power of high pressure in numerous scientific disciplines. Fundamental understanding in electronic structure, from simple electron gas to strongly-correlated systems, will be manifested through tuning of the pressure variable. New rules of crystal structure and superconductivity, for example, will be established across the Periodic Table in each pressure regime.   

Time-resolved measurements of structural changes in shocked crystals

Plane shock wave experiments provide a unique approach to examine compression induced structural changes in real time (sub-ns and ns resolution). Past studies have focused on time-resolved continuum measurements and the propagating wave profiles have been related to material thermodynamic states. Following a brief review of past continuum studies, recent experimental developments related to fast time-resolved optical spectroscopy and x-ray diffraction to examine structural changes at the microscopic level will be presented. Challenges associated with the use of these methods in shock wave experiments will be indicated. Specific examples will be described to demonstrate how continuum and microscopic results can be combined to gain detailed insight into shock wave induced structural changes in condensed matter. Directions for future work will be discussed. Work supported by DOE/NNSA.   

Martensitic phase transitions at the atomic length scale: Titanium Alpha to Omega

Martensitic phase transitions---diffusionless, first-order structural transitions occurring near the speed of sound---are abundant in both nature and technological applications from the earth's core to steels to shape memory alloys. Of particular interest to the aerospace industry are titanium alloys. However, pure titanium transforms under pressure to the brittle omega phase, a transformation that must be suppressed for technological applications. The theoretical understanding of this transformation involves finding the atomic pathway of the martensitic transformation. A systematic approach generates all possible pathways; they are successively pruned by energy estimates using elastic theory, tight-binding and {\it ab initio} methods. This general method reduces one thousand possibilities down to seven, and finally to the lowest energy pathway\footnote{D.R. Trinkle {\it et al.}, Phys. Rev. Lett. {\bf 91}, 025701 (2003).}. The lowest energy barrier pathway has a barrier four times lower than all others, and remains the lowest even when nucleation effects are considered. Molecular dynamics simulates the mobile interfacial boundary, and shows the transformation occuring at a fraction of the speed of sound. The resulting microscopic picture provides the starting point for understanding the effect of impurities and for the alloy transformations.   

Impurities Block the Alpha to Omega Martensitic Transformation in Titanium

Impurities control phase stability and phase transformations in nature: from shape memory alloys to steel to planetary cores. Experiments and empirical databases are still central to tuning the impurity effects. Missing is a broad theoretical underpinning. Consider, for example, the titanium martensitic transformations: diffusionless structural transformations proceeding near the speed of sound. Pure Ti transforms from ductile $\alpha$ to brittle $\omega$ at 9~GPa creating serious technological problems for $\beta$-stabilized Ti alloys. Impurities in the Ti alloys A-70 and Ti-6Al-4V suppress the transformation up to at least 35 GPa enhancing their technological utility as lightweight material in aerospace applications. These and other empirical breakthroughs in technological materials call for broad theoretical understanding. Impurities pose two theoretical challenges: The effect on \emph{the relative phase stability} and \emph{the energy barrier} of the transformation. {\it Ab initio} nudged-elastic band methods calculate both changes due to impurities. We show that interstitial O, N, and C retard the transformation while substitutional Al and V influence the transformation by changing the d-electron concentration. The resulting microscopic picture explains the suppression of the transformation in commercial A-70 and Ti-6Al-4V alloys. In general, the effect of impurities on relative energies and energy barriers is central to understanding structural phase transformations.   

Fermi Surface as a Driver for the Shape-Memory Effect in AuZn

Martensites are materials that undergo diffusionless, solid-state transitions. The martensitic transition yields properties that depend on the history of the material and if reversible can allow it to recover its previous shape after plastic deformation. This is known as the shape-memory effect (SME). We have succeeded in identifying the operative electronic mechanism responsible for the martensitic transition in the shape-memory alloy AuZn by using Fermi-surface measurements (de Haas-van Alphen oscillations) and band-structure calculations. Our findings suggest that electronic band structure gives rise to special features on the Fermi surface that is important to consider in the design of SME alloys.   

Session L3: Recent Work on Strongly Coupled Fermi Gases I

Fermionic Condensates

The realization of fermionic superfluidity in a dilute gas of atoms, analogous to superconductivity in metals, is a long-standing goal of ultracold gas research. In my talk I will present experiments where it has become possible to create a condensate of fermionic atom pairs. These pairs are regarded as generalized Cooper pairs in the crossover regime between BCS-type superfluidity and Bose-Einstein condensation (BEC). Beyond providing experimental access to this exciting crossover regime, a gas of ultracold fermionic atoms is a highly controllable system where experimenters can widely vary interactions and study dynamical behaviour. The experiments therefore open the intriguing possibility to address fundamental questions of modern solid state physics with an atomic physics system.   

Universal properties of Fermi gases near a Feshbach resonance

A resonantly-interacting degenerate gas of Fermi atoms provides a paradigm for strong interactions and impacts several disciplines, including condensed matter physics (high-temperature superconductivity), nuclear physics (universal interactions, quark-gluon plasma), high-energy physics (effective theories of strong interactions), and astrophysics (neutron stars). A feature common to all of these systems is that spin-up and spin-down particles ``strongly" interact, i.e., the zero-energy scattering length far exceeds the interparticle spacing. The atomic gas is an extremely flexible experimental system: Thanks to the Feshbach resonance phenomenon, the scattering length can be tuned to any value simply by applying an external magnetic field. I will describe experiments that focus on two phenomena stemming from strong interactions: (i) high-temperature superfluidity and (ii) universality, in which the system becomes independent of the microscopic details of the interaction. These phenomena are probed by studying the fundamental thermodynamics and mechanical properties of an optically trapped gas of fermionic lithium-6.   

Strongly Interacting Fermi Gases: Current Issues and Future Prospects

There has been rapid development in the study of interacting atomic Fermi gases last year. In this talk, I shall discuss the issues brought forth by current experiments with regard to the nature of the newly found pair condensate, the universal thermodynamic and dynamical features in strongly interacting regime, and new methods of probing strongly interacting physics not possible in solid state environment. In the last part of the talk, I shall discuss the exciting theoretical possibilities associating with the latest experimental progress on producing molecules with higher orbital angular momentum, and on strongly interacting Fermi gases in optical lattices. \newline \newline In collaboration with Roberto Diener.   

BCS-BEC crossover in strongly interacting Fermi gases

We discuss our treatment of the BCS-BEC crossover for trapped Fermi atoms, where the mutual interaction can be tuned from weak to strong coupling by means of a Fano-Feshbach resonance. A simple Hamiltonian describing fermions of two different species mutually interacting with a point-contact interaction leads to an accurate description of the systems that currently are the most widely studied experimentally. The resulting many-body problem is then solved by diagrammatic methods. The superfluid phase is described by a single-particle fermionic self-energy, where it is crucial that pairing-fluctuation effects are included on top of the BCS mean-field. The theory reduces to the Popov theory for dilute superfluid fermions in weak coupling and to the Bogoliubov approximation for the composite bosons in strong coupling (where bosonic molecules form as bound-fermion pairs). Excellent agreement is found by comparing our theoretical results with experimental data on cold trapped Fermi atoms as well as with recent QMC simulations, especially in the crossover region about the unitarity limit. Further developments of the theory will be also discussed.   

Session L4: Invited Symposium: The Role of Physics in NASA's Vision for Space Exploration

Icosahedral Order in Undercooled Metallic Liquids - Impact on the Crystal Nucleation Barrier and Thermophysical Properties

Over a half-century ago, Charles Frank argued that metallic liquids could be undercooled because of developing icosahedral short-range order (ISRO) in the liquid that is incompatible with the translational periodicity of crystal phases. Our recent high-energy x-ray diffraction and nucleation undercooling studies of electrostatically levitated droplets of a Ti-Zr-Ni liquid produced the first experimental proof of this hypothesis. In addition to coupling to the nucleation barrier for the ordered phase, the icosahedral order can significantly influence the thermophysical properties of the liquid. A sharp decrease in the specific heat that is correlated with the growing ISRO indicates a rapidly decreasing configurational entropy in the liquid, at temperatures far above the glass transition temperature. Surprisingly, our studies demonstrate that ISRO is evident even above the liquidus temperature in the Ti-Zr-Ni liquid as well as in liquid Ni. It is significantly distorted in liquid Ti, consistent with an increasing importance of the covalent character of the 3-d bonding, which frustrates the development of ISRO. Supported by NASA under contract NAG8-1682, and by the National Science Foundation under grant DMR 03-07410.   

Microgravity Research: A Retrospective of Accomplishments

During the early days of human spaceflight U.S. National Aeronautics and Space Administration (NASA) began giving researchers the ability to perform experiments under extremely low gravity conditions (microgravity). Early microgravity experiments were rudimentary and discovery driven. The limitations of such an approach were clear and in the early 1990s, NASA broadened its program significantly beyond those experiments that were destined to be flown to include a ground- based program that contained both experimental and theoretical investigations. The ground-based program provided a source of carefully designed microgravity experiments. This led to the program in the Physical Sciences Division that involved research in, for example, fluids, materials and low temperature physics. The impact of the microgravity research program has been the focus of a recent National Research Council report titled “Assessment of Directions in Microgravity and Physical Sciences Research at NASA.” We found that there have been numerous high impact ground-based and flight investigations. For example, NASA funding has been instrumental in elucidating the nature of surface-tension-driven fluid flows, dendritic crystal growth and the thermodynamics of phase transitions near critical points. Using this report as a basis, a discussion of the impact of microgravity research on the fields in which it is a part will be given.   

Advanced Systems for Air and Water Quality Monitoring in Long Duration Human Flight

Any space mission involving extended astronaut travel time must have an accompanying system for monitoring the quality of the onboard air and water. These systems must not only meet the detection criteria for undesirable species, at the detection limits set by NASA and the National Academy of Sciences. They must also meet generic requirements such as having low mass, volume, and power; requiring minimal astronaut assistance, and having minimal need for consumables. We will briefly review the criteria for acceptable air and water contamination levels. We will then review the monitoring methods presently in use, and those being developed. These methods include, for example, GCMS, ion mobility spectrometry, the ``electronic nose,'' infrared absorption, and solid phase extraction with colorimetry.   

Session L5: Emerging Devices and Materials for the Microelectronics Industry

The Search for New Information Processing technologies

Our society has benefited from the ‘Golden Age of Electronics’ for the last half century. The ubiquitous transistor, in its many manifestations, has enabled an explosion of capabilities in information processing, communications, and sensing that has spurred exponential growth in performance-benefit ratios. Much of the credit for this progress is due to the continued scaling of the silicon integrated circuit (IC) components and to the associated efficient fabrication processes that have made the IC affordable. There is a growing realization, from simple physics arguments, that as minimum features sizes approach the ten nanometer regime, scaling will very likely slow and eventually end. This doesn’t mean that the MOSFET will disappear, but more likely that it will need to be supplemented by other device and interconnect technologies if the exponential gains are to continue. In this talk we discuss the basis for the projected limitation of scaling of charge-based devices for logic and memory devices. We argue that a fundamental consideration for all devices, including those based on charge, relates to the capacity to manage heat generated by circuit operation. Our preference is for devices that operate at room temperature since the energy costs for cooling the devices must also be charged against the overall system energy consumption. (Cooling costs increase as a power of the difference between the ambient and the target temperature.) Therefore we seek new state variables to serve as an alternative to electrical charge for future information processing technologies. These technologies must provide the potential for sustaining exponential performance-cost benefits with time. The search must not only focus on device structures but on the underlying materials and process technologies that enable these structures. Indeed, to obtain extremely scaled CMOS, new materials and processes must also be developed. In this talk, we survey some of the candidates for replacements/supplements for/to the MOSFET and give a status report on the status of the search. We also briefly discuss the problem of design in the far nanometer regime where device variability is likely to be significant. What design constraints must be employed to ensure that manufacturing yields are high, given broad tolerance margins for the device characteristics? Variability is a growing problem in extremely scaled CMOS what is learned from these applications will likely benefit replacement technologies as well.   

Physics of Modern VLSI CMOS

The Integrated Circuit (IC) was invented in 1958, and modern CMOS was invented in 1980. The semiconductor physics that underlies the IC was discovered in the early part of the past century, and, by the early 60's, it was simplified and codified such that it could be used by engineers to design transistors of ever shrinking size and increasing performance. However, in the past 5-10 years, the ``engineering physics'' of the 60's is becoming increasingly inadequate. Empirical corrections are being made to allow for quantum and non-equilibrium Boltzmann transport effects. Moreover, as features in CMOS transistors reach atomic dimensions, continuum physics is no longer adequate, and devices must be designed increasingly, at the atomic level. In the past 30 years, transistor gate length has shrunk by a factor of 100X: from 10 um to 0.1 um. And it is expected to shrink by about another factor of 10X to 10 nm in the next 10-15 years. However, as transistors approach the end of scaling, the physics to design them will become increasingly complex: \begin{itemize} \item Gate oxide, which is today a few monolayers (10A) thick will be replaced with new materials with high dielectric constant. \item Metal gate electrodes will replace poly-Si, and the interface, which sets the effective work-function, needs to be understood. \item Carrier scattering in the inversion layer in the presence of increasingly high electric fields (horizontal and vertical) needs to be better understood. \item Tunneling will increasingly dominate transistor behavior. \item The discrete positioning of dopants will increasingly affect transistor performance. \item Transistors will become increasingly ballistic. \item Stress in the channel is increasing to the point where it has large impact on device performance. \item And new materials will be introduced into the Source/Drain and channel. \end{itemize} Each of these issues will be discussed, and the unresolved physics issues will be identified   

Challenges for Materials to Support Emerging Research Devices

The 2004 International Technology Roadmap for Semiconductors (ITRS) Emerging Research Devices Chapter has highlighted a number of emerging memory and logic devices with potential for application to future computing technologies. Digital devices operate by representing one of the 2 or more possible values of a logic ``state variable,'' and these values of the state variable are selected by the operation of the device responding to a stimulus. The operating characteristics of the device depend upon the materials and interface properties. Current state variables include: Charge transport and charge state, spin state, solid state phase, molecular charge transport state, quantum state (Qbit), flux quanta (RSFQ) field energy, and mechanical state. Devices that operate based on many of these states will need new materials, characterization and modeling techniques to measure and extract their properties at the nanometer scale. While many materials may be possible to synthesize with conventional deposition techniques, new chemical precursors or molecules may be required, but self assembly should be explored. As new nanometer scale materials are explored to fabricate these new device materials, existing metrology may need to couple with other stimuli to characterize the material and interface properties at these scales.   

Novel Materials for Organic and Thin Film Electronics

Pentacene is highest mobility organic semiconductor known. It forms a crystalline molecular solid that can be deposited in thin film form by either vacuum sublimation or spin-coating. In this talk I will present results of in-situ growth studies of the vapor growth of thin pentacene films on a wide variety of substrates, utilizing video-rate high resolution Low Energy Electron Microscopy (LEEM) and Photo Electron Emission Microscopy (PEEM). We find that the molecular orientation in the thin film depends directly on the electronic structure of the substrate surface (insulator, semimetal, or metal). In addition, epitaxial pentacene films can be grown on relatively weakly coupling substrates, such as semimetallic Bi, as well as insulating alkane Self-Assembled Monolayers on Si(001). The results will be illustrated with dynamic video movies of typical growth sequences.   

nanoHUB.org - Towards On-Line Simulation for Materials and Nanodevices by Design

Challenges in nanoelectronics are the merging notions of material and device. Device lengths have reached the nanometer scale, where material properties are defined. Detailed atomic composition such as strain, interface, doping, and size fluctuations need to be treated. Here the material science and device engineering communities meet on the common ground of quantum mechanics. Success will depend on bridging language and approach barriers between communities. The development of accepted community software will be a significant step.\newline One element of such codes is the NanoElectronic MOdeling Tool. NEMO 3-D enables the computation of strain and electronic structure in an atomistic basis for over 60 and 23 million atoms, corresponding to volumes of $(107nm)^3$ and $(77nm)^3$, respectively. NEMO 3-D runs on a serial and parallel platforms, local cluster computers as well as the NSF Teragrid. About 400,000 atoms are treated efficiently on a single 32bit CPU. NEMO uses an atomistic valence force field method (strain) and the empirical tight binding method (electronic structure). Quantitative simulations for quantum dots in the InAs/GaAs and Si/SiGe material systems have been performed. \newline The Network for Computational Nanotechnology (NCN) is in the process of developing new community and research codes for the analysis of nano-(electronic/mechanical/bio) devices. These tools are hosted on http://nanohub.org for on-line simulation use free-of-charge. Last year over 1,000 people performed about 64,000 simulations. 2,200 others viewed seminars and nanotechnology curriculum items. nanoHUB is being developed as a community resource that encourages on-line simulation, collaborations and nanotechnology education. \newline \newline Co-author: Mark S. Lundstrom   

Session L6: Presenting Current Research to Undergraduates: Physical Review Focus Authors

Controlling the Microscopic World with Holograms

An optical tweezer uses the forces exerted by a strongly focused beam of light to trap and move objects ranging in size from tens of nanometers to hundreds of micrometers. Since their introduction in 1986, optical tweezers have become a mainstay of research in biology, physical chemistry, and condensed matter physics. This presentation highlights recent advances that promise to take optical traps out of the laboratory and into the mainstream of manufacturing, medical diagnostics, and even consumer products. In particular, recently introduced techniques in computer-generated holography can create hundreds of simultaneous optical traps, each of which can be moved independently in three dimensions and can be transformed from force-exerting pincers into new all-optical tools such as torque-exerting optical vortices. By providing unprecedented access and control over the mesoscopic world, the next generation of single-beam optical traps also offers revolutionary new opportunities for fundamental and applied research.   

`Wavefunction' Imaging in Complex New Materials

Advanced scanning tunneling microscopy techniques now allow one to image not only the locations of the atoms in a material, but also the quantum mechanical wavefunction of the electrons which move through it. This means that one can explore directly the quantum mechanical environment within new electronic materials, learning how their exotic properties are generated at the atomic scale. I will discuss how these new `wavefunction' imaging techniques work and show several examples of studies in which they revealed startling new insights into what is actually occurring inside complex new materials.   

Sonoluminescence and other energy focusing phenomena

Fluids and solids that are driven off equilibrium do not return smoothly to the equilibrium state. Instead they can display a wide range of energy focusing phenomena. In sonoluminescence a sound wave passing through a fluid has its energy concentrated by 12 orders of magnitude to create ultraviolet picosecond flashes of light. For 30KHz sound waves the spectrum is a blackbody yet its size is so small as to confound theory. At the very low frequencies achieved with a water hammer the strength of a single flash can be up-scaled by 6 orders of magnitude so as to be visible to this audience, in a real time demonstration. In a ferroelectric crystal such as Lithium Tantalate the application of heat leads to the expulsion of electrons with energies that can exceed 100KeV. Upon striking a target x-rays are emitted. A crucial question relates to whether there exist experimental configurations in which these processes can be used to generate nuclear fusion. Energy focusing also plays a role in turbulence, where intermittency leads to the formation of unexpected structures. Static electricity generated by friction is another striking example of an energy focusing effect. In the 'barometer light' dragging glass through mercury at a speed of 1mm/sec leads to picosecond electrical discharges where the electrons are accelerated to over 1{\%} the speed of light. Experiments indicate that this effect is related to phenomena encompassed by everyday friction.   

Stock Market Crashes and Skipped Heartbeats: Complex Systems Follow Universal Rules

Stock market gyrations and heartbeat anomalies are not entirely random. They follow universal rules that physicists can understand. I will discuss some of these rules in a variety of complex systems, including social and computer networks, and the coding in DNA molecules. One theme is that extreme but rare events like stock market crashes are not entirely unexpected but fit quite normally into the proper statistical framework.   

Session L7: Modeling Large Scale Molecular Biological Data

Evolution and development: what can physics contribute

The genome contains both the 'parts list' for an organism, the genes, as well as the instructions about how to assemble it. Many of the current genome sequencing projects have been motivated by the desire to compare sufficiently similar organism, to discover what parts of the genome are functional, and how they define the differences between the organisms. A particularly fruitful place to study gene regulation (the `assembly manual') is development from egg to adult. Current progress in deducing developmental regulatory networks from the genome will be illustrated with the early, head-tail patterning in the fly embryo. The recent sequencing of a second species of fly, has allowed us to computationally screen for interesting differences in the early embryonic patterning and test these experimentally. This provides a first glimpse of how regulation evolves.   

Applying Logic Analysis to Genomic Data and Phylogenetic Profiles

One of the main goals of comparative genomics is to understand how all the various proteins in a cell relate to each other in terms of pathways and interaction networks. Various computational ideas have been explored with this goal in mind. In the original phylogenetic profile method, `functional linkages' were inferred between pairs of proteins when the two proteins, A and B, showed identical (or statistically similar) patterns of presence vs. absence across a set of completely sequenced genomes. Here we describe a new generalization, logic analysis of phylogenetic profiles (LAPP), from which higher order relationships can be identified between three (or more) different proteins. For instance, in one type of triplet logic relation -- of which there are eight distinct types -- a protein C may be present in a genome iff proteins A and B are both present (C=A$\cap $B). An application of the LAPP method identifies thousands of previously unidentified relationships between protein triplets. These higher order logic relationships offer insights -- not available from pairwise approaches -- into branching, competition, and alternate routes through cellular pathways and networks. The results also make it possible to assign tentative cellular functions to many novel proteins of unknown function. Co-authors: Peter Bowers, Shawn Cokus, Morgan Beeby, and David Eisenberg   

Genomic Signal Processing: Predicting Basic Molecular Biological Principles

Advances in high-throughput technologies enable acquisition of different types of molecular biological data, monitoring the flow of biological information as DNA is transcribed to RNA, and RNA is translated to proteins, on a genomic scale. Future discovery in biology and medicine will come from the mathematical modeling of these data, which hold the key to fundamental understanding of life on the molecular level, as well as answers to questions regarding diagnosis, treatment and drug development. Recently we described data-driven models for genome-scale molecular biological data, which use singular value decomposition (SVD) and the comparative generalized SVD (GSVD). Now we describe an integrative data-driven model, which uses pseudoinverse projection (1). We also demonstrate the predictive power of these matrix algebra models (2). \\ \\ The integrative pseudoinverse projection model formulates any number of genome-scale molecular biological data sets in terms of one chosen set of data samples, or of profiles extracted mathematically from data samples, designated the ``basis'' set. The mathematical variables of this integrative model, the pseudoinverse correlation patterns that are uncovered in the data, represent independent processes and corresponding cellular states (such as observed genome-wide effects of known regulators or transcription factors, the biological components of the cellular machinery that generate the genomic signals, and measured samples in which these regulators or transcription factors are over- or underactive). Reconstruction of the data in the basis simulates experimental observation of only the cellular states manifest in the data that correspond to those of the basis. Classification of the data samples according to their reconstruction in the basis, rather than their overall measured profiles, maps the cellular states of the data onto those of the basis, and gives a global picture of the correlations and possibly also causal coordination of these two sets of states. \\ \\ Mapping genome-scale protein binding data using pseudoinverse projection onto patterns of RNA expression data that had been extracted by SVD and GSVD, a novel correlation between DNA replication initiation and RNA transcription during the cell cycle in yeast, that might be due to a previously unknown mechanism of regulation, is predicted. \\ \\ (1) Alter \& Golub, {\it Proc.~Natl. Acad.~Sci.~USA} {\bf 101}, 16577 (2004). \\ (2) Alter, Golub, Brown \& Botstein, {\it Miami Nat.~Biotechnol.~Winter Symp.~2004} (www.med.miami.edu/mnbws/alter-.pdf)   

Protein interaction networks from literature mining

The ability to accurately predict and understand physiological changes in the biological network system in response to disease or drug therapeutics is of crucial importance in life science. The extensive amount of gene expression data generated from even a single microarray experiment often proves difficult to fully interpret and comprehend the biological significance. An increasing knowledge of protein interactions stored in the PubMed database, as well as the advancement of natural language processing, however, makes it possible to construct protein interaction networks from the gene expression information that are essential for understanding the biological meaning. From the \textit{in house} literature mining system we have developed, the protein interaction network for humans was constructed. By analysis based on the graph-theoretical characterization of the total interaction network in literature, we found that the network is scale-free and semantic long-ranged interactions (i.e. \textit{inhibit}, \textit{induce}) between proteins dominate in the total interaction network, reducing the degree exponent. Interaction networks generated based on scientific text in which the interaction event is ambiguously described result in disconnected networks. In contrast interaction networks based on text in which the interaction events are clearly stated result in strongly connected networks. The results of protein-protein interaction networks obtained in real applications from microarray experiments are discussed: For example, comparisons of the gene expression data indicative of either a good or a poor prognosis for acute lymphoblastic leukemia with \textit{MLL} rearrangements, using our system, showed newly discovered signaling cross-talk.   

Population Dynamics of Genetic Regulatory Networks

Unlike common objects in physics, a biological cell processes information. The cell interprets its genome and transforms the genomic information content, through the action of genetic regulatory networks, into proteins which in turn dictate its metabolism, functionality and morphology. Understanding the dynamics of a population of biological cells presents a unique challenge. It requires to link the intracellular dynamics of gene regulation, through the mechanism of cell division, to the level of the population. We present experiments studying adaptive dynamics of populations of genetically homogeneous microorganisms (yeast), grown for long durations under steady conditions. We focus on population dynamics that do not involve random genetic mutations. Our experiments follow the long-term dynamics of the population distributions and allow to quantify the correlations among generations. We focus on three interconnected issues: adaptation of genetically homogeneous populations following environmental changes, selection processes on the population and population variability and expression distributions. We show that while the population exhibits specific short-term responses to environmental inputs, it eventually adapts to a robust steady-state, largely independent of external conditions. Cycles of medium-switch show that the adapted state is imprinted in the population and that this memory is maintained for many generations. To further study population adaptation, we utilize the process of gene recruitment whereby a gene naturally regulated by a specific promoter is placed under a different regulatory system. This naturally occurring process has been recognized as a major driving force in evolution. We have recruited an essential gene to a foreign regulatory network and followed the population long-term dynamics. Rewiring of the regulatory network allows us to expose their complex dynamics and phase space structure.   

Session L9: Focus Session: Exchange Interactions and Magnetization

L3/L2 Branching Ratio for Rare Earth Compounds

Variation of the L$_{3}$/L$_{2}$ intensity ratio for rare earth compounds-- the so called branching ratio (BR) is an outstanding subjects in the field of magnetic X-ray scattering. In X-ray absorptions not influenced by magnetism, the ratio is about 2, which can be explained by the statistical number of 2p core electrons. However, in X-ray circular dichroism(XMCD) and X-ray resonant magnetic scattering (XRMS), BRs greater than 10 and less than 0.1 have been observed. We show by first principles calculation for hcp heavy rare earth metals that the 4f-5d exchange interaction and the spin-orbit interaction in the 5d band states are the key to understand the variation of BR. In this talk, we will also discuss quadrupole transitions, crystal fields, and hybridization of 5d states with the empty, highly polarized 4f states and the effects on the BR. We will compare our results with XRMS data obtained for RENi2MnGe2 samples, and describe general systematics.   

Orbital magnetization as a ground-state bulk property

The magnetic dipole moment of any finite sample is well defined, while it becomes ill defined in the thermodynamic limit, due to the unboundedness of the position operator. Effects due to surface currents and to bulk magnetization are not easily disentangled. The corresponding electrical problem, where surface charges and bulk polarization appear as entangled, has been solved about one decade ago by the modern theory of polarization, based on a Berry phase. We follow a similar path here, providing a bulk expression for orbital magnetization for any lattice-periodical, though time-reversal breaking, Hamiltonian. We therefore limit ourselves to cases where the macroscopic (i.e. cell-averaged) magnetic field vanishes. For crystalline insulators we express the bulk magnetization in terms of Wannier functions, and we then transform the expression into a Brillouin-zone integral involving the occupied Bloch orbitals. The gauge-invariance of the final expression is explicitly shown. Interestingly, the final expression remains well-defined even for metals, but it is not yet clear whether it is correct in that case.   

Tight-binding calculations of the orbital magnetization in chiral insulators

We present tight-binding calculations of the orbital magnetization in chiral insulators. Our investigations focus on two-dimensional periodic systems with broken time-reversal symmetry and zero Chern number, and on finite samples cut from such systems. Time-reversal symmetry is broken by threading magnetic fluxes through parts of the unit cell in such a way that the net magnetic field remains zero. Results for the calculated magnetization as a function of the flux show that, in the limit of large but finite systems, the orbital magnetization converges to its bulk value as computed in k-space using the formulation of the previous abstract. We also investigate the surface bandstructure, obtaining insight about the role of edge states in the circulation of the current. Possible extensions to non-zero Chern number will also be discussed.   

Quantum magnetic short-range order in the paramagnetic state

A specific quantum magnetic short-range order above the critical temperature for ferromagnets is identified and analyzed. Using quantum Heisenberg model calculations with various exchange interactions parameters of different spins, we found that the quantum spin effect significantly contribute to the magnetic short-range order at rather high temperatures. For a system of small spins such a large short-range order can appear without the abnormal susceptibility which has been previously reported. The influence of this quantum effect on the critical temperature and spin wave spectrum will be discussed. We have also studied the fluctuations of the magnitude of the magnetic moment, and believe that they can also be responsible for the observed Curie-Weiss behavior of the susceptibility above the Curie temperature in real metals.   

Magnetism in FeAl, Ni$_3$Al and Ni$_3$Ga within DFT: Importance of asymmetry in the Exact Exchange potential

We have calculated the magnetic properties of FeAl, Ni$_3$Al and Ni$_3$Ga with exact exchange DFT within the all-electron full-potential linearized augmented-plane-wave method, including core-valence interactions. The correct ground state for these materials is obtained in all cases: non-magnetic for FeAl and Ni$_3$Ga and ferromagnetic in Ni$_3$Al with a magnetic moment of 0.20 $\mu_B$ per formula unit, which is in excellent agreement with experiments. Both LDA and GGA fail to produce the correct magnetic ground state of all three compounds. This failure has been the subject of several investigations in the past. {\it Ad hoc} corrections to the LDA have been used to obtain the correct ground state for these materials, but are either not parameter free (LDA$+U$) or include dynamical variables (spin fluctuations) which are closer in spirit to the quasi-particle picture. We attribute the success of exact exchange to the strong asymmetry in the exchange potential. This should be a desirable feature for next generation approximate functionals.   

Application of the spin cluster methods for the description of the thermodynamics of the Heisenberg model.

We present a general formulation of the spin cluster methods applied to the calculation of thermodynamics of the Heisenberg model in terms of renormalized fields describing interaction between a cluster and its environment. The results of our calculations demonstrate that a pair cluster approximation reproduces Monte-Carlo and spin-dynamic results for the Curie temperature with a rather high accuracy. Such non-mean field systems as systems with the frustrated interactions and systems with a small number of nearest neighbors are investigated. Both classical and quantum Heisenberg model results are obtained. We discuss the general applicability and the enormous computational advantages of this approach. This work was supported by the Office of Basic Energy Sciences of the U.S. Department of Energy under Contract No. W-7405-ENG-82.   

Magnetism of nano-alloys with Stoner interaction enhanced elements

Rapidly decreasing grain size of data storage media stimulate research targeted on understanding the factors controlling performance of magnetic metallic alloys at sub 8 nm lengths scale. We discuss a few specific examples where it is necessary to consider atomic scale finite size effects and correspondingly to develop a quantitative model of magnetic interactions. We present results of modeling motivated by the advances in preparation and characterization of L1$_{0}$ FePt, FePd, and CoPt alloys as well research on the meta-magnetic transformation in FeRh to be usedthe Heat Assisted Magnetic Recording (HAMR) recording process. The use of a multi-scale modeling approach which combines a microscopic model of the magnetic interactions and statistical modeling/theory techniques enables us to investigate the thermomagnetic process for 2-8 nm nano- particles. The microscopic model of the magnetic interactions is calibrated with measurements of the temperature dependent magnetic properties for nano-particulate and granular FePt thin films. The analysis of these experimental results uncovers the mechanism of the large magnetic anisotropy as being dominated by the two-ion contribution. In the case of FeRh, isotropic exchange interactions mediated by the induced moment of Rh atoms is found to increase with thermal fluctuations. This, along with a large Stoner intra-atomic exchange parameter results in an unusual M (T) dependence. We show that proposed microscopic mechanism of the AF-FM transformation explains the well established observations. The proposed model of isotropic and anisotropic magnetic inter-atomic interactions mediated by the Stoner intra-atomic interaction enhanced elements Pt, Pd and Rh is shown to be capable of explaining the unsusual finite size and temperature dependent magnetic properties of these nano- alloys.   

Phonon dynamics of the Heisenberg chain with finite-frequency phonons.

Since the discovery of the inorganic Spin-Peierls compound CuGeO$_3$, one dimensional spin systems coupled to phonons have been studied intensively. While static properties are well understood, the dynamic phonon behavior is still unclear. We have studied the phonon dynamics of the spin $\frac{1}{2}$ Heisenberg chain with bond phonons, around the structural phase transition which occurs as a function of spin-phonon coupling. We have employed Quantum Monte Carlo simulations based on stochastic series expansion (SSE), at almost zero and at finite temperature. The dynamic properties have been obtained by mapping the SSE to a continuous time path integral. At zero temperature we find that the quantum phase transition is of the central peak type as inferred before by Sandvik et al[1]. The renormalisation of the main phonon branch, however, depends strongly on the phonon frequency. As a function of temperature at fixed coupling, we find both a central peak for lower and phonon softening for higher spin-phonon coupling. This behavior is similar to the 3 dimensional case. [1] A. W. Sandvik et al., Phys. Rev. Lett. 83, 195 (1999)   

Exchange interaction and bonding in CuO from first-principles

The understanding of the chemistry of Cu-O interactions is an oustanding open issue in solid state physics in view of its relevance for high-T$_{c}$ superconductors, whose basic units are Cu-O chains or layers. Despite the clear experimental evidence that Cu-Cu spin coupling is strongly antiferromagnetic in the insulating parent compounds and in the doped superconducting phase, a consistent and realistic picture of bonding and magnetic ordering in real Cu-O compounds is still missing. Here we apply a self-interaction-free density-functional approach to investigate the complex interplay of bonding and magnetism in CuO. The puzzling apparently one-dimensional is explained by the presence of a single, spin-polarized d$_{x}^{2}$ character, which induces an antifferromagnetic ordering built up by ferromagnetic planar chains compensating, with period 2, along the $z$ axis. Despite the apparent one-dimensionality, the analysis of the exchange interactions reveals as many as 5 relevant parameters and shows that only a fully three-dimensional view provides a detailed understanding of magnetic ordering and low-energy excitations.   

Spin gap in coupled quarter-filled Hubbard ladders

We present DMRG calculations in quarter-filled ladder systems coupled to the lattice. Single and coupled ladders are considered, with model parameters obtained from first-principles band-structure calculations for $\alpha^\prime$-$NaV_2O_5$. The relevant Holstein coupling to the lattice causes zig-zag lattice distortions, concurrently with the formation of a charge ordered-state. The coupling to the lattice drastically reduces the critical nearest neighbor Coulomb repulsion $V$ and changes the critical exponent of the corresponding quantum phase transition. We find excellent agreement with experimental data for lattice distortion, charge gap, and charge order parameter at a $V^*$ which agrees with previous estimates. The spin gap extrapolates to zero on a single ladder and on two coupled ladders, in spite of a dimerization of spins in chain direction. A finite spin gap appears only on a system with periodicity of {\em four} ladders, as observed experimentally in $\alpha^\prime$-$NaV_2O_5$. The gap is of the right size at $V^*$ but, surprisingly, vanishes again at larger charge order.   

Collective modes in a Half-Metallic Ferromagnet

By using the Theory of Spin-polarized Fermi liquids, we study the spin dynamics of a model for a Half-metallic ferromagnet. We formulate the corresponding kinetic equation and solve it to determine the collective modes of the model. We calculate the velocities of these modes and also study the associated dispersion relations and spin stiffness. We compare the results obtained through this approach with the ones obtained through the application of the Green’s functions method. We discuss the results in the context of the currently available experimental data.   

The Truncated Polynomial Expansion Monte Carlo Algorithm for Spin-fermion Models: Application to Diluted Magnetic Semiconductors and Manganites

A system of fermions coupled to classical fields is common to a wide range of strongly correlated electron problems where the fermionic operators appear in the Hamiltonian involving only quadratic terms. A conventional approach to solve these kinds of models is by diagonalizing the fermions in the one-electron sector at finite temperature for a given configuration of classical fields. However, this results in a high computational cost as the computational complexity grows with the 4-th power of the size of the system. The Truncated Polynomial Expansion Monte Carlo Algorithm (TPEM), developed by N. Furukawa and Y. Motome (J. Phys. Soc. Jpn. \textbf{73}, (2004) 1482), replaces the exact diagonalization of the one-electron sector in these models and has a complexity that is linear with the size of the system. In this talk, I will discuss the performance and reliability of the method as well as the parallelization of the algorithm. I will also show novel applications of the TPEM to disordered systems in the context of diluted magnetic semiconductors and to finite Hund coupling models for manganites (G. Alvarez \textit{et al.}, submitted to Computer Physics Communications). I will discuss how the TPEM can drastically improve the study of those systems.   

Flat bands on partial line graphs

We introduce a systematic method to construct a lattice structure partial line graph. In the tight binding models on the partial line graphs, a flat band emerges on all over k-space. This method can be applied to any two- and three-dimensional systems. In addition to that, there is a large room to modify the lattice as the tight binding model on it has a flat band, i.e. we have many degrees of parameters, for examples, the on-site energy and~the hopping amplitudes.~We show several examples of the partial line graphs. It is expected that our method is also useful in giving a guide line for synthesizing materials with flat bands.   

Session L10: Focus Session: Magnetic Impurities in Semiconductors

Driving Mn into GaAs with an STM: Probing a Mn Acceptor

Using a low temperature scanning tunneling microscope (STM), we can induce an individual Mn adatom to substitute for a Ga atom in the GaAs (110) surface. The Mn atom occupies the Ga site while the Ga atom comes to the surface as a weakly bound adatom. Using STM manipulation the Ga adatom is moved away, leaving a Mn in its substitution position in the (110) surface layer. A Mn atom, in this configuration, gives rise to a strong in-gap level as probed by spatially-resolved STM spectroscopy measurements. The Mn-induced in-gap state has an unusual highly anisotropic spatial structure, which shares some characteristic features with other p-type dopants. The influence of tip-induced band bending on the spectroscopic measurements will be discussed. This talk will provide background for the talk following by Kitchen, \textit{et al}.   

Controlling Interactions between Mn Acceptors in the GaAs (110) Surface

Low temperature scanning tunneling microscopy (STM) is used to control the substitution of individual Mn adatoms into Ga sites in the GaAs (110) surface. In a Ga site, a Mn atom gives rise to a strong in-gap level with highly anisotropic character as probed by spatially-resolved STM spectroscopy measurements. Modifications to this in-gap resonance can occur when two Mn acceptors interact. The interaction of Mn acceptors depends upon both orientation as well as spacing, leading to strong bonding/antibonding-like states under certain configurations. Such measurements of interacting pairs of Mn can potentially provide information on their spin orientation.   

Mechanism for electric field driven single spin manipulation of a Mn dopant in GaAs

We show that the spin orientation of a $J=1$ (Mn + hole) complex in GaAs can be manipulated using only electrical control fields. The spin degeneracy of the compound spin can be split by a dc electric field due to inversion symmetry breaking. The resonances, corresponding to transitions between the split levels, can be driven by an ac electric field. As the bound hole has an anisotropic shape that depends on the compound spin orientation, we propose that the resonance of a single spin can be detected in the tunneling current with scanning tunneling microscopy. The visibility of the resonance is high, as the total (not spin-resolved) local density of states can change as much as 90\% for sites near the Mn dopant as the compound spin orientation is changed. This work is supported by ARO MURI DAAD19-01-1-0541 and DARPA QuIST DAAD-19-01-1-0650.   

Spatial structure of single and interacting Mn acceptors in GaAs

Ferromagnetic semiconductors such as Ga$_{1-x}$Mn$_{x}$As are receiving a lot of attention at the moment because of their application in spintronic devices. However, despite intense study of deep acceptors in III-V semiconductors such as Mn$_{Ga}$, little information has been obtained on their electronic properties at the atomic scale. Yet the spatial shape of the Mn acceptor state will influence the hole-mediated Mn-Mn coupling and thus all of the magnetic properties of ferromagnetic semiconductors such as Ga$_{1-x}$Mn$_{x}$As. This study presents an experimental and theoretical description of the spatial symmetry of the Mn acceptor wave-function in GaAs. We present measurements of the spatial mapping of the anisotropic wavefunction of a hole localized at a Mn acceptor. To achieve this, we have used the STM tip not only to image the Mn acceptor but also to manipulate its charge state A$^{0}$/A$^{-}$ at room temperature. Within an envelope function effective mass model (EFM) the anisotropy in the acceptor wave-function can be traced to the influence of the cubic symmetry of the GaAs crystal which selects specific d-states that mix into the ground state due to the spin-orbit interaction in the valence band. Comparison with calculations based on a tight-binding model (TBM) for the Mn acceptor structure supports this conclusion. Using the same experimental and theoretical approach we furthermore explored the interaction between Mn acceptors directly by analyzing close Mn-Mn pairs, which were separated by less than 2 nm. We will discuss some implications of these results for Mn delta-doped layers grown on differently oriented growth surfaces.   

The onset of Mn monomer and dimer adsorptions on GaAs(110)

Using the density functional VASP and FLAPW approaches, we studied the onset of Mn adsorption on GaAs(110). Large unit cells were used to explore the limit of monomer and dimer adsorbates. We found strong interplay between the magnetization and adsorption geometry, including substitution of Mn on the surface Ga sites. For Mn dimers, the nearest Mn-Mn distance is 8.1 {\AA}, a result which agrees with recent STM observations. These results are explained from electronic structures and lay a basis for further understanding of the mechanism of growth and magnetic ordering in(Ga,Mn)As, a prototype dilute magnetic semiconductor for spintronics applications.   

Luminescence properties of GaAs / AlGaAs quantum wells doped with Mn in the extreme dilution limit

Isolated Mn atoms in bulk GaAs have six degenerate spin states. This degeneracy will be lifted when a Mn impurity is located in a quantum well or quantum dot. AlGaAs / GaAs quantum wells have been grown via molecular beam epitaxy to investigate the optical properties of the included Mn impurities. Normal GaAs growth temperatures (590 C) are used except during low-temperature depositions (258 C) of sub-monolayer quantities of Mn. Drift and diffusion of Mn atoms through an AlGaAs barrier are exploited to dope the quantum well with Mn during the growth. Micro-photoluminescence imaging and spectroscopy indicates spatially localized emission centers consisting of multiple lines. The origin of these spectrally sharp lines will be discussed.   

Electronic states in Mn4+ ions in p-type GaN

There is interest in Mn doped GaN for high Tc ferromagnetic semiconductors. However, Mn in GaN forms deep levels in the band gap. In this study GaN was doped with both Mn and Mg to increase its p-type conductivity. The electronic states of manganese in $p$-type GaN were investigated using photoluminescence (PL) and photoluminescence excitation (PLE) spectroscopies. A series of sharp PL lines at 1.0 eV is observed in codoped GaN and attributed to the intra $d$-shell transition $^{4}T_{2}$(F)-$^{4}T_{1}$(F) of Mn$^{4+}$ ions$.$ PLE spectrum of the Mn$^{4+}$ [$^{4}T_{2}$(F)-$^{4}T_{1}$(F)] luminescence reveals intra center excitation processes via the excited states of Mn$^{4+}$ ions. PLE peaks observed at 1.79 and 2.33 eV are attributed to the intra-d-shell $^{4}T_{1}$(P)-$^{4}T_{1}$(F) and $^{4}A_{2}$(F)-$^{4}T_{1}$(F) transitions of Mn$^{4+}$, respectively. In addition to the intra shell excitation processes, a broad PLE band involving charge-transfer transition of the Mn$^{4+/3+}$ deep level is observed, which is well described by the Lucovsky model. As determined from the onset of this PLE band, the position of the Mn$^{4+/3+}$ deep level is 1.11 eV above the valence band maximum, which is consistent with prior theory using \textit{ab initio} calculations. Our work indicates 4+ is the predominant oxidation state of Mn ions in $p$-type GaN:Mn when the Fermi energy is less than 1.11 eV above the valence band maximum.   

SIC-LSD study of transition metal valencies in oxide materials

The electronic and magnetic properties of transition metal (TM) oxide materials are largely determined by the degree of localization of the TM d-electrons. With the self-interaction corrected (SIC) local spin density (LSD) approximation, we are able to differentiate between various localization/delocalization scenarios based on total energy considerations, and thus to determine the ground state valency onfiguration from the global energy minimum. Using the SIC-LSD, we studied the valencies of TM (Co, Mn) impurities in ZnO. We find the position of the TM(0/+) donor level to be such that the TM$^{2+}$ configuration is energetically most favourable both in n-type ZnO, and in ZnO without additional codopants, whilst in p-type ZnO one additional d-electron prefers to delocalize with the resulting TM$^{3+}$ groundstate configuration. We furthermore investigated the possibility of ferromagnetic order in the corresponding groundstate scenarios. Work supported in part by the Defense Advanced Research Agency and by the Division of Materials Science and Engineering, US Department of Energy. The Oak Ridge National Laboratory is managed by UT-Battelle LLC for the Department of Energy under Contract No. DE-AC05-00OR22725.   

Diluted Magnetic Semiconductors based on Cr-doped InN

Diluted magnetic semiconductors based on GaN have been widely studied following theoretical work that predicts room temperature ferromagnetism in these materials. However, there have been no reports yet of InN-based DMS materials. Here, we have studied the InN system doped with Cr and Mn. The films were grown on c-plane sapphire substrates by Plasma- Assisted MBE using a MOCVD-grown GaN buffer layer. X-Ray Diffraction rocking curve measurements confirmed single crystalline orientation with the FWHM of InN (0002) X-Ray rocking curve about 0.1\r{ }. PL measurements revealed a room-temperature bandgap of 0.8 eV and Hall measurements yield an n-type carrier concentration of about 5*10$^{19 }$cm$^{ -3}$ in both undoped InN as well as Cr-doped InN. While XRD measurements of the Mn-doped films reveal secondary phase formation, the Cr-doped films show no evidence of the formation of compounds such as CrN or Cr$_{2}$N. Magnetic measurements were made on the films using superconducting quantum interference device (SQUID) magnetometry. While the Cr-doped films show a clear magnetic hysteresis as well as remanence up to 300K, the Mn-doped films have less clear magnetic properties. Thus we present evidence of ferromagnetism in n-type, Cr-doped InN.   

Influence of Mn Distribution on Ferromagnetism in Magnetic Semiconductor Mn$_x$Ge$_{1-x}$

The ferromagnetism of Mn-doped Ge, grown with molecular beam epitaxy, is studied by controlling Mn distributions in the films via post-annealing and digital doping techniques. Randomly doped Mn$_{x}$Ge$_{1-x }$films exhibit a high concentration of Mn trapped at interstitial sites in Ge, and reveal two ferromagnetic transitions at $T_{C}$* and \textit{Tc}, respectively. A strong correlation between magnetic and transport properties is observed both at $T_{C}$* and $T_{C}$. Upon annealing as-grown films at a low-temperature, some interstitial Mn atoms are driven toward the surface of the film and even to the substitutional sites of Ge, as predicted by a theory and revealed by ion channeling and x-ray photoemission spectroscopy. This Mn redistribution leads to a large increase in ferromagnetism with both $T_{C}$* and $T_{C}$ shifting toward higher temperatures. Spatial control of Mn atoms along the growth direction is achieved in a Mn$_{x}$Ge$_{1-x}$/Ge digital heterostructure. Ferromagnetism enhancement is also observed in digital structures as compared to randomly doped material with same nominal $x$. The ferromagnetism variation is studied by changing undoped Ge spacer layer thickness and $x$ in doped Mn$_{x}$Ge$_{1-x}$ layer.   

Above Room Temperature Ferromagnetism in Mn-implanted Si

Utilizing the spin of the electron in semiconductor devices holds great potential to provide novel device structures. The integration of ferromagnetic materials into conventional semiconductors is necessary to achieve spintronic devices. Ion implantation is an attractive means for the fabrication of diluted magnetic semiconductors by integrating magnetic materials into existing CMOS electronic devices. We demonstrate that p-doped and n-doped Si crystals can be made ferromagnetic above room temperature through Mn-ion implantation. 300-keV Mn$^{+}$ ions were implanted at dose of (1-10)X10$^{15}$ cm$^{-2}$ reaching peak concentration of (0.1- 0.8) at.{\%} as measured through SIMS profiling. Ferromagnetic hysteresis loops were obtained using a SQUID magnetometer at temperature of (10-300) K, yielding a saturation magnetization of 0.1-0.3 emu/g-sample. The Curie temperature is found $>$400 K with carrier concentration dependence. The crystal structure has been investigated by RBS in the channeling mode and by TEM cross-section images analysis. In this study, we will report the effects of Mn concentration and post implantation annealing on the strength of the ferromagnetism and on the crystal composition.   

Electronic structure and magnetic properties of transition-metal doped Bi$_{2}$Te$_{3}$, Bi$_{2}$Se$_{3}$, and Sb$_{2}$Te$_{3}$ for diluted magnetic semiconductors

The semiconducting tetradymite-structure materials Bi$_{2}$Te$_{3}$, Bi$_{2}$Se$_{3}$, and Sb$_{2}$Te$_{3}$ serve as the basis for high-performance room-temperature thermoelectric devices. Recently, it was found that these materials act as diluted magnetic semiconductors (DMS) with T$_{c} \sim$ 10 K using a few percent doping of transition metal atoms ($T$ = Ti, V, Cr, Mn, Fe). Electronic structure calculations have been performed using the full-potential linear muffin-tin orbital (FP-LMTO) method to understand these materials magnetic properties. The $T$ atoms substitute at the much larger Bi/Sb sites which leads to isolated atomic-like states with very little crystal-field splitting and approximately 3+ valence. This leads to a high spin state with the magnetic moments essentially following Hund's rule. The position of the $T$ 3$d$ states in the band gap will be investigated by analysis of the density of states (DOS). The effects of lattice relaxation and the magnetic interaction of $T$ atoms in the unit cell will also be investigated.   

Session L11: Earth and Planetary Materials II

Thermochemical State of the Lower Mantle: New Insights from Mineral Physics

We report recent findings in the field of high-pressure mineral physics with important implications for Earth's lower mantle. We find that the two main constituents of the lower mantle, namely (Mg,Fe)SiO$_{3}$ -- magnesium silicate perovskite -- and (Mg,Fe)O -- magnesiow\"{u}stite --, undergo electronic transitions at lower mantle pressures, in which iron is transforms from the high-spin state to the low-spin state. The transformations should profoundly alter the thermochemical state of Earth's lower mantle. Minerals bearing high-spin iron have characteristic absorption lines in the near-infrared, hindering radiative conductivity at lower-mantle temperatures. These absorption lines shift in low-spin iron-bearing minerals to the visible range (green to violet), and their intrinsic intensities decrease; the minerals thus become transparent in the near-infrared and their radiative conductivity (and therefore total thermal conductivity) increases. Other issues at stake are that of iron partitioning between mineral phases in the bottom third lower mantle. The two transition pressures correspond to the bottom third of the lower mantle (70 GPa, 1700 km depth), and to the last 300 km above the core-mantle boundary (120 GPa, 2600 km depth); these regions have very special geophysical signatures, since chemical heterogeneities have been reported in the bottom third of the lower mantle, and that the bottom 300 km of Earth's mantle is constituted by the D'' layer. Our observations could provide a mineral physics basis for these two regions of Earth's lower mantle. The implications of these transitions on the dynamics of the lower mantle will be discussed.   

Elasticity of Deep-Earth Materials at High P and T: Implication for Earths Lower Mantle

Brillouin spectroscopy allows measurements of sound velocities and elasticity on phases of geophysical interest at high Pressures and Temperatures. This technique was used to measure the properties of numerous important phases of Earths deep interior. Emphasis is now on measurements at elevated P-T conditions, and measurements on dense polycrystals. Measurements to 60 GPa were made using diamond anvil cells. High temperature is achieved by electrical resistance and laser heating. Excellent results are obtained for polycrystalline samples of dense oxides such as silicate spinels, and (Mg,Al)(Si,Al)O3 --perovskites. A wide range of materials can now be characterized. These and other results were used to infer Earths average lower mantle composition and thermal structure by comparing mineral properties at lower mantle P-T conditions to global Earth models. A formal inversion procedure was used. Inversions of density and bulk sound velocity do not provide robust compositional and thermal models. Including shear properties in the inversions is important to obtain unique solutions. We discuss the range of models consistent with present lab results, and data needed to further refine lower mantle models.   

Probing lattice dynamics at high pressure with inelastic x-ray scattering

Inelastic X-ray scattering provides a direct measure of acoustic excitations within the crystalline lattice at pressures to 100 GPa and accuracies to better than a few percent. Inelastic X-ray scattering thus provides an extremely powerful method of determining the elastic moduli and lattice dynamics in earth materials at high-pressure. Furthermore, inelastic X-ray scattering can be performed on oriented single crystals as well as poly-crystals to provide another internal check of the method. We present measurements the single crystal elastic moduli and phonon dispersion curves of molybdenum in oriented single crystal samples to 40 GPa.   

High-to-low spin transition in magnesiumwustite under pressure

Mg-Fe substitution is most commonly seen in natural solid solution minerals. High resolution X-ray spectroscopy has recently demonstrated that the major lower mantle (LM) minerals undergo a high-to-low spin transition at LM pressures (23-135 GPa). Previous failures of standard DFT and ``LDA+U'' approaches to describe this phenomenon have hindered its investigation and consequences of fundamental importance to geophysics, such as heat transport in Earth?fs mantle. Here, using a rotationally invariant first principles formulation of LDA+U, where the Hubbard U parameter is computed in a internally consistent way, we report the first successful study of this transition in low solute concentration (Mg$_{1-x}$Fe$_x$)O, magnesiumwustite. This is believed to be the second most abundant phase of Earth's LM. This encouraging result appears to open for exploration a new class of problems of enormous significance to deep Earth geophysics.   

Post-perovskite transition in NaMgF$_3$

We have investigated through first principles computations the pressure-induced behavior of NaMgF$_3$. It has the same Pbnm perovskite structure as MgSiO$_3$, the major lower mantle phase. Likewise MgSiO$_3$ it displays the same post-perovskite transition. Static LDA calculations indicate this transition should occur shortly after 18 GPa and then decompose into NaF and MgF$_2$ above 40 GPa. Phonon dispersions and elastic moduli of the post-perovskite phase confirm its vibrational/mechanical stability. The existence of a post-perovskite transition at low pressures in this material makes possible experimental studies of this uncommon structure in a more easily accessible pressure range. Research supported by NSF/EAR 013533 (COMPRES), 0230319, and NSF/ITR 0428774 (VLab).   

Measurements of Thermal diffusivity anisotropy in the laser-heated diamond anvil cell

Heat transport within the Earth and planets is limited by the diffusive heat flow at thermal boundaries. Diffusive heat transport is controlled by a material property—thermal conductivity—that is currently not very well constrained for materials at the high pressures and temperatures relevant to planetary interiors. We have measured thermal diffusivity anisotropy of graphite in the laser heated diamond anvil cell, by examining the hotspot ellipticity generated by laser heating a highly oriented graphite crystals. At ambient pressures, the thermal diffusivity ratio inferred from hotspot ellipticity measurements is in good agreement with independent measurements. In addition, we provide the first measurements of pressure dependence of thermal diffusivity anisotropy of graphite. We compare the observed hotspot ellipticity with models of the heat flow behavior in the diamond anvil cell to examine the temperature dependence of thermal conductivity of graphite. These experiments provide a proof-of-concept for high pressure/ high temperature relative thermal diffusivity measurements in the laser heated diamond anvil cell, with applications for a wide variety of Earth and Planetary materials.   

First-principles calculation of lattice thermal conductivity of MgO

Lattice thermal conductivity of MgO has been calculated based on the first-principles total energy theory and the Boltzmann transport theory. In this study we consider the the anharmonic interaction up to the third-order. An efficient Brillouin zone integration technique is adopted to reduce the computational loads of calculating the phonon- phonon interaction terms in the linearlized Boltzmann equation. Our first-principle calculated results will be discussed with comparison to experimental data and some previous theoretical results.   

Thermal equation of state of bcc and hcp Fe: linear response quasi-harmonic lattice dynamics

Linear-response Linear-Muffin-Tin-Orbital calculations have been performed to understand and predict the thermal equation of state, elasticity, and phase stability of bcc and hcp Fe, for input into dynamic shock finite-element simulations. The phonon dispersion and phonon density of states have been calculated at different volumes and various c/a axial ratios for hcp structures, which show good agreements with available experimental data. The thermal conductivity and electrical resistivity at different pressure have been calculated. Free energy functional for bcc and hcp Fe has been derived, and has been further applied to establish the thermal equation of state, bulk modulus K$_{0}$, dK$_{0}$/dT, and thermal expansion coefficients under high pressures and temperatures. A detailed comparison with experiment has been made. For hcp Fe, the variations of $c/a$ ratios with temperatures and pressures have been predicted. The influence of anharmonic effects has been examined using tight-binding calculations. This work was supported by US Department of Energy ASCI/ASAP subcontract to Caltech , Grant DOE W-7405-ENG-48 (to REC).   

Single crystalline and aggregate elasticity of hcp cobalt at high pressure

The determination of elasticity at high pressure is singularly important for geophysics. In particular a comparison of single-crystalline and polycrystalline results is essential, since the various elements or minerals in Earth are present as textured aggregates.We report the first experimental determination of the complete elastic tensor of hcp cobalt under hydrostatic compression to 39 GPa by Inelastic X-ray Scattering (IXS). These results are complemented by an IXS study on polycrystalline cobalt up to 99 GPa, over the whole stability range of the hcp phase. Moreover the orientational averaging schemes and the micro-mechanical models describing the stress and strain relations of the interacting grains, currently employed to link the single crystal elastic moduli with the aggregate sound velocities in textured polycrystalline samples, will be discussed.   

Electronic topological transitions in high-pressure bcc metals

In 1960 Lifshitz$^{\dagger}$ pointed out that variations in the topology of the Fermi surface as a function of deformation could lead to so-called electronic topological transitions (ETT). We have carried out first-principles electronic structure calculations for the shear elastic constants of bcc vanadium, niobium, and tantalum as a function of pressure and find evidence of an ETT in each case. The transition is manifested in the form of a pronounced softening in the shear elastic constants over a particular pressure range that in turn coincides with a pressure-induced topological change in the Fermi surface. Specifically, the 3-fold degenerate $\Gamma_{25}^{\prime}$ pure $d$-state is unoccupied at low pressures but shifts to lower energy and passes through the Fermi energy as pressure is increased. We have explored the detailed nature of these transitions and suggest that they should appear rather generally in most metals. \\ \\ $^{\dagger}$ I. M. Lifshitz, Sov. Phys. JETP {\bf 11}, 1130 (1960).   

Investigation of the Phase Diagram of Carbon from first principles

The investigation of the phase diagram of carbon has been the subject of experimental research for several decades. Unfortunately progress has been slow due to the extreme temperature and/or pressure required to melt diamond. From the point of view of simulations, it is rather challenging to simulate melting lines under pressure completely from first principles, and only very recently elaborate tools to perform such simulations have been developed [1,2]. We present results for the high pressure portion of the carbon phase diagram, where we have determined new melting lines and predicted metallization pressure and temperature. Our results were obtained using ab-initio molecular dynamics, together with two phase simulation techniques and free energy calculations. This work was performed under the auspices of the US Department of Energy by the University of California at the LLNL under contract no W-7405-Eng-48. [1] S. Bonev et al., Nature 431, 669 (2004) [2] T. Ogitsu et al., Phys. Rev. Lett. 91, 175502 (2003)   

Electronic properties of carbon in the fcc phase.

The observation of a new carbon phase in nanoparticles obtained from Mexican crude oil having the face-centered-cubic structure (fcc) has been reported. However, more recently has been suggested that hydrogen is present in the samples forming CH with the zincblende structure. The structural and electronic properties of C(fcc) and CH(zincblende) are unknown. In the present work we have studied the electronic structure of C(fcc) and CH(zincblende) 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 analyzed the band structure, the local density of states (LDOS), and orbital population. We find that in contrast to graphite and diamond, both fcc carbon and CH with the zincblende structure exhibit metallic behavior. This research was supported by Consejo Nacional de Ciencia y Tecnolog{\'\i}a (Conacyt-M{\'e}xico) under Grants No. 43830-F, No. 44831-F, and No. 43828-Y.   

Session L12: Superconducting Properties

Superstructures in superconductors: The case of YBCO

Superstructures characterized by {\bf q}=(q$_x$, 0, 0) are observed throughout the phase diagram of yttrium-barium cuprates (YBa$_2$Cu$_3$O$_{6+x}$, YBCO); {\bf q} decreases with doping from $\frac12$ (2-unit-cell) in the heavily un-derdoped compound to $\frac15$ in the overdoped material. A 4-unit-cell superstucture is stable in the vicinity of optimal doping. The superstructures in YBCO correspond to short-range ordered regions of coupled atomic displacements on neighboring CuO, BaO, and CuO$_2$ planes, respectively. T-dependent measurements suggest that these ``nanodomains'' experience anharmonic thermal motion. These regions induce a long-range strain in the host, which manifests as ``bow-tie''-shape Huang diffuse scattering below $\sim$200 K. X- ray diffuse scattering results will be presented within the context of the oxygen ordering and the phase diagram.   

Measuring the London Penetration Depth in Aniso\-tropic Superconductors

We will show that by measuring the penetration depth in the conducting planes of an anisotropic superconductor and applying a magnetic field parallel to the conducting planes we can get rid of the signal coming from the vortices, and directly measure the London penetration depth, $\lambda_L$. Using a tunnel diode oscillator (TDO) in a dilution refrigerator, we have measured $\lambda_L$ verses magnetic field in CeCoIn$_5$ and found it to be linear, which is consistent with a d- wave order parameter. In the layered organic superconductor $\alpha$- (ET)$_2$HN$_4$Hg(SCN)$_4$, $\lambda_L$ verses magnetic field follows the shape predicted by BCS theory and an s-wave order parameter. The same measurement in $\kappa$-(ET)$_2$Cu(NCS)$_2$ is also consistent with an s-wave order parameter. The first two results are supported by other types of measurements, but the results for $\kappa$-(ET)$_2$Cu(NCS)$_2$ are puzzling because most other measurements suggest that there are nodes in its order parameter. We will discuss the possible reasons why $\lambda_L$ is not linear as a function of magnetic field in $\kappa$-(ET)$_2$Cu(NCS)$_2$. We will also discuss how the same measurements under pressure will sort help sort out the roles of impurities and inhomogeneity in these materials. In this context we will describe a new pressure cell we have designed for these TDO experiments, and our preliminary results. Work at Clark supported by NSF-DMR-0331272.   

Penetration Depth and Isotope Effect in Highly Overdoped YBCO

Magnetic penetration depth was measured for Y$_{1-x}$Ca$_{x}$Ba$_{2}$Cu$_{3}$O$_{7}$ samples in the range from slightly to highly overdoped carrier concentrations covering Quantum Critical Point (QCP) doping level p=0.2. Together with oxygen isotope exchange experiment in progress, the data should indicate the role of phonons in understanding of the QCP phenomena. Separation of each sample onto different particle size powders along with appropriate model allowed us to extract temperature dependences for in-plane and out-of-plane penetration depth thus defining anisotropy coefficient behavior for the range of highly overdoped compounds.   

Measurements of penetration depth anisotropy in MgB$_2$

The presence of multiple gaps in MgB$_2$ leads to a temperature dependent anisotropy of both superconducting length scales, the London penetration depth, $\lambda$, and the Ginzberg-Landau coherence length, $\xi$. Using a sensitive rf technique, the temperature dependence of both $\lambda_a$ and $\lambda_c$ is measured in single crystals of MgB$_2$. The temperature dependent anisotropy of the penetration depth, $\gamma_\lambda(T)=\lambda_{c}/\lambda_{a}$, calculated from measurements is in approximate agreement with that expected from calculations based on the measured superconducting gaps and band structure calculations. Torque and specific heat measurements are used to determine the anisotropy in $H_{c2}$ and the predicted convergence in the anisotropy of $\lambda$ and $\xi$ near $T_c$ is examined.   

Superconducting parameters of aluminum-lithium alloys

Superconducting transition temperatures T$_{c}$ near 1 K of single-phase fcc aluminum-lithium alloys, with 0 to 10 at.{\%} Li, have been determined through ac susceptibility data. Earlier calorimetric measurements above 2 K on the same samples yielded the Debye temperature $\theta _{D}$ and the density of states at Fermi level N(0) from the lattice specific heat coefficient and the electronic specific heat coefficient, respectively. Following the modified BCS expression, T$_{c}$ = 0.85$\theta _{D}$exp[-1/N(0)V], such experimentally derived T$_{c}$, $\theta _{D }$and N(0) values for each sample provide a measure of the electron-phonon interaction parameter V, which plays central roles in inducing the traditional superconductivity. Its value increases monotonically from 0.611 to 0.710 eV for 0 to 10 at.{\%} Li.   

Specific Heat of Superconducting HfV$_{2}$: Effect of Magnetic Fields

Specific-heat ($C)$ measurements on a single crystal of HfV$_{2} $ were made from 1 to 300 K in magnetic fields ($H)$ to 14 T along the [110] axis. The HfV$_{2}$ has a martensitic transition at $T_{M}$ = 118 K and becomes superconducting (SC) at $T_{c} \quad \sim $8 K. The $T_{c}$ and $C$ vary following repeated cooling cycles through the $T_{M}$ from room temperature. This variation is probably related to strains or fracturing or both from the cubic-to-orthorhombic transition at $T_{M}$. For zero field, $\Delta C(T_{c})$/\textit{$\gamma $T}$_{c}$ = 2.06, which indicates strong coupling, and is nearly independent of variations in \textit{$\gamma $}$_{n}$, $T_{c}$, and $C$ that result from the thermal cycling. The SC state $C$ can be fitted with the alpha model for strongly-coupled superconductors using an energy gap \textit {$\Delta $}(0)/k$_{B}T_{c}$ = 2.1; the associated electron-phonon coupling constant \textit {$\lambda $} = 1.5. Both of these parameters are similar to those of Pb. In the normal state, the Sommerfeld constant (\textit{$\gamma $}$_{n})$ depends on the thermal history: for $T_{c}$ = 8.0 K, \textit{$\gamma $}$_{n}$ = 42.1 mJ K$^{2}$ mol$^{-1}$ after the first cooling. From fits above $T_{c}$, the Debye theta \textit{$\Theta $}$_{D}$, characterizing the low-temperature lattice $C$ is 177 K following the first cooling through $T_{M}$.   

Unconventional Spin-Fluctuation Mediated Superconductivity in PuMGa$_5$ (M=Co, Rh)

The discovery of superconductivity at $T_c$=18.5 K in PuCoGa$_5$ and at $T_c=9$ K in PuRhGa$_5$ has generated renewed interest in Pu-based intermetallic compounds. The thermodynamic properties of PuMGa$_5$ (M=Co, Rh), such as the initial slope of the upper critical field and specific heat jump at $T_c$, are consistent with an electronic specific heat coefficient $\gamma \sim$ 50-100 mJ/mol-K$^2$ indicating moderately heavy fermion behavior in these materials. The normal and superconducting states of these two Pu-based superconductors are remarkably similar to those of the well-known heavy-fermion CeMIn$_5$ (M=Co, Rh, Ir) superconductors, in which superconductivity is mediated by antiferromagnetic spin fluctuations. We present electrical resistivity and nuclear spin lattice relaxation measurements on PuMGa$_5$ that indicates a single (spin-fluctuation) energy scale dominates the physical properties, suggesting that spin fluctuations control both the superconducting and normal states. We contrast these behaviors with those observed in isostructural UMGa$_5$ and NpMGa$_5$.   

Weak ferromagnetic order and possible high-T$_{c}$ superconductivity in the new RuCa$_{2}$PrCu$_{2}$O$_{8+\delta}$ ruthenium-cuprate

Weak ferromagnetic order with ordering temperature T$_{m} \quad \sim $ 47 K and possible superconducting transition with T$_{c} \quad \sim $ 37 K are observed for the new ruthenium-cuprate RuCa$_{2}$PrCu$_{2}$O$_{8+\delta }$ with the orthorhombic distortion of the tetragonal RuSr$_{2}$GdCu$_{2}$O$_{8+\delta }$-type (with T$_{m} \quad \sim $ 136 K and T$_{c}$(max) $\sim $ 65 K) phase. Anomalous temperature dependent magnetization M$_{m}$(T) in both field-cooled (FC) and zero-field-cooled (ZFC) modes and isothermal magnetic hysteresis M$_{m}$(B$_{a})$ below and above T$_{m}$ and T$_{c}$ indicates a very complicated coexistence and interplay between weak-ferromagnetic order and possible superconductivity.   

Inversion symmetry breaking superconductors: Re$_3$W and Re$_3$Mo

Superconductors that break inversion symmetry and have strong spin-orbit coupling are theoretically predicted to have many anomalous properties, including the development of a two-gap structure in the superconducting density of states. One recent example of such a material is CePt3Si, which has attracted much recent attention. We have begun to examine two other materials that meet these criteria, Re3Mo and Re3W, both of which crystallize in the $\alpha $-Mn structure that breaks inversion symmetry. Here we present X-ray diffraction, magnetization, resistivity, and specific heat data on both Re3Mo and Re3W. Characteristic parameters of the superconductivity are extracted, and the data are closely examined for any deviation from ordinary BCS behavior. Oak Ridge National Laboratory is managed by UT-Battelle, LLC, for the U.S. Dept. of Energy under contract DE-AC05-00OR22725.   

Elastic anomalies in HoNi$_2$B$_2$C single crystals

Temperature and magnetic field behavior of elastic properties of HoNi$_2$B$_2$C single crystal have been experimentally determined. The main result is a huge softening the velocity of C$_{66}$ mode in a very wide tempearture range, which can be adequately explained in terms of a theoretical model with the Jahn-- Teller interaction. This interaction is shown to be the driving force of the tetragonal- orthorhombic structural phase transition observed in HoNi$_2$B$_2$C previously. The H--T phase diagrams of this compound with magnetic field oriented along principal crystal directions have been revised by means of analysis of anomalies in the sound velocity and attenuation of the C$_{66}$ mode. New features of the phase diagrams are the critical points where several phase transition lines, separating magnetic phases, merge or maybe intersect. On the other hand, our experiments did not find manifestations of the superconducting phase transition in the acoustic properties.   

Co-existence of ferromagnetism and superconductivity in Ni/Bi bilayers

Thin films of Bi on ultra-thin Ni seed layers (2-15ML) exhibit a novel superconducting (SC) phase, with the ferromagnetism (FM) of Ni quenched for $<$ 2nm. We have prepared Al/Al$_2$O$_3$/Ni/Bi tunnel junctions with varying Ni thicknesses to study the competition between FM and SC. We unambiguously demonstrate that by tuning Ni thickness, the competition between FM and SC in Ni/Bi can be tailored. For narrow range of Ni thickness, the coexistence of a SC energy gap and conduction electron spin polarization are visible on the Ni side of the Ni/Bi bilayers is directly observed. We believe this represents one of the clearest observations of SC and FM coexisting. We have also performed extensive structural and electrical characterization of the Bi. Hall and resistivity measurements show a much higher carrier concentration and lower mobility compared to bulk Bi, and metallic R(T) behavior. These facts support the idea that an increased DOS at the Fermi level in this structure of Bi is responsible for the observed superconductivity. Supported by NSF grants.   

Effects of fluctuations and Coulomb interaction on the transition temperature of granular superconductors

We investigate the suppression of superconducting transition temperature in granular metallic systems due to (i) fluctuations of the order parameter (bosonic mechanism) and (ii) Coulomb repulsion (fermionic mechanism) assuming large tunneling conductance between the grains $g_T \gg 1$. We find the correction to the superconducting transition temperature for 3 $d$ granular samples and films. We demonstrate that if the critical temperature $T_c > g_T \delta$, where $\delta$ is the mean level spacing in a single grain the bosonic mechanism is the dominant mechanism of the superconductivity suppression, while for critical temperatures $T_c < g_T \delta$ the suppression of superconductivity is due to the fermionic mechanism.   

Session L13: Superconducting Nanostructures

A Superconducting Phase Gradiometer Made Out of DNA-Templated Nanowires

Continuous superconducting nanowires with diameters less than 10 nm can be fabricated by sputter-coating suspended DNA molecules. We have fabricated and measured pairs of such nanowires, using DNA as a mechanical template. It is found that a pair of nanowires connected in parallel between two superconducting thin-film electrodes acts as a phase gradiometer. We measure oscillations in the resistance and the critical current of the device with respect to magnetic field at various temperatures and bias currents. Surprisingly, the period of the magnetoresistance oscillations is not determined by the area of the loop formed by the pair of nanowires. Instead, it is determined by the phase difference along the edge of the leads created by screening currents. The results are in good quantitative agreement with our theoretical model, which predicts the phase gradients occurring in the system due to these currents and predicts the response of our double-wire device to such phase gradients.   

Coupled Array of Superconducting Nanowires

We present experiments that investigate the collective behavior of arrays of superconducting lead nanowires with diameters smaller than the coherence length. The ultrathin ($\sim$15nm) nanowires are grown by pulse electrodeposition into porous self-assembled P(S-b-MMA) diblock copolymer templates. The closely packed ($\sim$24 nm spacing) 1-D superconducting nanowires stand vertically upon a thin normal (Au or Pt) film in a brush-like geometry. Thereby, they are coupled to each other by Andreev reflection at the S-N (Pb-Au) point contact interfaces. Magnetization measurements reveal that the ZFC/FC magnetic response of the coupled array system can be irreversible or reversible, depending on the orientation, perpendicular or parallel, of the applied magnetic field with respect to the coupling plane. As found by electric transport measurements, the coupled array system undergoes an in plane superconducting resistive transition at a temperature smaller than the Tc of an individual nanowire. Current-voltage characteristics throughout the transition region are also discussed. This work was supported by NSF grant DMI-0103024 and DMR-0213695.   

Superconductivity in one-dimensional nanowires isolated from environment by on-chip resistors

To test the effects of dissipative environment on superconductivity in quasi-one-dimensional nanowires, we fabricated a series of samples in which the nanowire, connected to small-size thin-film superconducting electrodes, is isolated from the rest of the measurement circuit by means of on-chip Pt/C resistors. The resistors (about 50-100 k$\Omega )$ were made by focused-ion-beam-induced deposition of Pt. The nanowires were fabricated by deposition of MoGe on suspended carbon nanotube. Regardless the fact that nanowires are isolated from the environment, the temperature dependence of a wire resistance is similar to the one predicted by a theory of thermally activated phase slips.   

Matching Effect in Superconducting Pb Inverse Opal.

The type II superconductivity was observed in highly periodic three-dimensional lead inverse opal prepared by infiltration of a melted Pb in blue (D=160 nm), green (D=220 nm) and red (D=300 nm) opals and following extraction of SiO$_{2}$ spheres by chemical etching. The onset of a broad phase transition ($\Delta $T=0.3 K) measured by temperature dependence of magnetic moment and AC resistivity was shifted from T$_{c}$=7.196 K for bulk Pb to T$_{c}$=7.29 K. Upper critical field H$_{c2}$ (3150 Oe) measured from high-field hysteresis loops exceeds the critical field for bulk lead (803 Oe) four fold. Well distinguished matching affect in field dependence of magnetic moment confirmed by periodic pinning in magneto-resistivity measurements is complying with the lattice parameter of inverse opal structure.   

Iso-Dissipative Measurements of Little-Parks Oscillations on Ultrathin, Superconducting Films Perforated with Nano-pores

We have quench condensed ultra-thin Bi/Sb films on substrates with a regular (honeycomb) array of holes whose spacing is of the same order as the coherence length. The resulting films retain the same perforated structure as the substrate with a much longer penetration depth than previous studies [1]. Iso-dissipative measurments of magnetic field vs temperature have been made at several different fractions of the normal state resistance. The magnitude of the Little-Parks oscillations grows with decreased dissipation level, agreeing qualitatively with expectations for such a phase-coherent effect. In addition, the number of oscillations grow with hole size reflecting an increase in vortex pinning by the holes [2,3]. Our latest data will be presented and discussed within the context of the relative roles of amplitude and phase fluctuations for films near the Superconductor to Insulator Transition. We acknowledge the support of the NSF through DMR0203608 and an REU supplement. [1] U. Welp \textit{et al}., Phys. Rev. B, 66, 212507 (2002). [2] A. I. Buzdin, Phys. Rev. B, 47, 11416 (1993). [3] V. V. Metlushko \textit{et al}., Europhys Lett., 41 (3), 333 (1998).   

Investigation of superconductivity in AuSn nanowires

We have fabricated superconducting AuSn nanowires by electrochemical deposition in porous polycarbonate membranes. The diameter of the wires is in the range of 40-100nm with length of 6$\mu$m. By carefully adjusting the deposition parameters, we can fabricate nanowires containing different phases of Au-Sn alloy. Electrical resistivity measurements were done on the array of nanowires still inside the membrane using a two-point probe technique. We observed two phases, AuSn and AuSn4, exhibiting different superconducting properties as observed in the transition temperature, critical field, etc. Currently, we are trying to conduct resistivity measurements on single AuSn nanowire in a four-point configuration, using an electrical field-assisted assembly technique to align the nanowires. This work was supported by Penn State MRSEC NSF grant DMR 0213623.   

Superconducting wires of amorphous indium oxide

We present results of electronic transport measurements on disordered superconducting wires in the 1D limit. The wires are made of amorphous indium oxide (a:InO). They are templated on WS$_{2}$ nanotubes with a diameter between 20 and 120 nm, and stretching across an approximately 1 $\mu $m wide gap etched on the surface of a semiconductor. In earlier studies, a:InO was used extensively for studying the influence of disorder on superconductivity in 2D, including the interplay between disorder, superconductivity, and magnetic field. We have extended that work towards the 1D limit and see signatures of a 2D to 1D crossover in our measurements. The crossover is seen in resistance versus temperature data, as well as in magnetoresistance data.   

Anomalous Current-Voltage Characteristics of Submicron High-$T_c$ Superconducting Wires

We report anomalous non-linearities in the superconducting current-voltage characteristics of submicron YBa$_2$Cu$_3$O$_{7- \delta}$ wires. Submicron-wide and 100$\mu$m-long samples were fabricated using a chemical-free technique based on selective epitaxial growth. Both current-biased and voltage-biased measurements were made between 4.2 K and $T_c$, using pulsed signals to minimize Joule heating. $S$-shaped non-linearities were observed under voltage-biasing and sharp discontinuities were observed under current-biasing, in striking agreement with phase-slip phenomenology established for low-$T_c$ superconductors in quasi-1D geometries. For our quasi-2D high- $T_c$ wires, these observations indicate the formation of phase- slip lines transverse to the current.   

Destructive regime and step features in ultrathin doubly-connected superconducting Al cylinders

In doubly-connected superconductors, because of the fluxoid quantization, the supercurrent velocity, $v_s$, is modulated by the applied magnetic flux. The maximal $v_s$ is inversely proportional to the sample size, leading to a destructive regime in which superconductivity is lost around half-integer flux quanta even at the zero temperature limit, as the sample diameter, $d$, becomes less than the zero-temperature superconducting coherence length, $\xi (0)$. Our recent measurement shows that for ultrathin Al cylinders with $d<\xi (0)$, regular steps emerge in the resistance vs. temperature $R(T)$ curve as the destructive regime is approached. These resistance steps correspond to minima in $dR/dT$ and are approximately equally spaced in logarithmic scale. These features are not present for large cylinders. We also examined the effect of the size of the measurement currents and found that some steps disappeared at higher measurement currents. All these suggest that the regular step features observed in ultrathin cylinders are not due to the formation of phase slip centers. We will discuss the physical origin of these steps.   

Suppression of superconductivity in nanowires by bulk superconductors

Transport measurements were made on a system consisting of zinc nanowire array sandwiched between two bulk superconductors (Sn, In and Pb). It was found that the superconductivity of Zn nanowires could be unexpectedly suppressed by the existence of two mass superconducting reservoirs. The degree of suppression effect is found to closely depend on the diameter and length of the Zn nanowires, as well as the bulk materials (stronger with Sn and weaker with Pb). When a magnetic field (H) is applied and drives the bulk superconductors into the metallic state, the superconducting drop near T$_{c}$ of the Zn nanowires reappears or assumes its full magnitude, indicating the ZNWs have switched back to their superconducting state. Our systematic study demonstrates that this unexpected phenomenon is probably related to the one-dimensional character of zinc nanowires and the interaction of nanowires with the strong bulk superconducting reservoirs.   

The superconductor-insulator transition in ultrathin Pb: the effects of disorder, magnetic field, and magnetic impurities

Using ultrathin quench-condensed Pb films we have performed a systematic comparative study of the superconductor-insulator transition (SIT) driven by disorder ($d)$, magnetic field ($B)$, and magnetic impurities (\textit{MI}). The Pb films were quench-condensed at low temperature onto an Sb buffer layer. The $d$-tuned transition was studied by increasing the thickness of the same film in small steps and performing \textit{in situ} transport measurements. The film was driven back into the insulating state by a perpendicular magnetic field and then later in zero field by magnetic impurities deposited in small increments. We observed that the $d$- and \textit{MI}-tuned transitions showed similar features across the SIT, while the $B$-tuned transition appeared qualitatively different. In particular, the $B$-field induced a quasi-reentrant behavior near the critical field and activated transport immediately on the insulating side, indicating possible $B$-induced mesoscale phase separation across the $B$-tuned SIT. These are distinguishing features for the SIT in granular films and were absent in the $d$- and \textit{MI}-tuned transitions which exhibited sharp well-defined phase boundaries.   

Transport Properties of Superconducting Nanomeshes Fabricated Using Porous Aluminum Oxide Templates

It is known that critical superconducting phenomena in thin films are enhanced with the introduction of artificial pinning centers (APCs) into the films. Additionally, matching field anomalies in the film resistance and critical currents are observed. In this work, we have fabricated hexagonal arrays of holes in superconducting niobium thin films using porous anodic aluminum oxide templates. The hole diameters are 50 nm with an inner hole separation of 100 nm. Standard four-point measurements have been used to investigate the effect of the hole array on the longitudinal and transverse transport properties of the thin films. Results from these measurements will be presented. This work was supported by NSF grant nos. DMR-0080054 and NSF-0132534.   

Superconductor-insulator transition in 1-D nanowires of different lengths

It is unclear whether the superconductor-insulator transition (SIT) in thin superconducting wires is controlled by the wire's diameter, normal state resistance, or both. In order to distinguish between these possibilities we study the SIT in wires of different length. A well-defined SIT is only observed in homogeneous wires which are shorter than $\sim $200 nm. For these wires, the superconducting state is well described by the Langer-Ambegaokar-McCumber-Halperin theory of thermally activated phase slips whereas the insulating state can be explained by localization and electron-electron interactions in one dimension. Longer wires, though fabricated in the exact same manner, do not show a well-defined SIT and frequently display signs of non-uniformity such as double transition steps. Data obtained from resistance vs. temperature as well as differential resistance vs. bias current measurements will be presented which illustrate these observed traits.   

Superconducting Properties of Amorphous Tantalum Thin Films

We investigate superconducting properties of tantalum thin films. The films are deposited at a rate of 0.1nm/sec on quartz substrates by dc sputtering with an argon pressure of 4 mtorr. The samples are patterned to a Hall bar geometry with a shadow mask, and contacts are made with an indium press via gold pads that are pre-deposited on the substrates before the tantalum deposition. We found that with decreasing film thickness, the superconducting transition temperature ($T_c )$ of these films continuously decreases towards 0K. This is characteristic of the amorphous structure of the superconducting thin films, and in strong contrast to granular superconducting thin films where $T_c $ is nearly independent of film thickness. We also found that an exposure of these films to air at room temperature (at least for several months) does not cause any noticeable change in their superconducting properties. We present our results on superconducting properties measurements and structural studies on these amorphous tantalum films.   

Observation of the quantum capacitance in a Cooper-pair transistor

The effective capacitance of a single Cooper-pair box (SCB) can be defined as the second derivative of its energy with respect to gate voltage. This capacitance has two parts, the geometric capacitance, $C_{geom}$, and the quantum capacitance, $C_Q$. $C_Q$ is due to the anti-level crossing caused by the Josephson coupling energy $E_J$, and depends parametrically on the gate voltage. This capacitance, which is dual to the Josephson inductance, can be substantially larger than $C_{geom} $ as well as negative. To detect $C_Q$, we have measured the in- phase and out-of-phase rf-signal reflected from a Cooper-pair transistor (CPT), embedded in a resonant circuit. Under suitable biasing conditions the CPT acts as a SCB in series with a capacitance. It can be shown that the imaginary part of the reflected signal depends linearly on $C_Q$, and we can thus measure $C_Q$ directly from the reflected signal as a function of the gate voltage. The measured data agrees well with the theoretical prediction assuming that the system is in the ground state. We can extract the ratio $E_J/E_C$ for each of the two junctions in the CPT, where $E_C$ is the charging energy of the CPT.   

Session L14: Focus Session: Multifunctional Oxides I

Multifunctional Oxides: Growth and Integration

Recently, the formation of new classes of integrated structures based complex oxides has been pursued. The application of low temperature growth techniques have allowed the development of important oxides, such as lithium niobate, in forms not previously achieved through the processing of traditional bulk materials. The application of epitaxial processes to these systems can generate new function and levels of performance in optical devices. More intriguing applications are being developed through the integration of these materials directly with semiconductors. The deposition and thermal processing of complex oxide materials on semiconductor surfaces are complicated by many issues which impact the long term stability of these interfaces. These oxides are often characterized by a very large phase stability region leading to a broad range of stiochiometry, particularly in comparison to conventional semiconducting materials, a variety of types and concentrations of defects, and the possibility of ionic diffusion and conductivity. The general features of simple reaction systems will be reviewed and applied to the complex oxides. The development of other approaches based on recent advances and applications in materials integration techniques, such as wafer bonding, bypass many of these issues and can lead to a more general technology for the formation of new multifunctional devices.   

Second generation photocatalytic materials - anion doped TiO$_2$

TiO$_2$ is a wide-band gap semiconductor with band gap of ~3.0 eV. Recently an effective photoresponse in the visible-light region has been observed in anion doped TiO$_2$, which is a promising second generation photocatalyst. We contribute a theoretical understanding of this phenomenon. Our ab initio density functional theory investigations show that substitutional anion dopants incorporated into TiO$_2$ drastically affect the electronic structure of the material thus improving its photoactivity. The resulting smaller bandgaps (in the visible) predicted in this work agrees with the available experimental observations for larger anion concentrations around 5$\%$. We also address the effects of doping concentration on the photoresponse of this material.   

Quantum dynamics simulations of interfacial electron transfer in sensitized TiO$_2$ semiconductors

A mixed quantum-classical method combining {\it ab initio}-DFT molecular dynamics simulations with electronic relaxation calculations is used to investigate interfacial electron transfer in catechol/TiO$_2$-anatase nanostructures under vacuum conditions at finite temperature. The calculations demonstrate that the injection mechanism is accelerated by thermal nuclear motion. In particular, electron-phonon scattering leads to ultrafast adsorbate monolayer electron transfer and the disappearance of the anisotropic charge delocalization ({\it i.e.}, carrier diffusion) identified in frozen lattice studies, due to increased coupling between quasistationary molecular orbitals localized on the adsorbates and those orbitals delocalized throughout the semiconductor bulk. The results are particularly relevant to the understanding of surface charge separation in efficient mechanisms of molecular-based photovoltaic devices.   

Non-bolometric Photoresponse in Thin Films of Perovskite Manganites

We have studied the light induced resistance changes (photoresponse) in the thin films of several manganite systems that undergo insulator- metal transition. While the photoresponse in materials such as La $_ {1-x}$ Ca $_{x}$ MnO$_{3}$ is purely thermal (bolometric) in origin, we find that photoresponse of low Tc manganites that exhibit first order percolative transitions (e.g.La$_{0.7-y}$Pr$_{y}$CaMnO$_{3})$ is remarkably different. In these latter type of materials, we observe photoresponse that cannot be accounted for by thermal effects alone. The temperature dependence of the non-bolometric response suggests a light-induced reduction in the resistivity. We believe that the origin of the non-bolometric response is linked to the co-existence of the charge ordered insulating (COI) regions with the ferromagnetic metallic regions (FMM) in the low T$_{c} $ materials. Reduction in resistivity could arise due to the radiation induced switching of the COI regions into the FMM state. We will discuss the details of the non-bolometric response including its dependence on temperature, radiation intensity and frequency, its relaxational dynamics and the possible correlation with magnetoresistance. This work is supported by the NSF grants DMR-0116619 and DMR- 0348939, Undergraduate Research Grant from the College of Science and Mathematics, Towson University and the Federal work- study support for M. Overby.   

Multiferroic thin films

Multiferroic materials are both ferroelectric and ferromagnetic. This combination opens up new applications, as the magnetization can be addressed with an electric field and the polarization by a magnetic field. However, most multiferroic materials either have low polarizations and/or low transition temperatures, thus limiting their potential use in novel devices. We report on high quality epitaxial films of two candidate materials, namely BiFeO$_{3}$ and BiMnO$_{3}$. Both films have narrow X-ray rocking curves of around 0.04\r{ } and are highly insulating with resistivities in excess of 10$^{9}$ Ohm cm. The BiMnO$_{3}$ films are ferromagnetic (2.2 $\mu _{B}$/Mn) and the BiFeO$_{3}$ films show a very weak ferromagnetic magnetization of $<$0.05 $\mu _{B}$/Fe, in contrast to recent claims of an epitaxial enhancement to 1 $\mu _{B}$/Fe (Wang \textit{et al}., Science \textbf{299}, 1719 (2003)). In BiFeO$_{3}$ films we record a dielectric constant of 80 at room temperature. Interestingly, this figure is 2500 for BiMnO$_{3}$ suggesting good ferroelectric properties.   

Finite-temperature properties of ferroelectric PZT ultrathin films near the morphotropic phase boundary

Physical properties at nanometer scales in low dimensional ferroelectric structures are attractive fundamentally, as well as technologically. In this talk, a first-principles-based scheme allowing the computation of finite-temperature properties of complex ferroelectric (001) Pb(Zr,Ti)O$_3$ (PZT) thin films under different boundary conditions will be presented. The effects of uncompensated depolarizing fields and mechanical boundary conditions on the properties of films will be revealed. In particular, it will be shown that new ferroelectric phases, including unusual triclinic and monoclinic states can occur depending on the interplay between electrical and mechanical boundary conditions. Finally, multidomains and their formation mechanism and atomic characteristics in ultrathin ferroelectric films will be discussed. This work is supported by ONR grants N 00014-01-1-0365, N 00014-04-1-0413 and N 00014-01-1-0600 and NSF grants DMR-9983678 and DMR-0404335.   

Investigations on multiferroic characteristics of sol-gel synthesized (1-x) Pb (Fe$_{0.5}$Nb$_{0.5}$)O$_3$ – x PbTiO$_3$ thin films

Pb(Fe$_{0.5}$Nb$_{0.5})$O$_{3}$ (PFN) ceramics exhibit very high dielectric constant and therefore it is an attractive candidate for multilayer capacitors. From our earlier studies on bulk PFN ceramics we have found that the paraelectric to ferroelectric phase transition temperature (T$_{c})$ is diffused in nature with a dielectric constant $\sim $ 58000 (measured at 1 kHz) at T$_{c}$ ($\sim $390K). The process optimized perovskite PFN ceramics also exhibit room temperature ferromagnetic behavior, and therefore, PFN ceramics qualify as typical multiferroic materials. To promote the perovskite phase formation and reduce the loss tangent in PFN, we have synthesized solid solution thin films of PFN with PbTiO$_{3}$ (PT) deposited on platinized silicon and single crystalline strontium titanate substrates. The PT contents were systematically varied up to 40.0 at {\%} and the films were characterized in terms of their phase formation behavior, microstructure evolution, dielectric and ferroelectric properties. Unlike bulk ceramics, PFN thin films on strontium titanate substrates exhibited typical anti-ferromagnetic ordering with a Neel temperature $\sim $ 120K. The effect of PT contents on the magnetic ordering of PFN-PT thin films will be discussed.   

Morphological and Magnetic Properties of Pulse Laser Deposited Barium Hexaferrite Thick Films for Application in Microwave Circulators

Barium-hexaferrite thick films have been grown by Laser Ablation on MgO(111) and Si(111) substrates. For purpose of the experiment excimer laser was used with adjustable energy up to 1J. As a source of BaM a custom made high density BaM target was used. In order to investigate the crystallographic properties of the films XRD analysis was carried out. To characterize the magnetic properties, both MOKE and VSM were performed. SEM imaging provided the information for the overall topography of the films and their thickness as well.   

Theoretical Study of Gallium Oxide Clusters

Gallium oxide is an important semiconducting oxide with applications in the areas of optics and micro-electronics. It is, therefore, not surprising that considerable efforts have been made in the past to understand the structural, optical and electronic properties of gallium oxide. On the other hand, interest in studying the properties of nanostructures and nanoclusters of gallium oxide is relatively recent. Small clusters of gallium oxide can be taken as a prototype to understand the Physics and Chemistry of surfaces and nanostructures. In this talk, we report the equilibrium structure, bonding and electronic properties of small clusters of gallium oxide which are studied under the framework of Density Functional Theory. All the clusters studied so far have shown a preference for planar arrangement of constituent atoms. The ionization induced structural distortions in the neutral clusters are relatively small, except those for Ga$_3$O$_2$. In anionic (cationic) clusters, the added (removed) electron is shared by gallium atoms, except in case of GaO$_3$. All the isomers are dominated by ionic Ga-O bond. The HOMO-LUMO gap in oxygen deficient Ga$_m$O$_n$ clusters depends upon the gallium content, i.e., the gap will change with particle size even if $n/m$ is fixed. Ionization potential and electron affinity of these clusters have been calculated, for the first time, in this study, and both values increase with the increase in oxygen to metal ratio.   

Intermittent polaron dynamics: Born-Oppenheimer out of equilibrium

We consider the non-equilibrium dynamics of a molecular level interacting with local phonon modes in the case of a strong polaronic shift which prevents a perturbative treatment of the problem. Instead, we find that in an adiabatic regime when the electronic states react faster than the phonon modes it is possible to provide a fully non-perturbative treatment of the phonon dynamics including random noise and dissipation. The result shows intermittent switching between bistable states of the oscillator with an effective random telegraph noise.   

Session L15: Focus Session: Dilute Nitride Semiconductors: From Atoms to Devices II

Band Anticrossing Effects in Dilute Nitrides

Recent advances in nonequilibrium epitaxial growth techniques have led to successful synthesis of alloys of distinctly different semiconductors. An important class of such materials are Highly Mismatched Alloys (HMAs). These are compound semiconductors formed when metallic (electronegative) anions are partially replaced by isoelectronic, more electronegative (metallic) atoms. Dilute nitrides in which highly electronegative N atoms partially replace standard group V elements are the most prominent and extensively studied class of HMAs. Using Ga$_{1-y}$In$_{y}$N$_{x}$As$_{1-x}$ as a prototypical HMA, experimental and theoretical studies will be presented that show how all the unusual properties of these alloys can be explained by considering the interaction between highly localized states of substitutional N atoms and the extended states of Ga$_{1-y}$In$_{y}$As matrix in the Band Anticrossing (BAC) model. The interaction splits the conduction band into two nonparabolic bands resulting in large changes in the electrical and optical properties of these materials. The BAC model provides a consistent and quantitative description of experimentally observed data including the large band gap bowing, splitting of the conduction band, and increase enhancement of the electron effective mass. Also, it explains the mutual passivation effect in which group IV donors form nearest neighbor pairs with substitutional N atoms, which eliminates the activity of both species. Most recently we have found that the electronic properties of N-rich GaN$_{1-x}$As$_{x}$ (x$<$0.06) HMAs are determined by an anticrossing interaction between localized donor-like As states and the valence band states of the GaN matrix. The finding provides a basis for a description of the electronic structure of these alloys in the whole composition range.   

Penetration of dilute nitrogen states deep into the GaPN conduction band$^{\ast}$

We study the electronic structure consequences of perturbations caused by dilute N impurities in GaP by means of large supercell ($\sim$1700 atoms) calculations, using a fully atomistic empirical pseudopotential method. We find that numerous localized states are introduced by a single N atom and N clusters, not only close to the band edge but also throughout the GaP conduction band[1], up to $\sim 1$ eV above the conduction band edge. Many of these ensuing states have no counterpart in any of the previous simplified models, such as impurity band or band anticrossing models. These high energy N-localized states are essential for understanding of a previously puzzling observation of splitting of PLE intensity at the GaP $\Gamma_{1c}$ energy into two features, one blue shifting and the other staying pinned in energy with increasing N concentration. Our calculations also explain the observed build up of the featureless spectral intensity between the GaP $X_{1c} $ and $\Gamma_{1c}$ energies with increasing $x$, as being due to the $L-L$ like optical transitions from the states {\it below the VBM}. This work is done in collaboration with Paul Kent and Alex Zunger. \newline [1] S.V. Dudiy, P.R.C. Kent, and A. Zunger, PRB {\bf 70}, 161304 (2004). \newline $^{\ast}$ Supported by DOE-SC-BES-DMS.   

Effects of N incorporation on the electronic structure of GaNP: Origin of the 2.87 eV optical transition

Temperature dependent photoluminescence excitation (PLE) spectroscopy is employed to evaluate basic physical properties of the 2.87 eV absorption peak, recently discovered (I. A. Buyanova et al, PRB 69, 201303 (2004)) in the GaN$_{x}$P$_{1-x}$ alloys. Whereas appearance of this transition is found to be facilitated by incorporation of N and also H atoms, its intensity does not scale with N content. This questions a possible association of this feature with a N-related localized state. Based on the results of temperature dependent measurements, the involved state is concluded to have a non-$\Gamma $ character. Excitation of the known N-related localized states via this state is found to be non-selective, opposed to that between the N-related centers. The observed properties are shown to be hardly consistent with those predicted for the higher lying localized state of the isolated N atom derived from the $\Gamma $ conduction band minimum (CBM). Alternative explanations for the ``2.87 eV'' state as being due to either a t$_{2}$ component of the X$_{3}^{c}$ (or L$_{1}^{c})$ CBM or a level arising from a complex of N and H (in some form) are also discussed.   

Vibrational spectroscopy of N-H2 complexes in GaPN

The dilute III-N-V alloys have attracted much recent attention because of a large reduction of the band-gap energy that occurs for N concentrations of a few percent. The hydrogenation of these alloys gives rise to an increase of the band-gap energy, eliminating the effect of N [1]. Vibrational spectroscopy provides a powerful probe of the structures of the important N- and H- containing complexes in these materials. A previous study of the vibrational properties of GaAsN:H showed that the dominant N- and H-containing defect contains two weakly coupled N-H oscillators, a result that is inconsistent with an H$_{2}$* configuration that several theoretical groups have suggested to explain the properties of H in GaAsN and GaPN [2]. New results from an IR study of the N- and H-containing defects that are produced in GaPN by hydrogenation have led to a better understanding of the vibrational properties of N-H$_{2}$ complexes in the III-N-V alloys. This work is supported by NSF Grant DMR 0403641. 1. A. Polimeni \textit{et al.}, Phys. Rev. B \textbf{63}, 201204 (R) (2001). 2. F. Jiang \textit{et al.}, Phys. Rev. B \textbf{69}, 041309 (R) (2004).   

Effects of heavy nitrogen doping on the host band structure of GaP

A recently observed excitation peak in photoluminescence excitation (PLE) spectra of GaP$_{1-x}$N$_{x}$ epilayers, that remains pinned \textbf{\textit{below}} the \textit{$\Gamma $} point of the GaP$_{1-x}$N$_{x}$ with N concentration, is attributed to a transition from the valence band edge to either the $t_{2}$(X$_{3})$ or $t_{2}(L)$ conduction bands by Buyanova et al. [PRB 69, 201303(R), 2004]. A theoretical study based on an empirical pseudopotential band structure calculation offers an alternative explanation for the pinned peak claiming that it is produced by high energy N-cluster states, despite the fact that the calculated pinned peak is \textbf{\textit{above}} the \textit{$\Gamma $} point [Duidy et al., PRB 70, 161304(R), 2004]. Using absorption and PLE studies on free-standing samples, we show that this pinned peak is merely an artifact that arises from the GaP buffer layer and is not associated with the GaP:N epilayers. Also, we directly probe the host conduction band minimum (CBM) near $X_{1C}$ using absorption, which shows that a weak CBM absorption peak remains stationary up to nitrogen composition x = 0.1 {\%} before it is smeared out by the inhomogeneous broadening. This result further supports the conclusion that the absorption below the host CBM in heavily N doped GaP is primarily due to the formation of an impurity band consisting of broadened states of N pairs and clusters (Zhang et al, PRB 62, 4493, 2000).   

Investigations of the energy-fine structure related to different nitrogen nearest-neighbor environments in GaInNAs layers and GaInNAs/GaAs quantum wells

A formation of In-N bonds instead Ga-N ones in GaInNAs compound after annealing is one of the most interesting features of this compound. The aim of this paper is to investigate this issue for sets of different GaInNAs layers and GaInNAs/GaAs quantum wells by photoreflectance (PR) and contactless electroreflectance (CER). Five possible nitrogen nearest-neighbour environments, i.e. short-range-clusters, lead to five discrete band gap energies. In this work the temperature dependence of the band gap energy E(T) related to the individual clusters has been investigated. In addition, we show a first CER evidence of the energy-fine structure of the band gap for GaInNAs system. Besides GaInNAs samples series of GaNAs and GaNAsSb samples have been investigated in PR and CER. It has been shown that after annealing a blueshift of the band gap energy appear also for these samples. However, no energy-fine structure of the band gap energy has been observed for these compounds.   

Structure and local vibrational frequencies of the 2H complex in H-irradiated GaAs:N

Hydrogen irradiation of GaAs:N samples leads to a giant blue shift of the band-gap [1]. Insight on the H complex has recently been available through infrared studies [2], showing three distinct H-modes (3195, 2967, and 1447 cm$^{-1})$, all correspond to N-H bonds. The absence of the Ga-H bond, however, contradicts previously proposed low-energy H$_{2}$* model where one H is on Ga whereas the other is on N [3]. Analysis of the measured isotope shifts shows that the H complex in the H-irradiated GaAs:N samples should involve two coupled H atoms. Based on density functional calculations, we propose a new nitrogen-2H complex. Not only the model accounts for the observed giant blue shift due to H-irradiation, but the calculated H-vibrational frequencies (3207, 3052, and 1417 cm$^{-1})$ and isotope shifts are also in good agreement with experiment. Supported by the U. S. DOE/ BES and EERE under contract No. DE-AC36-99GO10337. [1] G. Baldassarri H. v. H., et al., Appl. Phys. Lett.\textbf{78}, 3472 (2001) [2] F. Jiang, et al., Phys. Rev. B \textbf{66}, 073313 (2002) [3] A. Janotti, et al. Phys. Rev. Lett. \textbf{89}, 086403 (2002)   

Ion Beam Synthesis of InAsN Nanostructures

We recently demonstrated the utility of ion-beam-synthesis for producing light-emitting GaAsN nanostructures [1]. Here, we report the ion-beam-synthesis of InAsN nanostructures, using low temperature N implantation into epitaxial InAs. 100keV N ion implantation, with a dose of 5x10$^{17}$cm$^{-2}$, leads to complete amorphization of a $\sim $300nm thick surface layer. Following annealing, this layer transformed into three layers: a nanostructure layer containing $\sim $5nm zincblende InN-rich InAsN crystallites within an amorphous matrix, a polycrystalline layer consisting of $\sim $100nm InAs-rich InAs:N crystals and amorphous domains, and layer of solid-phase epitaxially grown InAs. These results suggest that ion-beam synthesis is promising for producing InN-rich nanostructures or/and InAs-rich alloys. We will also discuss the effects of implantation and annealing conditions on the structure and properties of ion beam synthesized InAsN nanostructures. [1] X. Weng. R.S. Goldman, et al, J. Appl. Phys. 92, 4012 (2002); Appl. Phys. Lett. 85, 2774 (2004).   

The influence of growth temperature on the nitrogen incorporation into MBE-grown GaInNAs-on-GaAs epilayers

We have studied the influence of growth temperature (within the 410-470 $^{o}$C range) on the nitrogen incorporation into lattice-matched GaInNAs-on-GaAs epilayers grown by molecular-beam epitaxy under constant fluxes. It was found that, over the whole temperature range, nitrogen is incorporated both on substitutional sites and as dimers on Ga and As sites. On substitutional sites nitrogen is present in the form of N-Ga$_{4}$ clusters and, to a lesser extent, in the form of N-Ga$_{3}$In ones. Increasing the growth temperature reduces the amount of substitutional nitrogen and increases the ratio between the N-Ga$_{3}$In and N-Ga$_{4}$ clusters. At the same time, the band gap increases. The amount of nitrogen dimers also decreases with increased growth temperature but the ratio between nitrogen dimers and nitrogen substitutionals appears not to be affected by the growth temperature. The effects of annealing on the incorporated nitrogen are discussed in the paper.   

GaAs-based InGaAsN Lasers

We demonstrated 1262 nm high performance single mode InGaAsN lasers. 4 $\mu $m stripe ridge waveguide InGaAsN lasers were processed. For as-cleaved case, pulsed threshold current was only 15 mA (313 A/cm$^{2})$ for 1200-$\mu $m-long chips at room temperature (RT). The laser can work beyond 120 $^{\circ}$C. After AR/HR coating, pulsed emission was up to 250 mW at RT. For cw operation, the lasers show a very low threshold of 25 mA and maximum output was up to 40 mW for 1200 $\mu $m length chip at RT. All the emission above was kink-free and single mode. New InGaAsN quantum well (QW) structures were designed. Comparing with the conventional InGaAsN QW structures, photoluminescence (PL) investigations show a significant improvement. After 3000 sec of annealing at 700 $^{o}$C, the PL peak area is about 20 times higher while the wavelength keeps 25 nm longer. After 800 sec of annealing, the PL quenched slowly for the conventional structures because of the strain relaxation, while the PL of the new structures increased rapidly and show no saturation after 3000 sec of annealing.   

Session L16: Photonic Crystals I

Optimization of two-dimensional photonic bandstructure systems using steepest-descent algorithm

Conventionally, perturbation theory is used for the analytical study of small changes in a system. However, it can also be considered as an exact expression in differential form for the gradient of an objective function describing a system. Based on this idea, we have developed an efficient steepest-descent algorithm to optimize particular features of a photonic bandstructure system. We have applied the algorithm to optimize the size of the photonic bandgap at minimal dielectric contrast in two-dimensional photonic crystals on square and hexagonal lattices, obtaining non-zero complete gaps at very low dielectric contrast.   

Efficient finite-element Green's function approach for CD metrology of 3D gratings on multilayer films.

We present an efficient method for calculating the reflectivity of 3D gratings on multilayer films based on a finite-element Green's function approach. Our method scales like $N^2$ (k-space version) or $N \log N$ (real-space version), where $N$ is the number of plane waves used in the expansion. Therefore, it is much more efficient than the commonly adopted rigorous coupled wave analysis (RCWA) method, which scales like $N^3$. We demonstrate the effectiveness of this method by applying it to a 2D periodic array of contact holes on a multilayer film. We find that our Green's function approach is at least one order of magnitude faster than the RCWA approach when applied to typical contact holes considered in industry. For most cases, this method is efficient enough for application as a real-time critical dimension (CD) metrology tool.   

Photonic Crystal Defect Mode Analysis Using Discretized Vector Wannier Functions

We present a theoretical approach for calculating the photonic structure of defects in 2D photonic crystals. The central feature of our approach is the basis construction of local vector Wannier functions from the perfect crystal eigenstates. It has been proposed$^a$ that this basis be used to expand photonic crystal defect states analogous to the expansion in linear combinations of atomic orbitals of electronic structure of the ideal silicon vacancy.$^b$ These approaches rely on a small number of basis states local to the defect region. In this work, we replace the fourier expansion of the perfect crystal by a real-space description in vector finite-elements. This method allows the computation of the basis on the same grid as the perfect structure and a simpler defect eigenvalue problem. We present results that verify the eigenmodes of the crystal and examine defect modes. \newline $^a$K.M. Leung, J. Opt. Soc. Am. B \bf{10}, 303 (1993). \newline $^b$G.A. Baraff and M. Schluter, PRL \bf{41}, 892 (1978); J. Bernholc, N.O. Lipari, and S.T. Pantelides, PRL \bf{41}, 895 (1978).   

Tunable band gaps in two-dimensional semiconductor-dielectric photonic crystals

This paper reports the multiple band gaps in the two-dimensional semiconductor-dielectric photonic crystals of several compositions: semiconductor (dielectric) thin cylinders in the dielectric (semiconductor) background. We consider both square lattice and hexagonal lattice arrangements and compute extensive band structures using a plane-wave method within the framework of an efficient standard eigenvalue problem for both E- and H-polarizations. The whole range of filling fractions has been explored to claim the existence of the lowest (the so-called acoustic band gap) and multiple higher-frequency band gaps within the first thirty to forty bands for various compositions. The completeness of the existing band gaps is substantiated by computing the band structures via detailed scanning of the principal symmetry directions covering periphery as well as the interior of the irreducible part of the first Brillouin zone and through the computation of the density of states. In general, the composition made up of doped semiconducting cylinders in the insulating background is found to be the optimum case for both geometries. Such semiconductor-dielectric photonic crystals which are shown to possess huge lowest band gaps below a threshold frequency (the plasma frequency) have an advantage over the dielectric photonic crystals in the emerging technology based on the photonic crystals.   

Theory of Light Refraction at a Surface of a Photonic Crystal

In past studies on photonic crystal refraction, how the surface orientation affects refraction was largely unexplored. In this work, a general, analytic theory of light refraction is developed for a photonic crystal (PC) that has an arbitrary lattice type and surface orientation. A simple topological argument is presented to prove the equal partition of forward and backward propagating modes by an arbitrary plane in any periodic optical media. Furthermore, we have discovered the surface-dependent degeneracy of crystal modes. The current theory addresses light refraction by a {\it natural} quasi- periodic surface, in addition to an ordinary periodic surface. Particularly interesting is the transition from a periodic surface to a quasi-periodic surface, which could happen upon a slight change of surface orientation. Such a transition could lift the surface-dependent mode degeneracy, which can be observed as a small number of refracted beams split into an essentially infinite number of beams. Harnessing such an extremely sensitive phenomenon could lead to interesting applications in optoelectronics.   

Effective Index Model and Guided Modes in a Photonic Crystal Fiber

In the last few years, there has been intense work in photonic crystal fibers (PCF's). These systems can be obtained by surrounding the core of a normal fiber with a two-dimensional photonic crystal made of silica, with air holes running along the length of the fiber. To study the waveguiding properties of these fibers, the cladding surrounding the solid core is replaced by an effective homogeneous medium described by an effective refractive index. This effective index model has been used to explain some of the peculiar properties of these systems. However, using the effective index medium to calculate the number of guided modes in PCF's, it is necessary to know the radius of the core precisely. Because in PCF's there is no clear boundary between the cladding and the core, different values of the fiber's core have been used in the literature. In this work we calculate the waveguiding properties of PCF's solving the Maxwell equations by using the plane wave expansion and the supercell method. We calculate the propagation constant both, for the propagating modes in the PCF's and for the fundamental space-filling mode (FSM). The FSM is the fundamental mode of the infinite photonic crystal cladding when the core is absent. Our results predict single-mode behavior at higher values of the air holes radius when compared with those reported previously.   

Flat lens without optical axis: Imaging theory

We derive a general theory for imaging by a flat lens without optical axis. We show that the condition for imaging requires a material having elliptic dispersion relation with negative group refraction. The medium can be viewed as having an effective anisotropic refractive index. Imaging can be achieved with both positive and negative refractive indices, although the image quality can vary greatly and multiple images may be present. The Veselago-Pendry lens is a special case of the theory with isotropic negative refractive index of -1. Snell's law for group refraction is valid and leads to ray diagrams. Realizations of the imaging conditions using anisotropic media and inhomogeneous media, particularly photonic crystals, are discussed. Numerical examples of imaging and consequences for sub-wavelength imaging are also presented. Work supported by NSF and AFRL.   

Focusing by plano-concave lens using negative refraction

We demonstrate experimentally focusing of plane waves at microwave frequencies by a plano-concave lens using negative refraction. The lens was fabricated from a microwave dielectric photonic crystal acting as a left-handed metamaterial. The inverse experiment where the source is placed at the observed focal point was also performed and shows clearly an emerging plane wave. The focal point is observed to move with the radius of curvature of the lens. Different radii of curvature have different frequency ranges of focusing all of which lie in the second band frequencies along $\Gamma $-X propagation direction of the photonic crystal. The measured values of refractive index are in complete agreement with those determined from band structure calculations. Work supported by AFOSR and NSF-PHY-0098801.   

Negative Refraction and Subwavelength Lensing in a Polaritonic Crystal

We show that a two-dimensional polaritonic crystal, which is made of metallic rods which support plasmon oscillations, can act in a narrow frequency range act as a medium in which a negative refraction, and subwavelength lensing occurs. The lensing effect in our crystal obeys the image-distance relationship characteristic of an n = -1 material. We show, that surface polaritonic modes are excited on the surface of the lens, and that they facilitate restoration of the evanescent waves, which carry the subwavelength image information. We also demonstrate, that this can occur in the visible frequency range for a wide range of materials, including silver and aluminum rods, as well as carbon nanotubes. This flexibility should allow for an experimental demonstration of this phenomenon in the visible frequency range.   

Electron beam lithography based fabrication of omnidirectional photonic crystal structures for visible and near-infrared wavelengths

We describe the fabrication of a three dimensionally periodic photonic crystal structure with omnidirectional band gap for the near-IR and visible wavelength region using a technique of direct electron beam write coupled with multi-level alignment. Using this method we have successfully fabricated silicon as well as gold based Iowa State ``woodpile'' structure with lattice spacings as small as 550 nm. We tested the devices for their optical properties and we find that the data reveals features consistent with the photonic band gap.   

The Extended Plane Wave Expansion Method for 3D Anisotropic Photonic Crystals\footnote{Supported by NSF(through the Northwestern MRC)}

Conventional plane-wave expansion (PWE) methods, good for calculating such properties as photonic band gaps for materials with periodic structure, are very difficult for calculating a crystal with an interface. While the dispersion relation used by PWE does not restrict the wave vectors, {\bf k}, to be real, the complex {\bf k} are the important for interface calculations. Therefore, we modified the PWE to make it possible to easily calculate the complex {\bf k} (EPWE) both in the 2D isotropic and the general 3D anisotropic cases. Advantages gained include: (i) the frequency is initially given and regarded as a known variable, rather than as an argument, and can always be set to be a positive real number even for complex systems with real, imaginary, or complex frequency-dependent permittivity or permeability; (ii) from the complex {\bf k} results, the resonant feature of the transmittivity can be easily analyzed; (iii) since EPWE is extended from the PWE, it obeys the same dispersion relation, and their results will also be the same, when providing PWE the {\bf k} derived from EPWE; (iv) because the imaginary part of {\bf k} is associated with the reciprocal of the penetration depth, the shortest width of the crystal when it is treated as a single crystal is well-defined. As an example, we present 3D isotropic GaAs crossed square prisms and find a good correspondence between results of both methods.   

Negative Refractive Index Materials

The realization of dielectric and metallic periodical matrices in a bottom up fabrication procedure is proposed for realization of materials with negative refractive index (NIM) or pseudo NIM (PsNIM). The single elements of the periodical matrices (2D photonic crystal, 2D PC)) will be realized by high-resolution e-beam lithography and dry etching technique. Dielectric materials will be used for PsNIM approaches. Special importance in the case of metallic matrices (for NIM approaches) will be put on structures in the nanometre region to control the surface plasmon modes. By stacking of the 2D PC structures we will realize the 3D PC in a bottom up procedure with special consideration of adjustment of matrix modes (PC gap frequencies) and surface plasmon modes to control and enhance the NIM-effect. Furthermore a stacking of many as possible layers in the bottom up procedure is important for the reduction of the surface leakage rate of the electromagnetic field out of the NIM structure. Optical measurements with high resolution and time resolved measurements will be carried out with partners in narrow co-operation.   

Session L17: Structural and Optical Properties of Nanostructures

X-ray Scattering Studies of Monolayer Assembly of Zeolite Crystals

We characterized monolayer assemblies of zeolite crystals using x-ray reflectivity and diffuse scattering. They were prepared on Si wafers through two different types of molecular linkages, namely, through the direct linkage between the Si-tethered 3- chloropropyl (CP) groups and the surface hydroxyl groups of zeolites and through the linkage between zeolite- and Si- tethered CP groups via polyethylene imine (PEI) as the intermediate linker. X-ray reflectivity results clearly differentiated the two types of linkages and furthermore showed the thickness and the density of each component layer before and after the assembly of zeolite monolayers on substrates, demonstrating that this analytical technique can serve as a powerful tool to collect important information about the molecular linkages between the molecularly tethered microcrystals and substrates.   

Absorption and Photoluminescence in very small diameter Si Nanowires

Optical absorption and photoluminescence spectra on 3 sets of crystalline Si nanowires with most probable diameter 3.5 nm, 5.5 nm and 9 nm are presented. In the optical absorption spectra, apart from the direct gap absorption at Er1 $\sim $ 3.4 eV and Er2 $\sim $ 4.2 eV, we observed two additional strong peaks near 1.5 eV and $\sim $2.5 eV. The 3.4 and 4.2 eV peaks exhibit a weak but clear blue shift with decreasing wire diameter. Interestingly, the anomalous 1.5 and 2.5 eV peaks increase in intensity with decreasing nanowire diameter. It is also interesting that the 1.5 eV peak does not shift with decreasing wire diameter. This behavior leads us to tentatively assign this structure to Si-SiO2 interface states. On the other hand, the $\sim $2.5, 3.4 and 4.2 eV absorption bands exhibit a systematic blue shift with decreasing diameter. This behavior is consistent with a quantum confinement phenomenon. Structure in the photoluminescence is also observed at $\sim $1.5 and $\sim $2.4 eV. The origin of these low energy peaks will be described in terms of the band structure of Si nanowires.   

Interplay of Phonon Confinement and Thermal Phenomena on the 520 cm-1 Raman band in Very Small Diameter Silicon Nanowires

Results of Raman experiments that investigate the influence of laser flux and thermal anchoring on the asymmetric line profile $\sim $520 cm$^{-1}$ optical phonon scattering from small diameter Si nanowires are presented. At low laser flux $\Phi \quad \le $ 20$\mu $W/$\mu $m$^{2}$, the lineshape seems well described by a phenomenological lineshape function associated with phonon confinement due to Richter et al$^{1}$. However, at high laser flux $\ge $ 100 $\mu $W/$\mu $m$^{2}$, the Raman band takes on even higher asymmetry that is likely due to inhomogenous heating. The data at low and high laser flux can be explained quantitatively on the basis of fundamental Raman scattering theory. $^{1}$ H.$^{ }$ Richter, Z. P. Wang, and L. Ley, Solid State Communcations, \textbf{39}, 625 (1981) $^{\dag }$Work supported by the NSF NIRT program (DMR- 0304178).   

Thin film assemblies of silicon nanoparticles roll up into flexible nanotubes

We report on synthesis of flexible nanotubes made of a self-assembly of fluorescent silicon nanoparticles. When a colloidal dispersion of the Si nanoparticles in alcohol is submitted to an electric field, the particles are driven to one of the electrodes via eletrophoresis. We coat various surfaces with thin films of silicon particles. Upon drying, the film rolls up into uniform tubes. We used Atomic Force Microscopy (AFM) and a linear elasticity model to measure the young modulus of this film. It was found to be as flexible as rubber. These structures have potential applications for future enhanced biological recognition and sensing of toxins. Moreover, they are useful as catalysts, and in nano robotic applications.   

Sensitivity of dangling bonds to the presence of dopant atoms in hydrogen terminated silicon nanoclusters

Dangling bonds on the surfaces of semiconductor nanoparticles are expected to play an important role in the self-assembly of hybrid molecule-semiconductor nanoelectronic devices[G.P. Lopinski, D. D. M. Wayner, and R. A. Wolkow, Nature 406, 48 (2000)] and should also directly impact the electronic and transport properties of such nanostructures. We have studied the dangling bond on hydrogen terminated silicon nanoparticles, both analytically in the effective mass approximation and using a self-consistent Poisson-Schr\"odinger model that we previously developed.[Phys. Rev. B, to be published] This model allows us to make calculations on silicon structures containing many hundreds of silicon atoms, enabling us to explore (doped) silicon nanoparticles with dangling bonds. A dangling bond on a hydrogen terminated silicon surface is shown to behave qualitatively as an electronic acceptor, its energy level however depends on the occupation of the dangling bond state which in turn depends on the the temperature and strongly on doping of the silicon. We will present energies and wave functions for dangling bond states on different (doped) hydrogen terminated silicon surfaces.   

Formation of Micro Tubes from Strained SiGe/Si Heterostructures

We report the formation of arrays of micrometer-sized SiGe/Si tubes by releasing strained SiGe/Si heterostructures from substrates. The silicon oxide sacrificial layer is etched by hydrofluoric acid buffered with ammonia fluoride. Because of the dynamic curvature change of the bilayer, the etching process deviates from the conventional ransport-controlled regime to the kinetic-controlled regime. A slow symmetric and a fast asymmetric etching mode are identified. The fast mode is associated with asymmetric surface deformation. Large etch channels are induced and etching becomes reaction controlled. In the slow etching process, bilayers are symmetrically deformed and retain mostly the initial surface pattern. A crossover from the transport-controlled (symmetric) etching to kinetic-controlled (asymmetric) etching is observed when the size of the bilayers becomes much larger than the curvature radius. Dispersion in etch rate is directly related to the degree of asymmetry in surface deformation. Using a micro manipulator, SiGe/Si tubes are assembled onto a micro-strip line for radio-frequency characterizations.   

Optical and Electronic Characteristics of Germanium Quantum Dots Formed by Selective Oxidation of SiGe/Si-on-Insulator

A complementary metal-oxide-semiconductor (CMOS)-compatible method is proposed to form atomic-scale germanium (Ge) quantum dots ($<$10 nm) for application in single-electron (SE) devices or optical devices. The formation of Ge quantum dots is realized by the Ge atoms' segregation and agglomeration during thermal oxidation of Si$_{1-x}$Ge$_{x}$ alloys. The size and distribution of the Ge dots are determined by conditions of thermal oxidation process and Ge content in the alloys. The optical and electronic characteristics of Ge quantum dots were examined by $x$-ray diffraction, high-resolution transmission electron microscopy, cathodoluminescence spectroscopy, and spectroscopic ellipsometry. The dot size and crystallite morphology were strongly dependent on thermal oxidation conditions. Visible photoemissions from Ge dots were observed at room temperature and they exhibited pronounced blueshifts of peak energies with increasing oxidation time, which can be correlated to the change in dot size, shape, or crystalline structure transition. Compared to bulk Ge, the reduced refractive index and relevant blueshifts of band-structure critical points of Ge quantum dots, derived from spectroscopic ellipsometry, are also correlated to the nanocrystal size effects.   

Raman spectroscopy of free-standing Ge nanocrystals

Ge nanocrystals, with an average diameter of 5 nm, are grown in a silica matrix. Free-standing nanocrystals are obtained by selectively etching the oxide. Embedded nanocrystals experience considerable compressive stress relative to the bulk. The Raman line position of free-standing nanocrystals is redshifted by $\sim $6 cm$^{-1}$ relative to that of embedded nanocrystals, indicating relief of the compressive stress. Mixed surface/bulk vibrational modes between 125 and 250 cm$^{-1}$ and surface modes below 125 cm$^{-1}$ are observed by Raman spectroscopy on free-standing nanocrystals. These modes are not observed for the case of embedded nanocrystals. Exposed nanocrystals are stable under ambient atmospheric conditions after the formation of a thin, self-limiting native oxide layer. The effect of oxide layer thickness on the vibrational spectra of free-standing nanocrystals will be discussed. This work is supported in part by U.S. NSF Grant Nos. DMR-0109844 {\&} EEC-0085569 and in part by U.S. DOE under Contract No. DE-AC03-76F00098.   

Ge nanocrystals embedded in sapphire

Ge nanocrystals are formed in a sapphire matrix by ion implantation followed by thermal annealing. Transmission electron microscopy (TEM) of as-grown samples reveals that the nanocrystals are faceted. Notably, the matrix remains crystalline despite the large implantation dose and corresponding damage. Embedded nanocrystals experience up to 4 GPa of compressive stress relative to bulk, as measured by Raman spectroscopy of the zone center optical phonon. In contrast, nanocrystals embedded in silica are observed to be spherical and experience considerably lower stresses. Also, \textit{in situ} TEM reveals that nanocrystals embedded in sapphire melt very close to the bulk melting point (T$_{m}^{ }$= 936 $^{\circ}$C) whereas those embedded in silica exhibit a significant melting point hysteresis around T$_{m}$. This work is supported in part by U.S. NSF Grant Nos. DMR-0109844 {\&} EEC-0085569 and in part by U.S. DOE under Contract No. DE-AC03-76F00098.   

Structural and Optical Properties of Sn$_x$Ge$_{1-x}$ thin films and Quantum Dots

Sn$_{x}$Ge$_{1-x}$ layers and quantum dots (QDs) are of great interest as materials that could provide tunable direct band gaps, allowing completely group IV-based optoelectronic devices. These materials could be used in a wide range of applications such as emitters, infrared detectors, and thermophotovoltaics. However, substantial challenges remain in the growth and processing of these materials. We have grown Sn$_{x}$Ge$_{1-x}$ films by Molecular Beam Epitaxy (MBE), using low growth temperatures ($<$200$^{\circ}$C) in order to grow fully strained layers. X-ray diffraction, transmission electron microscopy, and Rutherford backscattering spectroscopy data indicate high-quality epitaxial films. Post-growth annealing was used to form QDs. Either QDs or quantum wires may be formed depending on annealing parameters. The effects of varying substrate temperature between 400C (wires) and 750C (QDs) on size and distribution of quantum structures were explored and will be discussed. Sn concentration (0-10{\%}) and film thickness (40nm - 200nm) were also varied. Optical properties probed by Fourier transform infrared spectroscopy (FTIR) will be presented. FTIR spectra clearly show the decrease in band gap of Sn$_{x}$Ge$_{1-x}$ layers with increasing Sn fraction up to 10{\%}. Photomodulated reflectance (PR) is another sensitive method for probing critical points in Sn$_{x}$Ge$_{1-x}$ band structure, and can detect both direct and indirect transitions. PR results for Sn$_{x}$Ge$_{1-x}$ layers will also be discussed.   

Raman Scattering from Surface Optic Phonons in Cylindrical and Rectangular Cross-sectional Semiconducting Nanowires†

Raman scattering from surface optic (SO) phonons has been observed and identified in cylindrical GaP and rectangular cross-section ZnS nanowires. We propose that the symmetry breaking mechanism which activates the SO phonon is a periodic modulation of the cross-sectional area along the nanowires. In the case of cylindrical GaP nanowires, Raman scattering from SO phonons in air at room temperature is observed at 394 cm-1, in between the first order longitudinal optic (LO) (401 cm-1) and transverse optic (TO) (367 cm-1), and downshift to 392 cm-1 in dichloromethane (?m=2.0) and 390 cm-1 in aniline (?m=2.56). Raman scattering from the ZnS nanowires in air at room temperature reveals a strong first-order LO mode at 346 cm-1 and two TO modes at 269 and 282 cm-1. The SO Raman band in ZnS is observed at 335 cm-1 in air, and downshifts to 328 cm-1 in dichloromethane and to 326 cm-1 in aniline. The position of the SO band in GaP and ZnS nanowires is consistent with a dielectric continuum model. Theoretical SO phonon dispersion for both cylindrical and rectangular cross-section nanowires is presented and compared to experiment. {\dag}This work was supported by the NSF NIRT program (DMR- 0304178).   

Fabrication of Magnetically Half-Coated Nanoparticles Using Molecular Beam Deposition

The top-down approach of building nanodevices can be combined with the bottom-up to create half-coated nanoparticles with well controlled magnetic, optical, electronic, and chemical properties. A type of half-coated nanoparticle particle that utilizes both optical and magnetic control is Magnetically Modulated Optical Nanoprobes (MagMOONs) (JN Anker, and R Kopelman, Appl. Phys. Lett., 82, 1102-1104 (2003).). MagMOONs emit modulated fluorescence or reflection intensities when externally manipulated by a magnetic field. We have fabricated functional MagMOONs by coating a thin layer of polycrystalline cobalt onto micro and nanospheres using molecular beam deposition (MBD). Additionally, vectorial magneto-optical Kerr effect was used to study the magnetization reversal of the coated spheres supported by a substrate. Hysteresis loop variations associated with the sphere size changes are observed and compared with the case of planar deposited films.   

Highly-Ordered Monolayers in a Hurry: Non-Equilibrium Pathways to High-Quality Superlattices

The formation of 2D superlattices has long been thought to occur at the end of evaporation when particle-particle, -solvent and -substrate interactions can induce spinodal phase separations. Surprisingly, recent experiments have shown that it is indeed possible to create highly-ordered nanocrystal monolayers by non-equilibrium processes such as drop drying. In these cases the monolayer self-assembly occurs on the liquid-air interface. We unravel details of the self-assembly mechanism by tracking the monolayer formation through its entire evolution using a combination of direct optical microscopy and transmission electron microscopy. Drop drying itself is a highly non-equilibrium phenomenon that is affected by solvent evaporation, mass transport inside the drop, drop geometry, environmental conditions, and other factors. Despite this complexity, we observed that highly-ordered monolayers form according to three different robust growth laws: linear, quadratic, and exponential in time. We delineate a phase diagram based on a simple geometric model that can be used to better understand the self-assembly of nanocrystal thin films from solution.   

Mechanical Properties of CdSe Tetrapods

CdSe tetrapods are novel nanoscale crystals with unique electric/optical properties, which makes them promising candidates for making nanocrystal based photovoltaic solar cells. However, their mechanical properties are still not well understood. We used atomic force microscopy to study their mechanical and electrical properties and try to discover the correlation between them. First of all, the AC mode images revealed that each arm of the tetrapods is about 150 nm long except the one that is along the surface normal (the vertical arm). They also showed that the tetrapods deposited on Si surfaces have already been pushed down by the capillary force caused by surface water layer. Additionally, the mechanical properties of CdSe Tetrapods were studied using force-volume technique. We were able to put the AFM tip right on the top of the vertical arm using this technique. We discovered that the tetrapods would undergo elastic deformation if the applied force was less than 52 nN. After we applied force more than 91 nN, the tetrapods would undergo plastic deformation and we started to observe the bending of the vertical arm. Applying a force more than 130 nN on top of the vertical arm would then completely destroy the tetrapods. Current Force Microscopy will be used to study the mechanical and electrical properties of tetrapods at the same time, which will be expected to give us more insight on the relationship between the electrical properties of CdSe tetrapods and their ``molecular'' structures.   

Library of Templates of Virtuall Small Clusters of Ga and In with As, P and V Atoms

Electronic energy level computations for small atomic clusters are feasible and provide a foundation for realization of a virtual (i.e., fundamental theory- based, computational) approach to synthesis of sub-nanoscale materials with pre-designed electronic and magnetic properties. In this study the Hartee-Fock (HF) and CI/CAS/MCSCF methods have been used to develop about 20 stable pre-designed and vacuum virtual clusters of Ga and In with As, P and V atoms that provide templates for experimental synthesis of the corresponding clusters in confinement and vacuum. The electronic energy level spectra (ELSs) and spin density distributions (SDDs) of the template clusters reflect influence of clusters' growth conditions, form and composition on formation and structure of their valence and conduction bands, the values of their optic transition energies, and collectivization of their spin density distributions. The ELSs and SDDs of the templates are compared and classified according to their possible use in development of sub-nanostructured materials for electronics, magneto-optics and spintronics.   

Session L18: Focus Session: Wide Band Gap Semiconductors II

Doping and defects in AlN and InN

First-principles calculations have significantly contributed to our understanding of defects and impurities in GaN. Here I will present new results for AlN and InN. UV light emitters require AlGaN alloys with high Al content, in which doping becomes increasingly problematic. In addition, bulk AlN is being considered as a substrate material, prompting an investigation of the origin of various absorption lines. For n-type doping, DX-center formation turns shallow impurities such as O, Si, or Ge into deep centers. I will present detailed configuration-coordinate diagrams that summarize the prospects of using these impurities for doping. Al vacancies, as well as their complexes with oxygen, can also occur, and I will discuss specific proposals for the optical absorption lines typically observed at 2.8 eV and 4.5 eV. Turning to InN, a major challenge is to control the n-type conductivity observed in nominally undoped material. Calculations show that point defects such as nitrogen vacancies are unlikely to be responsible for this unintentional conductivity. It is more plausibly caused by impurities such as oxygen. In the case of InN we have found that an even more common impurity can act as a shallow donor, namely hydrogen. Detailed results for the atomic and electronic structure of hydrogen in various configurations will be discussed.   

Effect of surface facets on the efficiency of InGaN/GaN quantum wells grown by molecular-beam epitaxy

The pronounced enhancement of indium incorporation efficiency and quantum efficiency for InGaN/GaN quantum wells due to rough, faceted surface of the GaN templates is reported. We studied the growth of InGaN/GaN quantum wells by RF plasma MBE on two types of GaN templates, i.e. MOCVD GaN templates and ammonia- MBE GaN templates. The latter was grown in situ with a growth system equipped for both ammonia- MBE and RF plasma MBE. Unlike the smooth (0002) surface of GaN templates grown by MOCVD, the surface of the templates grown by ammonia-MBE is defined by {\{}10-1$m${\}} pyramidal facets causing significant surface roughness. Possible mechanisms for the enhanced indium incorporation efficiency due to these surface facets, such as the possible indium migration to the extremities of the facets forming quantum dots, are discussed. We also investigated InGaN/GaN quantum wells grown on selectively grown GaN micro-pyramids with well-defined {\{}10-12{\}} or {\{}10-11{\}} facets. Micro-PL measurements aimed at resolving the emissions from the quantum wells on the facets and quantum dots at the tips of the micro-pyramids will be discussed.   

Electron scattering in bulk InN

Recent improvements in the quality of single-crystal InN have made controlled doping studies possible. Irradiation by a variety of beams (electron, proton, He) can be used to produce n-type bulk crystals of InN with carrier concentrations ranging from mid-10$^{17}$ cm$^{-3}$ to more than 10$^{20}$ cm$^{-3}$. We have calculated electron mobilities in InN, incorporating the standard mechanisms of acoustic and optical phonons, Coulomb scattering, and scattering from resonant defect states, and compared them with room temperature experimental values. The mobility in samples with carrier concentrations below the 10$^{18}$ cm$^{-3}$ level is limited by scattering from optical phonons and Coulomb centers. At concentrations between 10$^{18}$ cm$^{-3}$ and 10$^{20}$ cm$^{-3}$, the mobility is dominated by Coulomb scattering alone. At the highest concentrations, above 10$^{20}$ cm$^{-3}$, scattering from defects with energy levels resonant with the conduction band begins to play an important role. Our results also suggest that the defects from which the electrons originate may be multiply-charged. This work was partially supported by the US DOE under Contract No. DE-AC03-76SF00098.   

Surface and Bulk Properties of InN

InN presents challenges and opportunities uncovered during the last 3 years. The bandgap of 0.65eV is much smaller than UV bandgaps for GaN or AlN. InN shares similar chemical and radiation resistant properties of these other wurtzite III-nitrides. In contrast, control of InN electrical conductivity is significantly more challenging. The greatest difference is seen in band bending at the free surface. In place of surface depletion, InN exhibits surface accumulation of electrons at sheet densities of 3-5x1013 cm-2 accompanied by a large surface electric field. The valence band to Fermi level energy difference is found to be 1.2eV (XPS) to 1.5eV (EELS,CV). Undoped InN is n-type. Electron densities are below donor impurity densities and fall with increased thickness. A large dislocation density, which falls with thickness, may play a role in creating the bulk electrons. Low mobility electron transport occurs in defective regions near the lattice mismatched interface with GaN or AlN buffer layers, while low defect density regions have mobility exceeding 2000 cm2/Vsec with non-degenerate carrier densities near 1x1017 cm-3. Electron effective mass is 0.045m0 in the minimum of a non-parabolic gamma conduction band. Velocity-field relations from single particle spectroscopy show velocity overshoot and negative differential mobility. THz wavelength radiation from short pulse laser excitation is generated from the surface of InN and GaN/InN interfaces due to large electric fields. InN electro-chemical sensing occurs by surface chemical interaction with InN surface electron accumulation. P-type doping of InN and GaInN with Mg is inferred by temperature variable conductivity, but surface electron conductivity dominates net Hall polarity.   

A1(LO)phonon in degenerate InN semiconductor films

We have studied the A$_{1}$(LO) structure of InN thin films from a low (n$_{e}$=6.7x10$^{17}$/cm$^{3}$ ) to a very high (n$_{e}$=9.6x10$^{20}$/cm$^{3 })$ carrier concentration using Raman scattering experiments. Theoretically we investigated this structure using a wavevector dependent dielectric function$\varepsilon (q,\omega )$ which takes into account the coupling of longitudinal-optical (LO) phonon and electrons with non-parabolic energy dispersion. Phonon-plasmon interaction cannot explain the origin of this structure. However, phonon interaction with electron-hole pair excitations forms a well-defined structure in $Im\varepsilon (q,\omega )^{-1}$ which emerges from the electron hole pairs spectrum when higher-energy coupled-mode becomes Landau damped. With increasing values of q, this structure moves towards the experimental value. This peak structure is formed by a weaker (relative to the plasmon) interaction between the LO-phonon and electron hole pair excitations. Experimentally it is observed that the energy of this structure increases with increasing value of electron density.   

Near resonance enhanced Raman scattering and room temperature photoluminescence in highly degenerate InN films

We report the results of near resonance enhanced Raman scattering and room temperature photoluminescence (PL) studies on highly degenerate (carrier concentration, $n_{e} \quad >$ 3 $\times $ 10$^{19}$ cm$^{-3})$, wurtzite InN films grown on $c$-plane sapphire substrates by plasma source molecular beam epitaxy. At room temperature, carrier concentration dependent strong PL emission is observed in the 1.4-1.8 eV range. These films show strong resonance enhanced first and second order Raman scattering under 785 nm (1.58 eV) excitation energy and not with 514.5 nm (2.41 eV) excitation, suggesting large shifts in the optical absorption edges due band filling effects in these highly degenerate InN samples. The PL emission peak energies and their dependence on the carrier concentration are consistent with observed optical absorption edges. The present results are compared and contrasted to the data on single crystalline, low degenerate InN films which show a bandgap energy of $\sim $0.7 eV.   

Demonstration of strong photoluminescence in the deep-green/yellow region from InGaN/GaN multiple quantum wells grown on native AlN substrates

Relatively intense deep-green/yellow photoluminescence emission at $\sim $570 nm is demonstrated for InGaN/GaN multi quantum well structures grown on native AlN substrates, showing potential to extend commercial III-N LED technology to longer wavelengths. Temperature- and excitation-power dependent photoluminescence and cathodoluminescence results show the presence of alloy compositional fluctuation in the active region despite the lower strain expected in the structure. This is contrary to what has been observed for lower In content InGaN/GaN MQWs on bulk GaN substrates with sharp interfaces and little alloy compositional fluctuation in the InGaN layers. Detailed optical characterization results will be presented .   

True LDA Band Gaps of Wurtzite and Cubic Indium Nitride

We report the calculated band gap of wurtzite and cubic indium nitride (InN). Our ab-initio computations employed a local density approximation (LDA) potential and the linear combination of Gaussian orbital (LCGO) formalism. The implementation of the Bagayoko, Zhao, and Williams (BZW) method led to \textit{true LDA band gaps of 0.88 eV and 0.65 eV for wurtzite and cubic indium nitrides}, respectively. When available, recent experimental electronic structures, density of states, band gaps, and effective masses agree with our findings. We discuss the need for the BZW approach in LCAO calculations purporting to implement the initial, density functional theory that is concerned with the description of \textit{only the ground state.}   

Room Temperature Photoluminescence and Absorption in InAlN/AlN/Sapphire quantum dot structures

We have grown InAlN self-assembled quantum dots (QD) on a AlN epitaxial layer with the average diameter of the QDs was as small as 20 nm and detected strong QD photoluminescence. The samples were grown by Plasma Source Molecular Beam Epitaxy on sapphire (0001) substrates. A 150 nm AlN buffer layer was grown at 400$^{\circ}$C and a 200-700 nm InAlN at 500$^{\circ}$C. In-situ RHEED scan mode measurements were used to define RHEED intensity oscillations, strain profiles, and coherence length profiles; they confirmed Stranski-Krastanov 3-dimentional regime of the film growth. The In composition of QDs is estimated 0.8 and 0.4 from the photoluminescence (PL) spectrum. Microscopic PL spectra were obtained using the 514.5 nm line of an Ar+ laser as excitation source. The laser spot was about 2-3 $\mu$m in diameter. We observe a reproducible double peak of very sharp and strong photoluminescence with FWHM of 1 meV at room temperature. We attribute these photoluminescence peaks to electron confinement in nano-hillocks of the InAlN film by the strong electric field of piezoelectric and spontaneous polarization; our model calculations of the localization energies agree with the experimental data.   

Effects of Stoichiometry on Electrical and Optical Properties of InN

Indiumnitride is the least developed semiconductor among the group III -- nitride compounds. Although the commonly produced material has high defect concentrations it already exhibits high electron mobility which indicates a great potential for future high speed electronic applications. Recently, a series of research efforts has been focused on the clarification of the fundamental bandgap of InN. The variations in the bandgap measurements were mainly attributed to the Burstein-Moss energy shift, the presence of oxide precipitates, possible indium clusters and other stoichiometry related effects. The influence of the indium to nitrogen flux ratio on the electrical and optical properties of InN, grown by molecular beam epitaxy (MBE) is systematically investigated and presented in this paper. A sudden increase in the electron concentration was observed for the highest indium flux. Simultaneously, a red shift in the photoluminescence peak energy was recorded. A correlation of these findings with changes in the materials chemistry and/or the presence of defects such as indium clusters will be presented.   

Coherent longitudinal optical phonon and plasmon coupling in the near-surface region of InN

Coherent phonon spectroscopy of a high-quality InN epitaxial layer is carried out using time-resolved second-harmonic generation. Only coherent longitudinal optical phonon and plasmon coupling mode at 447 cm$^{-1}$ can be resolved in the spectrum. Its frequency shows no dependence on the photoinjected carrier density up to 1.5$\times $10$^{19}$ cm$^{- 3}$. This phenomenon is attributed to the hybridization of coherent A$_{1} $(LO) phonon with the intrinsic cold plasma accumulated in the near-surface region of InN, where the plasma density could reach the order of 10$^{20} $ cm$^{-3}$, much higher than the bulk carrier concentration, 1$\times $10$^ {18}$ cm$^{-3}$, determined by Hall effect measurement..   

Plasma-Assisted Molecular Beam Epitaxy Grown InN Epifilm

High quality InN epitaxial layer is grown by plasma-assisted molecular beam epitaxy. Substrates used are c-plane sapphire and (111) Si wafer. Various characterizations are performed to investigate the crystal structure, compositions, electrical and optical properties. Hall measurements yield unintentional doping concentration in a range of 10$^{18}$ -- 10$^{20}$ cm$^{-3}$. Hall mobility reaches 1000 cm$^{2}$/Vs. High resolution x-ray rocking curve gives a full-width-at-half-maximum of $\sim $1000 arcsec for InN (0002). No oxygen signal can be detected with electron probe micro-analysis. Field emission scanning electron microscopy and atomic force microscopy show the flatness of the film surface. Raman scattering spectroscopy reveals Raman modes only from the hexagonal phase of InN. Extensive photoluminescence measurements are carried out to explore the bandgap of InN. Discussion on the results will be reported in detail.   

In-plane optical anisotropy in In$_{x}$Ga$_{1-x}$N/GaN multiple quantum wells induced by Pockels effect

We have investigated the crystal orientation dependence of optical properties in In$_{X}$Ga$_{1-X}$N/GaN multiple quantum wells. The spectral peaks and intensity of the micro- photoluminescence signal for different crystal orientations were found to have sixfold symmetry. Quite interestingly, the refractive index, obtained from the interference pattern, also varies with the crystal orientation. The 60 degree periodic anisotropy of electronic transitions as well as optical parameters was interpreted in terms of the Pockels effect induced by the strong built-in field in nitride heterojunctions. The linear dependence of the change of the refractive index on electric field is consistent with the prediction of the Pockels effect. Our result provides an alternative solution to improve the designs of photonic and electronic devices based on nitride semiconductors.   

Optical properties of InGaN quantum dots

Standard nitride field effect transistors rely on the polarization based 2DEG generation on GaN/AlGaN interfaces. The disadvantage of this structural approach (sensitivity against surface charges) shall be overcome by this approach of the quaternary InAlGaN material composition. With this approach we intend to realize 2DEG at zero polarization field thus controlling charge and mobility in the channel completely by doping the (In)AlGaN barrier and adjusting the spacer layer thickness. This approach should reduce the surface charge sensitivity dramatically open up a new degree of freedom in choosing optimized parameters for nitride FETs.   

X-ray Spectroscopy of InN heavily irradiated with He

We show that irradiation of InN with 2 MeV He ions produces a highly conducting n-type material. Electron concentration saturates at about 4$\times$10$^{20}$ cm$^{-3}$ for the ion dose of 800 $\mu$C. Nitrogen K-edge soft x-ray absorption (XAS) and emission (XES) spectroscopy is used to investigate modifications to the conduction band (CB) and valence band (VB) electronic structure of InN containing these very high concentrations of free electrons and defects. XAS, a probe of unoccupied CB states, shows a depletion of states near threshold absorption corresponding to free-carrier filling of the CB, and the creation of two new peaks for irradiated InN that correspond to $(i)$ the N-vacancy defect level, and $(ii)$ the formation of N-pairs. XES, a probe of occupied states, shows additional emission above the VB maximum, not present in non-irradiated InN, resulting from filled CB states and from elastic scattering. The elastic scattering intensity shows an enhancement for photon excitation at the localized defect level and the non-elastic CB emission is consistent with a band gap narrowing of $\approx$0.4 eV arising from free-carrier electron-electron and electron-ionized defect interactions. The results provide additional support for previously reported low energy gap and large Burstein-Moss shift in heavily doped InN \footnote{J. Wu {\it et~al.}, Phys. Rev. B {\bf 66}, 201403 (2002).}.   

Session L19: History of Physics

Physics at Fisk University

Fisk University was chartered in 1866 to educate former slaves at the end of the civil war. The physics department was started in 1931 under the chairmanship of Dr. Elmer Imes, Fisk 1903, a research physicist in the field of infrared (IR) spectroscopy. After Imes' death in 1941, one of his early physics majors, James Lawson, became chair and soon obtained a research IR instrument from the University of Michigan. By the early 1950's Fisk's IR research findings began to be published in the scientific journals and Fisk graduate students began to read the results of their M.A. thesis at the meeting of the Southeastern Section of the American Physical Society (SESAPS). This active participation in SESAPS in the mid-1950's was the impetus which caused SESAPS to switch its meetings from segregated to unsegregated facilities. During the next four decades physics at Fisk University expanded to include the annual Fisk Infrared Institute (FIRI) and the formation of strong research collaborations with Oak Ridge National Laboratory, Vanderbilt University, Bordeaux University (France), and NASA. Our presentation will expand on these issues and also include a discussion of the ``McCarthyite Problem'' of the mid-fifties as it impacted both Fisk University and the physics department.   

Atempts to link Quanta \& Atoms before the Bohr Atom model

Attempts to quantize atomic phenomena before Bohr are hardly ever mentioned in elementary textbooks.This presentation will elucidate the contributions of A.Haas around 1910. Haas tried to quantize the Thomson atom model as an optical resonator made of positive and negative charges. The inherent ambiguity of charge distribution in the model made him choose a positive spherical distribution around which the electrons were distributed.He obtained expressions for the Rydberg constant and what is known today as the Bohr radius by balancing centrifugal energy with Coulomb energy and quantizing it with Planck's relation $E=h\nu$. We point out that Haas would have arrived at better estimates of these constants had he used the virial theorem apart from the fact that the fundamental constants were not well known. The crux of Haas's physical picture was to derive Planck's constant h from charge quantum $e$ , mass of electron $m$ and atomic radius. Haas faced severe criticism for applying thermodynamic concepts like Planck distribution to microscopic phenomena. We will try to give a flavor for how quantum phenomena were viewed at that time. It is of interest to note that the driving force behind Haas's work was to present a paper that would secure him a position as a Privatdozent in History of Physics. We end with comments by Bohr and Sommerfeld on Haas's work and with some brief biographical remarks.   

Personal Recollections of Albert Einstein

My grandparents were good friends of Albert Einstein in Berlin. Later my parents also were on friendly terms with him. I had the opportunity to meet Einstein four times after my parents and I came to the United States in 1940. My parents and I, on occasion, had correspondence with Einstein and took a few photos of him. Albert Einstein had considerable influence on my development and style of doing physics, as I will discuss.   

Ugo Fano, Enrico Fermi, and spectral line shapes

Ugo Fano's 1961 paper on spectral line shapes$^1$ was recently ranked as the third highest in citation impact of all papers published in the entire Physical Review series.$^2$ In the course of preparing an article for a NIST Centennial volume,$^3$ I became interested in the history of the results presented in Fano’s seminal paper, and will present my findings in this talk. An amusing sidelight concerns the role played by Enrico Fermi in the development of the famous ``Fano profile'' formula. I had been told this story by Fano when I was his graduate student, but uncertain of my recollection of the details, I did not publish it in his obituary.$^4$ I later learned that the archives of the Royal Society of London contain Fano's own written version of the tale, which will be presented in this talk. The story sheds light on the nature of Enrico Fermi's interactions with his students, and confirms accounts concerning the way in which he did his theoretical work.$^5$ \\ $^1$ U. Fano,``Effects of Configuration Interaction on Intensities and Phase Shifts,'' {\em Phys. Rev.} {\bf 124}, 1866-1878 (1961)\\ $^2$ S. Redner, physics/0407137 (2004)\\ $^3$ http://nvl.nist.gov/pub/nistpubs/sp958-lide/116-119.pdf\\ $^4$ C. W. Clark, {\em Nature} {\bf 410}, 164 (2001)\\ $^5$ F. Rasetti, in {\em Collected Papers, vol. I}, E. Fermi (University of Chicago Press, 1962), p. 178 \\   

Citation Statistics From More Than a Century of Physical Review

The statistics of citations from all Physical Review journals for the 110-year period 1893 until 2003 are studied. Basic properties of the citation distribution are discussed. It is found that the growth of citations is consistent with linear preferential attachment. The time evolution of citations are also investigated. There is a positive correlation between the number of citations to a paper and the average age of citations. Citations from a publication have an exponentially decaying age distribution; that is, old papers tend to not get cited. In contrast, citations to a publication are consistent with a power-law age distribution, with an exponent close to -1 over a time range of 2-20 years. Finally, strong bursts of citations, as well as other dramatic features in the time history of citations to individual publications, are identified.   

Sarah Frances Whiting: Foremother of American Women Physicists

Sarah Frances Whiting taught physics and astronomy at Wellesley College for 40 years. After receiving her A.B. degree in 1864, she taught math and classics at a girls' secondary school, and independently studied the science that had attracted her interest. Named to the faculty of Wellesley before its opening in 1876, she attended MIT as an unenrolled guest and observed Edward Pickering's work in establishing the first instructional labs in the U.S. She established the second, which were the first for women students, on returning to Wellesley in 1878. In 1879 she began teaching astronomy also, and built both departments and programs. Rather than celestial observation, she emphasized photometry and spectroscopy. She was instrumental in procuring support for a college observatory, completed and dedicated in 1900, which she directed until her retirement in 1916. Tufts awarded her an honorary Sc.D. in 1905.   

A half-century ago physicists missed a major public service opportunity, costing the human race widespread chronic illness and many deaths!

Radar$-$pulsed microwave (MW) radiation$-$helped the Allies win World War II but health concerns soon arose. Alerted to a syndrome resembling {\it mild radiation poisoning},$^1$ a worried M.D. surveyed radar-exposed workers, finding a high incidence of internal bleeding, 2 leukemia cases in 600 radar operators, 2 brain tumor cases in a 5-man MW research team and many complaints of headache. He sent his report$^2$ to the Pentagon in 1953. Alarmed Navy officers convened a meeting$^3$ [mostly of electrical engineers (EEs)] to identify a safe level of MW exposure for servicemen. Biophysicist Herman Schwan attended, playing a major role in establishing 10 mW/cm$^2$ as a {\it thermally safe} MW exposure limit. The IEEE became sole sponsor of ANSI C95 [an early health standard for radiofrequency (RF) exposure] with {\it negative long-term consequences for human health!} I review RF health standards development since 1953, comparing what physicists might have done, had {\it they}$-$not EEs$-$had this responsibility! [See also my technical abstract.] $^1$ N.H. Steneck, {\bf The Microwave Debate}, Cambridge, MA: MIT Press, 1984; p. 33. $^2$ J.T. McLaughlin, {\bf A Study of Possible Health Hazards from Exposure to Microwave Radiation} (Hughes Aircraft, Culver City CA, Feb. 9, 1953). $^3$ {\bf Biological Effects of Microwaves}, meeting minutes (Navy Dept. Conference, Naval Medical Research Institute, Bethesda MD, Apr. 29, 1953).   

History of the Wave Structure of Matter (WSM)

The puzzling structure of the electron is due to the belief that it is a discrete particle. Einstein deduced this impossible since Nature's properties do not match the discrete particle. Clifford, 1876, rejected discrete matter and suggested a WSM. Schroedinger, 1937, proposed to eliminate discrete particles writing: \textit {What we observe as material bodies and forces are nothing but shapes and variations in the structure of space. Particles are just schaumkommen}(appearances). Mach's principle of inertia, 1883, first recognized a role of the space medium. Theory was developed by Milo Wolff, 1990-04, and Geoff Haselhurst (SpaceAndMotion.com) using the Scalar Wave Equation to find solutions that form a quantum-wave structure with all the electron's properties plus the Schroedinger Equation. Carver Mead, 1999, applied the WSM to design Intel micro-chips correcting errors of Maxwell's magnetic Equations. New applications of the WSM are concerned with matter at molecular dimensions: nanotechnology, new alloys and catalysts, the mechanisms of biology and medicine, molecular computers and memories.   

Session L20: Focus Session: Properties of Complex Oxides and Interfaces II

Direct observation of the formation of polar nanoregions in Pb(Mg$_{1/3}$Nb$_{2/3}$)O$_3$ using neutron pair distribution function analysis

Using neutron pair distribution function (PDF) analysis over the temperature range from 1000~K to 15~K, we demonstrate the existence of local polarization and the formation of medium-range, rhombohedrally ordered polar nanoregions (PNRs) in a prototypical relaxor ferroelectric Pb(Mg$_{1/3}$Nb$_{2/3}$)O$_3$. We estimate the volume fraction of the PNRs as a function of temperature and show that this fraction steadily increases from 0 \% to a maximum of $\sim$ 30\% as the temperature decreases from 650~K to 15~K. Below T$\sim$200~K the PNRs start to overlap as their volume fraction reaches the percolation threshold. We propose that percolating PNRs and their concomitant overlap play a significant role in the relaxor behavior of Pb(Mg$_{1/3}$Nb$_{2/3}$)O$_3$.   

Electrical and Dielectric Properties of ACu$_3$Ti$_4$O$_{12}$ Compounds

We studied frequency and temperature dependences of impedance and permittivity of several compounds in the system ACu$_3$Ti$_4$O$_{12}$ (A=Ca, Bi$_{2/3}$, Y$_{2/3}$, La$_{2/3}$) in the ranges of 10$^{-1}$$\sim$10$^{6}$ Hz and -150$\sim$200 $^\circ$C, respectively. We found all compounds investigated displayed similar electrical and dielectric properties, namely, they all have two electrical responses in the impedance formalism and a Debye-like peak in the permittivity formalism, besides, their dielectric constants are independent of frequency and temperature in a wide range. We explained the experimental results in terms of a two-layer model with conducting grains partitioned from each other by poorly conducting grain boundaries. We attributed the two electrical responses in the impedance formalism to the grain and grain boundary effects, respectively, while the Debye-like peak in the permittivity formalism to a Maxwell-Wagner relaxation.   

The Dielectric Properties of Complex Oxides: CaCu$_{3}$Ti$_{4}$O$_{12}$ and Other Perovskites

Detailed studies of the properties of ceramic CaCu$_{3}$Ti$_{4} $O$_{12}$ (CCTO) have clarified the physics of this interesting material. The unusual dielectric relaxational properties of CCTO are explained in terms of a capacitive-layer model, as for an inhomogeneous semiconductor consisting of semiconducting grains and insulating grain boundaries. The kinetics of the main (low T) relaxation reveal that two different thermally- activated processes in the CCTO grains control the dynamics. A higher T relaxation is determined by grain boundary conduction. Both Nb and Fe doping lower both the dielectric constant, $\varepsilon \prime $, and loss, but Fe doping leads to the more dramatic effects; 3 at.{\%} Fe removes the anomalous $\varepsilon \prime $ (T) response making CCTO an intrinsic dielectric. The intrinsic $\varepsilon \prime $ ( $\underline{\sim }$75) and its T dependence are shown to be largely determined by a low-lying soft TO phonon. At low T, cubic CCTO transforms into an antiferromagnetic phase at T$_{N }$= 25 K. T$_{N}$ decreases significantly with Fe doping. Analysis of the high T dependence of the magnetic susceptibility provides insight into the role of Fe. Finally, an $\varepsilon \prime $ (T) anomaly associated with the onset of antiferromagnetic order has been discovered providing evidence for coupling between the polarization and sublattice magnetization. Possible origin of this coupling is discussed. We have also observed large dielectric constants and relaxations similar to those in CCTO in doped single crystals of KTaO$_{3}$.   

Optical Homogeneous Linewidths and Spectral Diffusion at 1.5 microns in Mixed Er$^{3+}$:Eu$^{3+}$:Y$_{2}$SiO$_{5}$ Studied by Photon Echo

Er$^{3+}$-doped materials are important for spectral hole burning applications at 1.5 micron communication wavelengths, including analog signal processing and laser frequency stabilization. Doping Er$^{3+}$:Y$_{2}$SiO$_{5}$ with Eu$^{3+}$ is shown to broaden the inhomogeneous linewidth of the $^{4}$I$_ {15/2}$ - $^{4}$I$_{13/2}$ transition without significantly broadening the homogeneous linewidth. This maximizes bandwidth in real-time analog signal processing applications without compromising resolution. Photon echo and stimulated photon echo decays between 1.5 K and 5.5 K were measured along with angle- dependent Zeeman spectra and site-selective absorption and emission. Detailed modeling of observed spectral diffusion induced by spin dynamics considered Er$^{3+}$-Er$^{3+}$ dipole interactions driven by direct-phonon processes. The model describes and explains observed behavior and predicts behavior vs. magnetic field, crystal temperature, Er$^{3+}$ dopant concentration, and crystal orientation. $^{*}$ Currently at University of San Francisco $^{**}$ Currently at University of South Dakota   

In-situ Spectroscopy on Erbium doped Lithium Niobate during Domain Inversion

Lithium niobate (LiNbO$_{3}$) has found wide application due to its favorable acousto-optical, electro-optical, and nonlinear optical properties. For many applications it is crucial to create ferroelectric domain patterns. With the most common technique, using electrodes patterned by optical lithography, periodically poled components with period lengths down to 3${\mu}$m could be fabricated. Smaller structures, however, are difficult to achieve with present techniques. For the investigation of novel domain inversion techniques, which potentially offer the feasibility to create smaller structures, it is vital to probe in-situ if domain inversion is occurring with high spatial resolution. In an earlier work we have shown that the emission of the Er$^{3+}$ ion is different for the as grown and domain inverted part of the sample this. In this work we will show that it is possible to probe the Er$^{3+}$ defect in-situ during domain inversion offering the potential to study the spatial extension of a moving domain wall and to develop an active feedback system for electron beam or light-induced domain inversion. We found significant differences between a moving domain wall and a static domain wall in terms of their spatial extension and their Raman spectra.   

Atomic Scale Catalysis: Structure and Reactions of Chromium Oxide Species on Transition Aluminas

Chromium-supported transition aluminas are widely used for de-hydrogenation of alkanes, yet an understanding of the structure and relevant chemical reactions on atomic scale is lacking. With first-principles calculations and Z-contrast scanning transmission electron microscopy observations, we show that dispersed CrO$_x$ species supported on transition aluminas are critical for catalytic activities. The atomic-scale species induce dissociation of alkanes whereas crystalline chromium oxide inhibits such dissociation. Contrary to prior models, chromium atoms do not participate in the catalytic reactions, but act as binding centers to which active oxygen atoms are bound. We also show that $\eta$-alumina is more efficient to support the CrO$_x$ species while nucleation of crystalline Cr$_2$O$_3$ is facilitated on $\gamma$-alumina. The difference is attributed to the different vacancy distributions in $\eta$- and $\gamma$-aluminas.   

Growth and spectroscopic characterization of epitaxial dielectrics on SiC

SiC has attracted much attention as a promising wide-bandgap semiconductor material due to its excellent physical and electrical properties. In SiC-based devises, inclusion of an AlN buffer layer improves the performance due to its good lattice match with SiC substrate and relatively low misfit with the III- nitride layers. In this work, we demonstrate the deposition of HfO$_2$, a well known high-k gate dielectric material for silicon based devices, by an atomic layer deposition process on SiC and AlN/SiC stacks. During the thin-film growth, the crystallinity of the epitaxial layers is monitored by in-situ reflection high-energy electron diffraction (RHEED) and the thin film composition is characterized by in-situ x-ray photoelectron spectroscopy (XPS). To examine the characteristic electrical properties, capacitance-voltage and current-voltage measurements are performed on the metal-insulator-semiconductor (MIS) capacitors. The correlation between growth conditions, stoichiometry and crystallinity of the eptaxial layer, and device performance will be discussed. Finally, we present first- principle calculations of the valence band structures of AlN and SiC as well as band alignment at the interface, in comparison with the experimentally determined band alignment by XPS.   

Layered Tunnel Barrier Lowering in High-k Heterostructures using Bias-dependent Internal Photoemission Spectroscopy

Layered tunnel barriers have been proposed as replacements for SiO$_{2}$ for use as injection barriers in nanocrystal floating-gate memories. Due to the predicted larger change in conductance as a function of injection voltage, such barriers are expected to enable nanosecond programming and erase times with archival data retention times. We have utilized internal photoemission (IPE) spectroscopy to study barrier height lowering of Al$_{2}$O$_{3}$/HfO$_{2}$/SiO$_{2}$/Si layered tunnel barrier structures over a wide range of applied biases. 15 nm layers of Al$_{2}$O$_{3}$ and HfO$_{2}$ were grown on n-Si substrates by atomic layer deposition. The IPE results are analyzed using a simple electrostatic model to yield effective barrier heights and overall band alignments across the entire voltage range. Using this technique we demonstrate substantial barrier lowering ($\sim $0.75 eV) for Si-compatible dielectric heterostructures, and we discuss the application of these barriers to improved speed and reliability of floating gate nonvolatile memory devices.   

Energies of 4f$^{N}$ and 4f$^{N-1}$5d States Relative to Host Bands in Rare-earth-doped Fluorides

Energies of 4f$^{N}$ states relative to crystal band states were measured for rare-earth ions in the optical host materials LiYF$_{4}$, Na$_{0.4}$Y$_{0.6}$F$_{2.2}$, and LaF$_{3}$ using x-ray photoemission spectroscopy. Spectra were modeled to determine the valence band maximum and 4f$^{ }$electron binding energies in each material. These results were combined with 4f$^{N}$ to 4f$^{N-1}$5d transition energies to determine 5d binding energies for the lowest levels of excited 4f$^{N-1}$5d configurations. While 4f$^{N}$ ground-state energies vary within several eV of the valence band maximum for different rare-earth ions in each host, the lowest 4f$^{N-1}$5d states have similar energies and are several eV below the bottom of the conduction band. A simple model accurately described 4f$^{N}$ and 4f$^{N-1}$5d binding energies across the entire series of rare-earth ions. These results improve the understanding of optical materials for lasers, phosphors, and spectral hole burning applications for optical signal processing and data storage.   

Theoretical study of orbital and lattice structure of $MnF_3$: the origin of orbital ordering

Orbital ordering in $MnF_3$ is studied with first-principles theory. Mathematical description of $e_g$ states within pseudospin formalism shows the importance of electron-electron interactions that oppose conventional electron-phonon interactions (e.g.: Jahn-Teller effects). Results obtained with LDA+U give stable ground state with experimentally observed orbital ordering, that cannot be explained solely in terms of electron-phonon interactions. Instead, the resulting orbital ordering is a consequence of competition between electron-phonon and electron-electron interactions. Our quantitative conclusion can be directly verified in future experiments.   

Hydrogen-Bond Nature in Isolated Hydrogen-Bonded Material h-MeHPLN and h-BrHPLN Studied by Neutron and X-ray Diffraction

MeHPLN and BrHPLN are hydrogen-bonded dielectric materials. Though their crystal structures are essentially the same, their phase transition schemes are significantly different. MeHPLN undergoes a phase transition at 41 K, while h-BrHPLN does not show any phase transitions. When the hydrogen atom in the hydrogen-bond of BrHPLN is replaced with a deuterium atom (d-BrHPLN), successive phase transitions occur. In this study neutron and X-ray diffraction experiments were performed on h-MeHPLN and h-BrHPLN to elucidate the difference about the phase transition from the structural aspect. From the experimentally obtained electron and nuclear distributions, a local disordered electronic dipole moment was found in the hydrogen bond region at room temperature. The phase transition of h-MeHPLN was found to occur with the ordering of the moment. Meanwhile, the moment in h-BrHPLN remains disordered until very low temperature, suggesting a tunneling motion of the hydrogen atom.   

Session L21: Focus Session: Intracellular Calcium Dynamics in Myocytes

Investigating a Period-Doubling Bifurcation in Cardiac Tissue Using Alternate Pacing

The action potential duration (APD) of cardiac cells undergoes a period-doubling bifurcation when the pacing rate (PR) is increased, resulting in a period-2 behavior called alternans. Studying the susceptibility of cardiac tissue to alternans is crucial because alternans can lead to ventricular fibrillation and sudden cardiac death. One way to study this behavior is to alternate the PR from beat-to-beat, which results in beat-to-beat alternation in APD. Recent mathematical models predict that these small beat-to-beat changes in PR will result in divergent beat-to-beat variations in APD near the period-doubling bifurcation. Thus, the appearance of divergent behavior during alternate pacing can uncover the tissue's propensity to alternans. In an experiment to test this hypothesis, we observed beat-to-beat APD variations that are only a fraction of the beat-to-beat change in the PR, despite proximity to the bifurcation point. This study demonstrates the discrepancy between experiment and theory, which may be due to changes in ionic concentrations and wave propagation.   

Spatiotemporal dynamics and control of alternans in cardiac tissue with short-term memory

Alternans is an abnormal cardiac rhythm that is a precursor of fibrillation. Recently, an amplitude equation describing spatiotemporal dynamics of alternans in a one-dimensional cable [1] was derived using a model that assumes the current action potential duration (APD) depends on the previous diastolic interval (DI). However, experimental work has shown that cardiac tissue is more accurately described by models that contain some degree of ``memory,'' where the current APD depends on preceding APD's and DI's. We add memory to the amplitude equation and find that it adds a new parameter to the equation which governs the onset of alternans. We also find that memory affects the ability to control spatially concordant alternans, but has no effect on the ability to control discordant alternans. Analytical results are verified by simulations using the Fenton-Karma model. [1] B. Echebarria, A. Karma, \textit{Chaos}, \textbf{12}:923 (2002)   

Excitation-contraction coupling gain and cooperativity of the cardiac ryanodine receptor: a modeling approach

During calcium-induced-calcium-release, the ryanodine receptor opens and releases large amounts of calcium from the sarcoplasmic reticulum into the cytoplasm of the myocyte. Recent experiments have suggested that cooperativity between the four monomers comprising the ryanodine receptor plays an important role in the dynamics of the overall receptor. Furthermore, this cooperativity can be affected by the binding of FK506 binding protein and hence modulated by adrenergic stimulation through the phosphorylating action of PKA. This has important implications for heart failure, where it has been hypothesized that ryanodine receptor hyperphosphorylation, resulting in a loss of cooperativity, can lead to a persistent leak and a reduced sarcoplasmic reticulum content. Here, we report on a theoretical model that examines the cooperativity via the assumption of an allosteric interaction between the four subunits. We find that the level of cooperativity, regulated by the binding of FK506 binding protein, can have a dramatic effect on the excitation-contraction coupling gain and that this gain exhibits a clear maximum. These findings offer a simple explanation of heretofore conflicting data from different species and allows for an evaluation of the aforementioned heart failure scenario.   

Antiphase calcium oscillations in astrocytes via inositol (1,4,5)-triphosphate regeneration.

In cultured astrocytes, antiphase oscillations in the intracellular free calcium concentrations have been observed in nearest neighbor cells that are coupled through gap junctions. A mathematical model is used to investigate physiologic conditions under which diffusion of the second messenger inositol (1, 4, 5)-triphosphate (IP3) through gap junctions can facilitate synchronized antiphase calcium oscillations. Our model predicts antiphase oscillations in both calcium and IP3 concentrations if a) the gap junction permeability is within a window of values and 2) IP3 is regenerated in the astrocytes via Phospholipids-C$\delta $. This result sheds new light on the current dispute on the mechanism of intercellular calcium wave propagation since it provides additional evidence for a partially regenerative mechanism as the model excludes synchrony in the absence of IP3 regeneration.   

Noise-induced ectopic activity in a simple cardiac cell model

Ectopic activity in the form of premature ventricular contractions (PVCs) is relatively common in the normal heart. Although PVCs are normally harmless, sometimes but rarely PVCs can generate spiral waves of activation through interaction with other waves of activation, potentially progressing to ventricular tachycardia, followed by ventricular fibrillation and sudden cardiac death. Clusters of PVCs have been found to be significantly more dangerous than isolated PVCs. We model PVC generation by applying triggers (noise) to the generic FitzHugh-Nagumo model as substrate, and study the effects the noise level and excitability. We find: exponential waiting time behavior at fixed parameter levels; no evidence of clustering at fixed parameter levels; and a sharp increase in PVCs as excitability approaches the auto-oscillatory threshold or noise increases beyond a similar threshold. This produces sharp increases in theoretical rates of PVC-induced fibrillation, consistent with results of A Gelzer et al. in animal models. Partially supported by the NSF and NIH.   

Diffusive transport through the myocardium of pharmacological agents placed in the pericardial space

The classical understanding of the pericardial sac is as a fluid-filled space surrounding the heart. Since it is a self-contained space, it can be viewed as a reservoir and therapeutically used as a drug container to deliver agents to the myocardium. It is only recently that experimental techniques for safe delivery of agents to the pericardial space have been developed. In this work, we present a quantitative model of the key biophysical processes affecting the distribution through the myocardium of a substrate delivered to the pericardial sac. By direct analysis of experimental data on pericardial delivery of agents to the porcine heart and comparison with computational results, we determine quantitatively for the first time values for fundamental physical parameters, such as effective diffusion constant and washout rate, for small and large molecular weight test agents in the myocardium. We comment on the efficacy of this mode of drug delivery to the myocardium, thereby aiding in the development of agents and methods of delivery that achieve various therapeutic goals.   

Best Stiffness for Striation: Effect of Matrix Stiffness on Myocytes and Stem Cells

Contractile myocytes provide a test of the hypothesis that cells sense their mechanical as well as molecular microenvironment, altering expression, organization, and/or morphology accordingly. Here, myoblasts and stem cells were cultured on collagen strips attached to glass or polymer gels of varied elasticity. MyoD expression and morphology peaks on gels with stiffness typical of normal muscle (passive Young's modulus $E \quad \sim $9-15 kPa). While fusion of myoblasts into myotubes occurs independent of substrate flexibility, myosin/ actin striations emerge later only on gels with the same tissue-like $E$. On glass and much softer or stiffer gels, including gels emulating stiff or fibrotic muscle, cells do not striate. In addition, myotubes grown on top of a compliant bottom layer of glass-attached myotubes (but not softer fibroblasts) will striate, whereas the bottom cells will only assemble stress fibers and vinculin-rich adhesions. Unlike sarcomere formation, adhesion strength increases monotonically versus substrate stiffness with strongest adhesion on glass. These findings have major implications for in vivo introduction of stem cells into diseased or damaged striated muscle of altered mechanical composition.   

Session L22: Metalloenzymes: Structure and Function

Experimental Studies of Structure, Function, and Coherent Oscillations in Biomolecules

Femtosecond coherence spectroscopy can be used to prepare and monitor coherent states of biological samples such as heme proteins. Following laser pulse induced ligand photolysis of myoglobin, the (initially planar) heme group is left far from its final product state equilibrium geometry. This leads to coherent oscillations of those modes composing the reaction coordinate for diatomic ligand binding and dissociation. Coherence studies, along with ``white light'' continuum measurements of the spectral dynamics, show that the timescale for diatomic ligand dissociation is much shorter than the 150fs period of the Fe-histidine vibration (the Fe-histidine bond constitutes the sole covalent linkage between the heme and protein material). Recent measurements of the effects of temperature and sample condition on the coherent motions of the heme and on the ultrafast geminate rebinding of various diatomic ligands are also reported. Investigations of heme model compounds, in the absence of the protein material, show that the spectrum of low frequency heme modes can be altered by the choice of sample conditions. The studies of the heme model compounds also allow the diatomic ligand rebinding barrier to be separated into ``proximal'' and ``distal'' contributions that can be separately determined.   

Investigations of the low frequency modes of Fe-porphyrin systems and perturbations induced by axial ligation

We investigate the effect of a 2-methyl imidazole (2-MeIm) ligand on the low frequency ($<$250cm$^{-1})$ vibrational spectrum of iron protoporphyrin IX (Fe-PPIX). This compound is designed to mimic the active site of histidine ligated heme proteins. We use femtosecond coherence spectroscopy to probe the reduced species of Fe-PPIX with and without the 2-MeIm ligand. We notice important changes in the lowest frequency region ($<$50 cm$^{-1})$ of the spectrum, along with the expected disappearance of the 2-MeIm-Fe mode at 216cm$^{-1}$ (in the FePPIX model) when the 2-MeIm ligand is absent. Overall, these observations suggest that a low frequency mode observed near 20 cm$^{-1}$ is associated with the imidazole ligand and that the anharmonic heme doming mode, associated with the Fourier components in the power spectrum near 40 cm$^{-1}$ and 80 cm$^{-1}$, can be affected by axial ligation.   

Quantitative Vibrational Dynamics of Iron in Porphyrins

We compare quantitative experimental (nuclear resonance vibrational spectroscopy - NRVS) and theoretical (density functional theory - DFT) approaches to characterize the vibrational dynamics of the $^{57}$Fe atom in CO-ligated porphyrins designed to mimic the active site of heme proteins. NRVS yields the frequencies, amplitudes, and directions of the Fe vibrations. These measurements confirm many aspects of the DFT predictions, suggesting that the latter provides a reliable description of the observed modes. We will discuss the character of normal modes for Fe(TPP)(1-MeIm)(CO), including a series of modes involving Fe motion in the plane of the heme, Fe-Im modes, Fe-ligand modes, and reactive modes.   

First-Principles Hartree-Fock Investigation of Electronic Structure and Hyperfine Properties of Deoxyhemoglobin

The electronic structure of Deoxyhemoglobin is studied using the Unrestricted Hartree--Fock Cluster Procedure considering as representative the entity involving the porphyrin ring, the Fe$^{2+}$ ion and the proximal histidine. The positions of the atoms are taken from X-ray data. The calculated electronic structure of the spin S=2 system is used to derive the magnetic hyperfine fields, nuclear quadrupole interaction parameters and the Mossbauer isomer shifts for the $^{57m}$Fe. The results are in good agreement with experiment, providing support for the accuracy of the calculated isotropic and anisotropic components of the spin density and electronic charge density near the $^{57m}$Fe nucleus. Results for the hyperfine properties of $^{14}$N, $^{13}$C, $^{1}$H and $^{2}$H nuclei will also be discussed. (*) Present Address: Dept. of Physics, Uppsala University, Sweden (**) Present Address: M. D. Anderson Center, Houston, Texas (***) Also: Dept. of Physics, University of Central Florida, Orlando, Florida   

Time-resolved heme protein intermediates

To determine the enzymatic mechanisms of heme proteins, it is necessary to identify the intermediates along the catalytic pathway and measure the times of their formation and decay. Resonance Raman scattering spectra are especially powerful for obtaining such information as the electronic structure of the heme group and the nature of the ligand coordinated to the heme iron atom may be monitored. The oxygen intermediates of two physiologically important enzymes will be presented. Nitric oxide synthase (NOS) uses oxygen to convert arginine to NO and citrulline; and cytochrome c oxidase (CcO) reduces oxygen to water to support oxidative phosphorylation. The fate or the oxygen in each of these enzymes has been followed by resonance Raman scattering. In NOS the oxygen is slowly converted to an activated species that then reacts fast, whereas in CcO the oxygen is rapidly converted to a reactive species that subsequently reacts slowly. The properties of the intermediates and the origin of the differences between these enzymes will be discussed.   

Density Functional Theory / Nudged Elastic Band Investigation of the Hydroxylation Reaction Mechanism Catalyzed by P450cam

We have calculated the complete minimum-energy reaction path for the hydroxylation of camphor by the P450 enzyme from Pseudomonas putida using the nudged elastic band method of Jonsson and co-workers[1]. Single-point force and energy calculations on pathway images were performed at the hybrid density functional level of theory (B3LYP) with large basis sets for the iron atom (6-311+G) and O$_{2}$ ligand (6-31+G*) on a ~100 atom active site extracted from a recent high-resolution crystal structure[2]. Our model includes the heme group liganded to both Cys357 and dioxygen and we also include Thr251 and Asp252, which have been shown to significantly affect product yield by mutational studies[3]. We find that, upon transfer of the 2nd electron to the active site, the Fe-O$_{2}$ moiety is unstable and decays to a Fe-OOH- intermediate via a Asp252-H$_{2}$O proton transfer chain. The barrier for dioxygen cleavage and the identity of the reactive species will be discussed. [1] H. Jonsson, G. Mills, K.W. Jacobsen, in Classical and Quantum Dynamics in Condensed Phase Simulation, World Scientific (1998). [2] I. Schlichting, et al., Science 287, no. 5458, p. 1615-1622 (2000). [3] R. Davydov, et al., J. Am. Chem. Soc., 123: 1403-1415 (2001).   

Models of the photosynthetic oxygen-evolving centerand electronic structure of Mn clusters

It is known that the oxidation of water to O$_2$ in green plants is associated with a tetramanganese complex of the photosystem II (PSII) protein-cofactor complex but the exact structure of the oxygen-evolving center (OEC) remains unknown. The recent X-ray spectroscopic studies suggest that the OEC contains a cubane-like Mn$_3$CaO$_4$ center linked to a forth Mn by an oxo-bridge. Using ab-initio methods we carry out geometry optimizations for a few models of the OEC and study their electronic properties. We consider the cubane-like Mn$_3$CaO$_4$ center, a funnel-like Mn4 core with Ca ligand and a synthetic Mn$_4$O$_6$ core structure. Possibilities for binding sites and eventual reaction paths of the water splitting are explored. Manganese clusters are recognized also as single-molecule magnets. Therefore we investigate spin properties of ground and excited states of these clusters. High spin ground states of Mn$_2$O$_2$ and Mn$_4$ compared to a low spin ground state of Mn$_4$O$_4$ illustrate a competition between ferromagnetic and antiferromagnetic ordering.   

Pre-Crystallization State of Ferritin at Low Temperature

In the course of a systematic exploration of the crystallization kinetics and conditions of the protein ferritin using x-rays, we discovered an unexpected new state of aggregation of the protein at low temperature. This new state was found to form reversibly below the freezing point of the solution. Using Small Angle X-ray Scattering (SAXS), we studied the properties of solutions of ferritin upon cooling and found that ferritin molecules form clusters of varying size and structures depending on the temperature and the thermal history of the sample. Simulations of the SAXS patterns were made using various cluster structures and these show that clusters of roughly 50 molecules form upon freezing. The structure was found to be similar to the FCC structure of macroscopic ferritin crystals which leads us to the conclusion that these clusters may be precursor states to the crystallization of ferritin.   

Ab initio studies of [Fe$_{4}$S$_{4}$]$ ^{2+/3+}$ clusters in metalloprotein MutY

Iron sulfur clusters are present in the DNA repair protein MutY in a region highly homologous in species as diverse as E. Coli and Homo Sapiens, yet their function remains unknown. In MutY, this mixed valence cluster exists in two oxidation states, [Fe$_ {4}$S$_{4}$]$^{2+/3+}$, with the stability depending upon the presence of DNA. We have studied the electronic structure and stability of these clusters using density functional theory, in particular the local orbital based SIESTA program. Our calculation shows that the energy difference between 2+ and 3+ forms is within the range of 0.1eV, which suggests that the redox process is reversible. We use this to propose a possible redox mechanism for modulating the rate for scanning for oxidized G-A mismatches in DNA by MutY \footnote{M. Slutsky and L.A. Mirny, preprint q-bio.BM/0402005 at http://arxiv.org.}. We note that this redox modulation mechanism for site recognition scanning may have broader generality.   

Session L23: Coherent Structures, Pattern Formation and Spatio-Temporal Chaos

Effect of weak dissipative disorder on front formation in pattern forming systems

We study the effect of weak disorder in the linear gain coefficient in pattern forming systems described by the cubic-quintic nonlinear Schr\"{o}dinger equation. We calculate the distribution functions of the front position and amplitude and find that both distribution functions are strongly different from Gaussian. We show that the distribution of the front amplitude is singular at the maximum value of the amplitude, while the distribution of the front position has a lognormal form. These predictions are in very good agreement with our numerical simulations. Implications of the results for other types of dissipative disorder are discussed.   

Counting intrinsic localized modes in an antif\-erromagnet

Intrinsic localized modes (ILMs), also called discrete breathers or lattice solitons, are responsible for energy localization in the dynamics of discrete nonlinear lattices [1]. Here we report on the observation of countable ILMs in an atomic lattice by means of a novel nonlinear energy magnetometer [2]. The instrument first produces frequency locked ILMs in the spinwave spectrum of an antiferromagnet and then measures the four wave mixing signal emitted by the sample versus time. This technique makes observable in nonlinear emission the small number of ILMs that remain locked to the driver in steady state. The disappearance of each ILM registers as a step in the time dependent emission power with the surprising result that the energy staircase of ILM deexcitation is uniquely defined. These experiments identify a new direction where future applications may lead to smart materials and directed energy transfer. 1. A. J. Sievers and S. Takeno, Phys. Rev. Lett. \textbf{61}, 970 (1988). 2. M. Sato and A. J. Sievers, Nature \textbf{431}, Nov. 25 (2004).   

Correlation of intrinsic localized mode properties with sample temperature

Intrinsic localized modes (ILMs) in a quasi-1D antiferromagnetic lattice may be eternalized with a moderate powered microwave driver at a locking frequency below the AFMR. These locked ILMs are dynamical sources of nonlinearity in the sample and can therefore be detected in emission by four wave mixing [1]. The emission signal decays in steps at reproducible times as individual ILMs are unlocked from a driver. We have discovered that an unlocked ILM may be recaptured by increasing the amplitude of the driver. To examine the role of the sample temperature on this locking phenomenon the frequency of the driver has been amplitude modulated from 100 Hz to 50 kHz. Our experimental results show that the ILMs are not able to lock to the driver if the sample temperature is unable to follow the modulation frequency. 1. M. Sato and A. J. Sievers, Nature \textbf{431}, Nov. 25 (2004).   

Collective excitations of charged dust grains in dusty plasma lattices

\emph{Dusty Plasmas} (or \emph{Complex Plasmas}) are large ensembles of interacting particles, consisting of electrons, ions and massive, heavily charged, micron-sized dust particulates. The presence of the latter modifies the plasma properties substantially and allows for new charged matter configurations, including liquid-like and solid (quasi-\textit{crystalline}) states (Debye crystals). One- dimensional (1d) dust crystals are formed in discharge experiments, where the electrode \emph {sheath} electric fields and electrostatic interactions constitute a highly nonlinear environment. The nonlinear aspects of horizontal (longitudinal, acoustic mode) as well as vertical (transverse, optical mode) motion of charged dust grains in a (1d) dust crystal are discussed. Different types of localized excitations, predicted by nonlinear wave theories, are reviewed and conditions for their occurrence (and characteristics) in DP crystals are discussed, in a continuum approximation. Dust crystals are shown to support nonlinear \emph{kink-}shaped supersonic solitary excitations, related to longitudinal dust grain displacement, as well as modulated \emph{envelope localized modes} associated with either longitudinal or transverse oscillations. Furthermore, the possibility for highly localized \emph{discrete breather}-type excitations to occur is investigated, for first principles. The relation to known results on atomic chains and on strongly- coupled dust layers in gas discharge plasma experiments is discussed.   

Formation of Suncups on Snowfields Exposed to Solar Radiation

A mathematical model is proposed to explain the ablation hollows (suncups) that are observed on snowfields exposed to intense solar radiation. The model is derived by first expressing the distribution of scattered sunlight in the snow in terms of the local slope and curvature of the surface. From this expression, we use a perturbation method valid in the limit of weak surface topography to obtain a differential equation for the snow surface morphology. The resulting non-linear equation is the Kuramoto Sivashinsky equation except with the addition of a conservative non-linear term. In simplified form the equation is: $\partial _t h=cF\left( {\left\langle \ell \right\rangle \nabla ^2h-\left\langle {\ell ^3} \right\rangle \nabla ^4h+\left( {\nabla h} \right)^2+\left\langle {\ell ^2} \right\rangle \nabla ^2\left( {\nabla h} \right)^2} \right)$ where $\left\langle {\ell ^n} \right\rangle $ is the spectral average of the $n^{th}$ power of the photon diffusion length. Multiple scattering from one concave part of the surface to another is treated self consistently. Numerical solutions of this equation with published values for the optical properties of snow yield spontaneous ordered patterns with a characteristic length of 25-50 cm and characteristic formation time under full solar illumination of 5-15 days, depending on the microstructure of the snow. The spontaneous pattern consists of a hexagonal array of parabolic valleys separated by sharp ridges that closely resemble suncups in size, shape and growth time.   

Pattern formation through alternation of dynamics in a nonlinear optical system

We will discuss the experimental observation of a new mechanism for pattern formation in spatially extended nonlinear systems, namely the alternation of dynamics. In this experiment we employ a nonlinear optical device known as a liquid crystal light valve (LCLV) that has been placed in an optical feedback loop. The LCLV arrangement has been studied for over a decade and exhibits a broad range of spatiotemporal behavior. We have found that by alternating one of the parameters of the system (the light intensity) between two values we can generate strong patterns. There is no patterning observed in either of the two states corresponding to each value of the parameter. Thus, alternation of the parameter is crucial. We will discuss the conditions under which patterning occurs and relate our experimental observations to recently proposed theory.   

Weakly vs Highly nonlinear front dynamics

We analyse the nonlinear dynamics of one-dimensional unstable fronts. Our main finding is the existence of two types of dynamics weakly, or highly nonlinear, which respectively lead to continuous or discontinuous morphological transitions. Based on a multi-scale analysis, we list the possible weakly nonlinear equations and determine some of the most relevant ones. We then show that dynamics is not weakly nonlinear, but highly nonlinear in many cases relevant to specific systems (e.g. when an instability occurs in the vicinity of thermodynamic equilibrium in a conserved system). Highly nonlinear equations are explicitly derived, and exhibit unexpected symmetries. The resulting dynamics is discussed. Explicit applications to pattern formation during Molecular Beam expitaxy are presented.   

The Platonic Ideal of Stalactite Growth

The chemical mechanisms underlying the growth of cave formations such as stalactites are well-known, yet no theory has yet been proposed which successfully accounts for the dynamic evolution of their shapes. Here we consider the interplay of thin-film fluid dynamics, calcium carbonate chemistry, and CO$_2$ transport in the cave to show that stalactites evolve according to a novel local geometric growth law which exhibits extreme amplification at the tip as a consequence of the locally-varying fluid layer thickness. Studies of this model show that a broad class of initial conditions is attracted to an ideal shape which is strikingly close to a statistical average of natural stalactites. A linear stability analysis shows is used to explain the instability of this state to the formation of centimeter-scale ripples, as commonly seen on a wide range of speleothem surfaces.   

Transient evolution of dendritic crystal tips and sidebranch structures

Dendritic crystal growth is an inherently non-local problem in pattern formation. Although the theory for steady state tip growth is fairly well established, real dendrites also contain a rich sidebranching structure that interacts with the diffusion field. I will report on experimental studies of the emergence of dendritic crystals that explore how the crystal growth speed, tip size, and sidebranch spacing interact as they evolve towards their steady state values. Starting with a small, nearly spherical seed of NH$_4$Cl held in unstable equilibrium in supersaturated aqueous solution, the temperature is dropped an amount $\Delta T$ ranging from 0.1$^{\circ}$C to 1.5$^{\circ}$C and the subsequent growth is recorded. Even before the steady state tip shape is established, the beginnings of the first sidebranches can be observed, and the dendrite quickly approaches steady state behavior. Once the steady state has been established, the temperature is again changed, and the evolution of the growth speed, tip size, and sidebranch spacing towards the new steady state values will be discussed.   

Instabilities and motion of tilt grain boundaries in three dimensional stripe patterns

Unlike two dimensional tilt boundaries in stripe phases for which stationary solutions are known to exist, the three dimensional case is generally unstable. We study the appropriate amplitude equations in the weakly nonlinear regime close to onset, and find a finite wavenumber, anisotropic instability with wavevector along the grain boundary plane. The characteristic wavelength is larger than that of the base stripe pattern. Our study reveals that this new three dimensional instability originates from a phase perturbation of the base periodic modes, as well as from the cross coupling between orthogonal base modes around the grain boundary region. Our results are in agreement with experimental findings in three dimensional lamellar diblock copolymers.   

Length Scales of Chaotic patterns near the onset of of Electroconvection in the Nematic Liquid Crystal I52

We report experimental results for Electroconvection of the nematic Liquid Crystal I52 with planar alignment and a conductivity of $1.0\times10^{-8}\, (\Omega\,{\rm m})^{-1}$. The cell spacing was $19.4\, \mu {\rm m}$ and the driving frequency was 25.0 Hz. Spatio-temporal chaos consisting of a superposition of zig and zag oblique rolls evolved by means of a supercritical Hopf bifurcation from the uniform conduction state.\footnote{M. Dennin, G. Ahlers and D. S. Cannell, Science, {\bf 272}, 388 (1996).} For small $\epsilon \equiv V^2/ V_c^2 -1$, we measured the correlation lengths of the envelopes of both zig and zag patterns. These lengths could be fit to a power law in $\epsilon$ with an exponent smaller than that predicted from amplitude equations. The disagreement with theory is similar to that found previously for domain chaos in rotating Rayleigh-Benard convection.\footnote{Y. Hu, R. E. Ecke and G. Ahlers, Phys. Rev. Lett. {\bf 74}, 5040 (1995).}   

An Anomaly in the Domain Chaos State of Rayleigh-B\'enard Convection with Large Aspect Ratio

Rayleigh-B\'enard convection-patterns exhibit a type of spatio-temporal chaos known as domain chaos (DC) at the onset of convection when the sample rotates fast enough about the vertical axis. DC is characterized by domains of straight rolls that chaotically change their orientation and size due to the K\"uppers-Lortz instability.\footnote{G. K\"uppers and D. Lortz, J. Fluid Mech. {\bf 35}, 609 (1969).} However, in a large aspect ratio $\Gamma\equiv r/d=82$ cylindrical sample, where $r$ is the radius of the cell and $d$ is the cell thickness, we observed DC in the sample center, surrounded by an annulus of radial rolls populated by occasional defects reminiscent of undulation chaos.\footnote{K. E. Daniels, B.B. Plapp, and E. Bodenschatz, Phys. Rev. Lett. {\bf 84}, 5320 (2000).} This was unexpected because smaller samples do exhibit domain chaos throughout and the weakly-nonlinear theory that describes the supercritical bifurcation to chaos is expected to be more applicable as $\Gamma$ increases. One possible explanation is that the centrifugal force, which is neglected in the theory, plays an important role.\footnote{A. Jayaraman and H. Greenside (private communication).}   

Meandering of the large-scale circulation of turbulent convection in a cylindrical cell

The large-scale circulation (LSC) in cylindrical cells of aspect ratio $\Gamma \equiv D/L = 1$ ($D =$ diameter, $L = $ height) filled with water at a mean temperature of 40$^\circ$C and heated from below was studied for Rayleigh numbers $R$ in the range $10^9$ to $10^{11}$. We measured the temperatures of the cell side-wall as a function of time $t$ at eight azimuthal locations on the horizontal mid- plane and from them deduced the azimuthal orientation $\theta(t)$ of the LSC. We found that $\theta(t)$ varied irregularly in time. Although it had a preferred value, on average there was a long-term continuous rotation of the LSC. From the data for $\theta(t)$ we derived $\dot\theta \equiv \Delta\theta/\Delta t$ ($\Delta t$ is the time interval between measurements). The time averages of $\dot \theta(\theta)$ gave a deterministic force $-\partial V/\partial \theta$ corresponding to a potential of the form $V = V_0 [-\cos(\theta - \theta_0) + v_1 \theta]$, and its probability distribution-function $P_ {\dot \theta}(\dot\theta)$ yielded a Langevin force $f(t)$. Integrations of the corresponding stochastic model equation $\partial \theta /\partial t = - \partial V / \partial \theta + f(t)$ produced time series $\theta(t)$ and distribution functions $P_{\theta}(\theta)$ remarkably similar to the experimental data. We attribute $f(t)$ to the action of the turbulent background fluctuations on the LSC, and found that its intensity depended on $R$.   

Lyapunov exponents for small aspect ratio Rayleigh-Benard convection

Positive Lyapunov exponents and their corresponding eigenvectors have been computed numerically for small aspect ratio, 3-D rotating Rayleigh-Benard convection cells with no-slip boundary conditions. The parameters are the same as those used by Ahlers and Behringer (PRL 40, 1978) in their seminal work on aperiodic time dependence in Rayleigh-Benard convection cells. Our work confirms that the dynamics in these cells truly is chaotic as defined by a positive Lyapunov exponent. The time evolution of the Lyapunov eigenvector in the chaotic regime will also be discussed.   

Patters in particles on free surfaces

Particles floating on a free surface above a turbulent flow tend to cluster because of the upwelling and downwelling motions in the fluid underneath. The types of patterns that form depend on the stretching and contraction rates of the flow as well as on the correlation times of the flow. With increasing divergence of the flow a transition from elongated structures to point like ones can be induced. Unexpectedly, we find that the presence of correlations can weaken the clustering of particles.   

Dynamic Buckling and Fragmentation in Brittle Rods

We present experiments on the dynamic buckling and fragmentation of slender rods axially impacted by a projectile. By combining the results of Saint-Venant and elastic beam theory, we derive a preferred wavelength $\lambda$ for the buckling instability, and experimentally verify the resulting scaling law for a range of materials including teflon, dry pasta, glass, and steel. For brittle materials, buckling leads to the fragmentation of the rod. Measured fragment length distributions show two peaks near $\lambda/2$ and $\lambda/4$. The non-monotonic nature of the distributions reflect the influence of the deterministic buckling process on the more random fragmentation processes.   

Strongly non-Gaussian statistics of optical soliton parameters in multichannel transmission due to delayed Raman response

We study the effects of delayed Raman response on soliton dynamics in optical fiber transmission systems with multiple frequency channels. Taking into account the quasi-random nature of pulse sequences in different channels and the collision induced energy exchange we show that soliton propagation in a given channel under many collisions with solitons from other channels is described by a perturbed stochastic nonlinear Schr\"odinger equation with weak disorder in the linear gain coefficient. As a result, the soliton amplitude becomes a random variable with a lognormal distribution. The cross frequency shift is also lognormally distributed and the self frequency shift is a random variable that is not self-averaging. We conclude that this disorder is potentially more harmful than other types of disorder that are usually considered in optical fiber transmission. Our predictions are in very good agreement with extensive numerical simulations employing importance sampling.   

Session L24: Focus Session: Friction, Fracture, and Deformation III

Microparticle Manipulation Using Inertial Forces

Manipulation (transport, positioning, separation, or removal) of micro- and nanoparticles has become an increasingly vibrant field of research. We present a simple method to transport a large number of microparticles in parallel. Piezoelectric shear plates are used to excite asymmetric shear waves which are coupled into a substrate. At the surface of the substrate, linear motion of particles is induced due to inertial forces on the particles and the stick-slip effect. While the approach is very versatile and can be used for a wide range of particle and substrate combinations, it is selective to the particle mass and the surface chemistry. In addition to the study of particle transport, the tribological behavior of the particles on the surface can be investigated by applying a symmetric waveform to the piezo, which allows the probing of the static friction force between the particle and substrate. A simple dynamic model to describe the behavior will be discussed. The frictional behavior of particles on chemically functionalized surfaces will be presented.   

Contact area dependence of frictional forces: Moving adsorbed antimony nanoparticles

Despite its daily-life importance, the fundamentals of friction are still insufficiently understood. In particular, the interplay between friction, adhesion, ``true'' contact area, and crystalline structure at the interface is an issue of current debate. In this work, antimony nanoparticles grown on highly oriented pyrolytic graphite and molybdenum disulfide were used as a model system to investigate the contact area dependence of frictional forces. This system allows to accurately determine both the interface structure and the effective contact area. Controlled translation of the antimony nanoparticles was induced by the action of the oscillating tip in a dynamic force microscope. During manipulation, the power dissipated due to tip-sample interactions was recorded. We found that the threshold value of the power dissipation needed for translation depends linearly on the contact area between the antimony particles and the substrate. Assuming a linear relationship between dissipated power and frictional forces implies a direct proportionality between friction and contact area. Particles smaller than 10000~nm$^{2}$ in size, however, were found to show lower dissipation than expected, which might be explained by structural lubricity.   

Intrinsic Friction Analysis of the Glass Forming Process of Polymer Films

Many modern and future technological applications involve ultrathin polymer films with a thickness below the 100-nanometer scale, where statistical bulk averaging is jeopardized and interfacial constraints dictate transport properties. In such confined polymeric systems, transport properties strongly depend on molecular relaxation and structural phases that deviate from the bulk. This is particularly relevant in thermally assisted nanoindentation processes near the glass transition temperature, Tg. In this paper, an elaborate isothermal friction-velocity analysis is introduced, as a material distinctive characterization tool that provides fundamental insight into the glass forming process. It is the glass forming process in constrained thin films that leads to a non-monotonous Tg-profile, which is responsible for a strongly film thickness dependent effective modulus during nanoindentation. The presented study involves ultrathin polystyrene films that serve as model systems in a thermomechanical NEMS storage application designed to circumvent the superparamagnetic limit associated with magnetic data storage.   

Molecular dynamics simulation of crack propagation in highly cross-linked polymers under uniaxial deformation

The strength of the interface between a structural adhesive and a solid surface is a fundamental issue. We study fracture in highly cross-linked polymer networks (e.g. epoxy) bonded to a solid surface using large-scale molecular dynamics simulations. An initial crack is created by forbidding bonds to occur on a fraction of the solid surface up to a crack tip. The time and length scales involved in this process dictate the use of a coarse grained bead-spring model of the epoxy network. In order to avoid unwanted boundary effects, large systems of up to a million particles are used. Stress-strain curves are determined for each system from tensile pull molecular dynamics simulations. Comparison with standard fracture mechanics will be presented. The dependence of the interfacial fracture energy on film thickness and on the ratio of the width of the unbonded solid surface to film thickness will be described.   

Reactive MD simulations of deformation and failure in cross-linking polymers.

We have developed a computational framework for studying the behavior of polymer systems simultaneously undergoing strain deformations and polymerization reactions, such as epoxy resins and other reacting polymeric systems. Using our simulations we predict system properties, such as yield strength, toughness and ultimate failure of the system at various stress regimes, and examine how these properties are affected by variations in reaction rates and curing conditions. Accordingly, we use this framework to optimize the design of autonomously healing polymer matrix composites, containing di-cyclo-penta-diene (DCPD) as the healing agent. Polymerization reactions occur on time scales, currently inaccessible by conventional MD. We bridge this chasm by coarse graining real monomers as soft interacting beads. Timescale mapping between the coarse-grained simulations and a reactive MD model of atomically detailed systems with reacting monomers such as DCPD, is achieved using diffusion timescale matching. Reaction processes in the coarse-grained simulations are accounted by a MC scheme, in which we can adjust the probability of reaction and the geometric aspects reflecting the reaction mechanisms in a particular polymer system.   

Short-Range Exponential Repulsive Force Between Randomly Rough Surfaces

Using a Surface Forces Apparatus we have studied the effects of surface roughness on the interaction forces and deformations of polymeric surfaces. We measured the force-distance functions on approach and separation of two rough surfaces which, on approach, exhibited an almost perfect exponentially repulsive force-distance regime, and a weak adhesion on separation. Random roughness may be a prerequisite for the exponential force regime, which appears to be due to the local compressions (micro- or fine- grained deformations) of the surface asperities. The resulting characteristic decay lengths were fitted to common surface roughness parameters obtained by Atomic Force Microscopy to draw possible correlations. The coarse-grained (global) deformations of the initially curved surfaces appear to be Hertzian.   

Oscillating viscoelastic JKR contacts

Adhesion of micron-scale probes with model elastomers was studied with a depth-sensing nanoindenter under oscillatory loading conditions. Force-displacement curves were highly reversible, consistent with Johnson-Kendall-Roberts (JKR) behavior. However, experiments revealed striking differences between the measured tip-sample interaction stiffness and the theoretical prediction from the JKR relationship. Measured stiffness was always greater than zero, and varying probe radius or polymer modulus resulted in stiffness curve shapes remarkably similar to Maugis’ JKR/DMT transition curves. These apparent paradoxes are resolved by considering viscoelasticity of an oscillating crack tip. Under well described conditions determined by oscillation frequency, sample viscoelasticity, and Tabor’s parameter, an oscillating crack tip will neither advance nor recede. Thus, contact size is fixed at any given instant, and experimentally measured stiffness is equal to the punch stiffness. For fixed oscillation frequency, transition between JKR and punch stiffness can be brought about by increasing probe radius, decreasing sample modulus, or varying frequency. Comparisons of experiments and theory will be presented. Storage modulus and surface energy measured from nanoscale JKR results were compared to calculated and measured with conventional nanoindentation and JKR force-displacement analyses. With this method it is possible to make localized mechanical property measurements for contacts with diameters smaller than the optical limit. \newline \newline Collaborators: S.A. Syed Asif (Hysitron, Inc.); K.L., Johnson, J.A. Greenwood (Cambridge Univ., UK)   

Cantilever tilt compensation for variable-load atomic-force microscopy

In a typical atomic-force microscope (AFM), the cantilever forms an angle with respect to the sample surface. This tilt is important for contact mode experiments, because the free end of the cantilever (constrained to move along the surface) displaces laterally as applied load varies. As a result, the AFM tip makes contact with a different point on the surface at each load. These positions lie along the surface projection of the lever's long-axis. The amount of relative tip-sample displacement is proportional to load and is shown to be substantial. Thus, care should be taken when performing load-dependent contact mode experiments, such as friction versus load or force versus separation measurements, when it is required that the tip scan the same line or remain on the same position at each load. We present a method that compensates for in-plane tip-displacement versus load, based on a simple geometric calculation that depends only on the range of vertical motion. We demonstrate the successful use of this method on surfaces with nano-scale inhomogeneities and show that the tip can remain localized over a large load range.   

Carbon, Hydrogen, and Silicon-containing Solid Lubricants

The development of micro-sized devices has prompted the need for protection of the surfaces of these devices. Amorphous carbon films (a-C and a-CH), doped carbon films, and self- assembled monolayers (SAMs) are all possible candidates for the passivation and lubrication of these devices. The fundamental problem associated with controlling friction and wear is a lack of understanding of the underlying atomic-scale chemical and physical processes that govern them. Extensive molecular dynamics (MD) simulations have been done that have examined the compression and friction of model hydrocarbon SAMs, amorphous carbon- and silicon-containing films both with and without hydrogen. We have examined the contact forces present at the interface between a tip and pure, or mixed-length, SAMs during sliding. Compression and shear-induced polymerization have also been modeled in unsaturated hydrocarbon films. In addition, we have also done simulations that analyze the mechanical and tribological properties of a-C, a-CH, a-C-Si, and a-C-Si-H films. Some of our recent results will be discussed.   

Nanotribology of high performance amorphous carbon films

High performance carbon films are attracting a great deal of interest as candidate materials to improve the tribological characteristics of mechanical parts ranging from the macroscale to the nanoscale. We have investigated the nanotribology and the surface chemistry of two types of diamond-like carbon (DLC) films. One, known as tetrahedral amorphous carbon (ta-C), is grown with Pulsed Laser Deposition (PLD). It is essentially hydrogen-free and contains a high (up to $\sim $80{\%}) fraction of sp3-bonded carbon. The other is grown with Plasma Immersion Ion Implantation and Deposition (PIIID). This film is a hydrogenated DLC with a lower fraction of sp3-bonded carbon (30-50{\%}). The nanotribology is characterized quantitatively with atomic force microscopy (AFM) utilizing silicon AFM tips coated with diamond, DLC, or ta-C. The surface chemistry is characterized via near edge x-ray absorption fine structure (NEXAFS) spectroscopy. We will discuss how doping the DLC and annealing the ta-C affects the nanotribology and surface chemistry of these films. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.   

Metallic Adhesion in Atomic-Size Junctions

We report high resolution simultaneous measurements of electrical conductance and force gradient between two sharp gold tips as their separation is varied from the tunneling distance to atomic-size contact. The use of atomically sharp tips minimizes van der Waals interaction, making it possible to identify the short-range metallic adhesion contribution to the total force. [Phys. Rev. Lett. 93, 116803]   

Incipient Plasticity During Indentation of a Well Characterized 3 Nanometer Radius Tip

We present the results of nanoindentation testing of a well characterized tip geometry with a spatial scale easily matched by existing atomistic simulation. The atomically defined tungsten asperity of 3 nm radius was fabricated and imaged by field ion microscopy and brought into contact with a Au(111) terrace in ultra-high vacuum conditions. The mechanical evolution of the asperity contact under cyclic indentation testing was monitored by a simultaneous load-displacement and electrical current-displacement measurement. Load displacement curves of the pristine surface showed multiple discrete plastic (``pop-in'') events during loading, with energies consistent with the nucleation of individual defects. During unloading, we observe reverse plasticity and complete self-healing of the induced defect. Both the qualitatative behaviour and measured energy dissipation values are in agreement with recent molecular dynamics simulations of incipient plasticity in metallic asperity contacts.   

Molecular Dynamics Simulations of Nanoindentation and Nanoscratching of $\beta$-SiC

We present the results of molecular dynamics simulations of nanoindentation of the Si-terminated (001) surface of $\beta$-SiC by a diamond tip. In particular we investigate the dependence of the critical depth and pressure for the elastic-to-plastic transition as a function of indentation velocity, tip size, and workpiece temperature. Our simulations were carried out using the Tersoff potential, which accurately reproduces the lattice and elastic constants of $\beta$-SiC. Over the range of indenter sizes used in our simulations, both the critical pressure and indentation depth decrease with increasing indenter size. In contrast, the critical indentation depth for the elastic-to-plastic transition does not depend on the indenter velocity. For indentation depths beyond the critical depth, the pressure saturates at 100 GPa, which corresponds to the experimental pressure at which $\beta$-SiC transforms to the rocksalt structure. An analysis of the pair-correlation function and bond-angle distribution as a function of indenter depth supports our conjecture that the observed plastic behavior is related to the onset of a phase transition from the zinc-blende structure to the rocksalt structure. Results for nanoscratching of the (100) surface of $\beta$-SiC are also presented.   

Session L27: Focus Session: Carbon Nanotubes: Devices

Advanced Sensors based on Carbon Nanotube Networks

Single wall carbon nanotubes (SWNTs) are useful materials for a variety of electronic applications. The relative chemical passivity and environmental robustness of SWNTs suggests that they could be suitable materials for environmental sensors. Kong et al. * have demonstrated sensing of biological and chemical analytes from liquid and gaseous ambient, respectively. We have extended this work in several ways. An important criterion for sensors is the noise level, which sets a lower limit on sensitivity. It is shown that one of the weaknesses of nanoscale devices prepared from discrete SWNTs, high 1/f noise, is greatly ameliorated by the use of interconnected random networks of SWNTs to make large, or ``macro'' scale, devices. While changes in conductance can be used to indicate the presence of adsorbates of volatile analytes, it is shown that a capacitor configuration, utilizing the carbon nanotube network (CNN) as one electrode, leads to a much more sensitive, responsive, and accurate detector, suitable for use with a wide range of materials. In general, the capacitance of such a detector is proportional to the ambient fraction of equilibrium vapor pressure times the dipole moment of molecules constituent in ambient. Thus, saturated atmospheres of extreme low-vapor pressure polar materials (e.g., explosives) can induce responses greater than below-saturation atmospheres of highly volatile materials, or even saturated atmospheres of non-polar materials. This response can be further enhanced by the deposition of self-assembled monolayer or ultra-thin polymer coatings on the CNN or device substrate. Use of such modifications allows specificity, by comparing responses from each of a set of modified sensors to challenge. *J. Kong, N.R. Franklin, C.Ahou, M.G. Chapline, S. Peng, K. Cho, and H. Dai, Science \underline {87}, 622 (2000).   

Tunable Carbon Nanotube Tunneling Devices for Wireless Communications

Carbon nanotube could be one of the best candidates for a nanoscale emitter, a receiver, and a mechanical switch, which are essential components for the future application of high-speed wireless communications. Tunable nanoscale resistors, capacitors, inductors, and mechanical resonators can be implemented with carbon nanotubes. It is originated from the excellent properties of carbon nanotube such as structure dependent metallic properties, pseudo-one-dimensional transport characteristics and electronic structure, hollow structure, extremely high mechanical strength with high aspect ratio, good thermal conductivity, chemical inertness, etc. In this study, we studied carbon nanotubes as an emitter, a receiver and an electromechanical oscillator from suspended carbon nanotubes on a device array for the wireless communications. The suspended carbon nanotube arrays are fabricated by directly and laterally grown carbon nanotubes on the multilayer electrode arrays with a field effect transistor structure. The characteristics of carbon nanotube transmitter, receiver, and mechanical oscillator are studied using an impedance analyzer as a function of frequency and gate voltage modulation.   

Impedance Spectroscopy, High Frequency Scanning Gate Microscopy and Local Memory Effect of Carbon Nanotube Transistors

Successful implementation of carbon nanotube field effect transistors (CNFETs) as nanoelectronic devices requires reliable techniques for the characterization of their local electronic properties. At low frequencies, this goal was achieved by using recently developed scanning probe techniques such as Scanning Gate Microscopy (SGM). The extension of these techniques to high frequency is important because, although the low frequency performance of CNFET has been greatly improved, little is known about their high frequency behavior. We will present impedance characteristics of CNFET devices for ac-frequencies up to 15MHz. We also extend SGM to frequencies up to 15MHz, and use it to image changes in the impedance of CNFET circuits induced by the SGM-tip gate. Results of both experiments are consistent with a simple RC parallel circuit model of the CNFET, with a time constant of 0.3 microsec. We also use the tip gate to show that charge injection from the single wall nanotube into the substrate, which is responsible for the memory effect, can be induced at specific locations along the tube length. This result is a strong indication that CNFET-based memory cells may be miniaturized to dimensions far below the micrometer scale of current devices.   

Nanowire and nanotube transistors with surrounding gates

We will present fabrication and electronic transport studies of novel nanowire and nanotube transistors with surrounding polymer-electrolyte gates. These devices are based on nanowires/nanotubes contacted by source/drain electrodes atop Si/SiO$_{2}$ substrate. Vias were etched through the SiO$_{2}$ layers, followed by refilling LiClO$_{4}$/poly(ethylene oxide). The silicon substrate is thus in electrical contact with the polymer electrolyte, therefore forming the surround gate for the transistors. Electronic characterization revealed well-enhanced transconductance for both nanowire and nanotube transistors, with operating gate voltage reduced to 1V. In addition, intriguing negative differential resistance has been observed with surround-gated nanowire transistors, which can be attributed to the much enhanced gate dependence.   

Single-nanotube Devices from Purified HiPCO Material

We have developed a purification process that retains the remarkable electronic properties of single walled carbon nanotube (SWNT) material. Nanotubes grown by the HiPCO method (High Pressure catalytic decomposition of Carbon monOxide) are purified and suspended as single tubes and small bundles in a surfactant solution. SWNTs are deposited on functionalized substrates and contacted by electrodes. The resulting circuits consist of high quality metallic and semiconducting nanotubes that are apparently unaffected by the purification process. Circuits made from raw HiPCO material have vastly inferior device parameters indicating the crucial role of the purification process. We show how source-drain current measurements as a function of temperature and backgate voltage can be used to determine the energy gap of a semiconducting nanotube in a field effect transistor geometry. This work represents significant progress towards the goal of producing complex integrated circuits from bulk-grown SWNT material. This work has been partially supported by NSF (Grants DMR 00- 79909 (MRSEC); DMR-0203378), by NASA (NAG8-2172), and the Petroleum Research Fund. DJ acknowledges support from an NSF- funded IGERT Fellowship Grant DGE-0221664 administered through Penn's Center for the Science and Engineering of Nanoscale Systems.   

Thin Film Carbon Nanotube FETs with Polymer Electrolytes

Thin film transistors of single-walled carbon nanotubes were operated with polymer electrolyte as gate media. Nearly ideal gate efficiencies allow low-voltage operation with the absence of the common hysteresis problem observed in back-gated carbon nanotube FETs yielding a reliable and simple method for measuring the device characteristics. Furthermore, the conduction type (p/n-type) of the devices can easily be controlled by varying the electrolyte media. Effects such as charge transfer between polymer and nanotubes, and tube-tube interactions in arrays of nanotubes will be discussed.   

Long, Suspended Metallic and Semiconducting Carbon Nanotube Devices

We have fabricated devices consisting of individual suspended carbon nanotubes (CNTs) spanning trenches up to 120$\mu $m long and 500$\mu $m deep. The carbon nanotubes are grown via chemical vapor deposition over existing gold or platinum electrodes, forming complete electronically addressable devices without exposure of the CNTs to resists or etchants. These CNT devices allow study of the intrinsic transport properties of the nanotubes without disorder induced by the substrate or chemical residues from conventional lithography. The use of a mobile probe as a gate electrode allows identification of metallic and semiconducting nanotubes. We present the growth and fabrication procedures along with transport measurements on these long, suspended CNTs.   

A Three-Terminal Carbon Nanorelay

Three-terminal nanorelay structures were fabricated with multiwall carbon nanotubes (MWNTs). The nanotube relays were deflected by applying a gate voltage until contact (mechanical and/or electrical) was made with a drain electrode, thus closing the circuit. It was possible to achieve multiple switching cycles, showing that carbon nanotubes are suitable and practical systems for developing nanoelectromechanical devices of this kind.   

Carbon Nanotubes as the Protective Coating in Space Propulsion Systems

We have evaluated carbon nanotubes as the protective coatings against ion erosion in space propulsion systems. The space exploration program faces enormous challenges to achieve improvements in safety, cost, and speed of missions. Electric propulsion (EP) thrusters are recognized as far more efficient than chemical thrusters. However, an electrode sputter erosion process limits the lifetimes of these EP devices. Inspired by their impressive cohesive energy and stiffness, we have tested carbon nanotubes (CNTs) as the protective coating. We compare CNTs to CVD diamond, carbon and BN films as exposed to the exhaust beam of a Hall-effect thruster. We found that only CVD diamond films and VA-MWNTs survived erosion by 250 eV ions. Analysis by field emission scanning electron microscopy, backscattered electron imaging, and Raman spectroscopy indicate that these VA-MWNTs were bundled at their tips before the erosion. An erosion mechanism was then formulated and verified by a series experiments with Xe propellant at an ion current density of 5 mA/cm$^{2}$. We found that VA-MWNTs are eroded in a nonlinear rate. Our result suggests that catalysts on the VA-MWNTs are responsible for this erosion and their removal could further enhance the resistance of VA-MWNTs against ion erosion.   

Session L28: Phonons in Metals

A Study of the Cross--Over Temperature between the Adiabatic and Non--Adiabatic Contributions to the Electron--Phonon Free Energy in Na, K, Al, and Pb

We calculate the electron--phonon contribution to the free energy and entropy for four elemental metals, Na, K, Al, and Pb, using realistic phonon spectra and pseudopotentials for temperatures between $0 \leq T < 1.5 \,\, T_{melt}$. We show that the non--adiabatic contribution dominates at low temperatures whereas the adiabatic contribution dominates at high temperatures. We calculate the cross--over temperatures between the two contributions which is roughly between 0.5 and 0.8 $T_{melt}$. Where we are able to compare, we find good agreement with experiment.   

Calculation of phonon dispersion relations and softening in photo-excited bismuth

The phonon dispersion relations for equilibrium and photo-excited bismuth are calculated using density functional theory, combined with constrained density functional theory. The dependence of phonon frequency on photo-excited electron-hole plasma density is found for modes throughout the Brillouin Zone. The results are in excellent agreement with available neutron scattering data for the equilibrium occupation of electronic bands. We find the effect of phonon softening by the electron-hole plasma to be larger in the optical modes than in the acoustic modes.   

Compositional variation of the phonon dispersion curves of bcc Fe-Ga alloys

Inelastic neutron scattering techniques have been used to measure the phonon dispersion curves of bcc Fe-Ga alloys as a function of Ga concentration. We observed that the phonon frequencies of every branch decrease significantly with increasing Ga concentration with the softening being more pronounced for the T$_{2}$[110] branch and for the L[111] in the vicinity of $\xi $ = (2/3, 2/3,2/3). The slope of the T$_{2}$[110] branch was found to decrease linearly with increasing Ga concentration and to extrapolate to zero at approximately 27 at.{\%} in agreement with the results of sound velocity measurements. As the Ga concentration increases, a splitting of the T$_{1}$[110] branch is observed, an effect characteristic of diatomic lattices.   

First-principles elastic constants and phonons of $\delta$-Pu

Elastic constants and zone boundary phonons of $\delta$-Pu have been calculated within the density-functional theory. The electronic structure is modeled by disordered magnetism utilizing either the disordered local moment or the special quasi-random structure techniques. The anomalously soft C$^ {prime}$ as well as a large anisotropy ratio of $\delta$-Pu is reproduced by this first-principles model. Also the measured phonons for $\delta$-Pu compare relatively well with their theoretical counterpart at the zone boundary.   

Phonon Entropy of Alloying in Dilute Vanadium Alloys

We investigate the entropic effects associated with changes in the phonon modes of vanadium upon dilute substitutional alloying. Using inelastic neutron scattering, we have measured the phonon DOS and the phonon entropy of mixing in V - 6\%X, with X a transition metal impurity. We study trends for impurities across the d-series and down several columns of the periodic table. We show that for Ni, Pd and Pt impurities, the phonon entropy of alloying is large and negative, and in the case of Pt it results in a negative total entropy of mixing for 6\% impurities. A Born-von Karman model was used to invert the experimental DOS curves and showed that the phonon stiffening down this column is associated with an increases in 1NN longitudinal inter-atomic force-constants. The changes in the phonon DOS for impurities across the 3d series are also correlated with the previously measured changes in the superconducting temperature $T_{c}$. Ab-initio DFT simulations were used to compute the effect of impurities on the electronic and phonon properties of vanadium, and are compared to the experimental results. This work was supported by DOE through the BES Grant DE-FG03-0346055 and BES-MS, W-31-109-ENG-38.   

Ultrasonic study of $Gd_{5}(Si_{2}Ge_{2})$ elastic properties

$Gd_{5}(Si_{2}Ge_{2})$ undergoes a magnetic-martensitic transition near room temperature modifying its symmetry from an orthorhombic to a monoclinic structure. A giant magnetocaloric effect ($\Delta T/\Delta B\sim 8K/2T$) and a colossal strain (up to 10000ppm) can be induced both thermally and magnetically. Due to low hysteresis ($<$2K,$<$0.5T), the material has a potential for energy efficient refrigeration and actuation uses. The acoustic phonon properties of the $Gd_{5}(Si_{2}Ge_{2})$ single crystals, grown by tri-arc pulling technique were studied by echo-pulse ultrasonic probing. For the first time we have measured room temperature velocities of longitudinal and transverse sound waves. The measured diagonal elastic constants in the monoclinic phase are: $c_{33}=1.36\times10^{12},c_{44}=5.17\times10^{11},c_{55}=3.39\times10^{11}$dyne/cm$^{2}$. Here x and z are Cartesian axes parallel to crystallographic directions a and c; the later coincides with a two-fold rotation axis of the crystal.Work in Ames is supported by the US DOE.Work in NHMFL is supported by the In-House Research Program, NSF and State of Florida.   

Phase locking transitions in arrays of coupled anharmonic oscillators

The Chaotic Dynamics of Coupled Oscillator arrays with cubic anharmonicity is studied. Our model includes damping terms and external time dependent forces. These Coupled oscillators are either phase-lock or behave chaotically or hyperchaotically, depending upon the magnitude of their inter-oscillator coupling strength and the frequency of the applied external force. The associated Lyapunov exponents are determined indicating the type of attractor, and phase diagrams are presented.   

Optical Conductivity of Weakly Ferromagnetic Metals

The frequency and temperature dependences of the contributions to $\sigma(\omega)$ from impurity and phonon scattering are calculated using an extension to the paramagnon model due to Hirsch$^1$. This model includes both an on-site repulsion and a nearest neighbor ferromagnetic coupling. The Debye model is used for the phonons. The dependences of these contributions on the parameters of the model are presented. \vskip 8pt \noindent $^1$ J. E. Hirsch, Phys. Rev. B {\bf 40}, 2354 and 9061 (1989).   

Direct measurement of the electron-phonon relaxation rate in thin metal films

We have used normal metal-insulator-superconductor (NIS) tunnel junctions for ultrasensitive thermometry at sub-Kelvin temperatures. With the help of these thermometers, we have developed an ac-technique to measure the electron-phonon (e-p) scattering rate directly, without any other material or geometry dependent parameters, based on overheating the electron gas. The technique is based on Joule heating the electrons in the frequency range DC-10 MHz, and measuring the electron temperature in DC. Because of the nonlinearity of the electron-phonon coupling with respect to temperature, even the DC response will be affected, when the heating frequency reaches the natural cut-off determined by the e-p scattering rate. Results on thin Cu films show a $T^{4}$ behavior for the scattering rate, in agreement with indirect measurement of similar samples and numerical modeling of the non-linear response.\footnote{L. J. Taskinen, J. M. Kivioja, J. T. Karvonen, and I. J. Maasilta, phys. stat. sol. (c) {\bf 1}, 2856 (2004). },\footnote{J. T. Karvonen, L. J. Taskinen, I. J. Maasilta, phys. stat. sol. (c) {\bf 1}, 2799 (2004).}   

Electronic polarization in quasilinear chains

Starting with a finite $k$-mesh version of a well-known equation of Blount, we show how various definitions proposed for the polarization of a long chain are related. Expressions used for infinite periodic chains in the `modern theory of polarization' are thereby obtained along with a new single-particle formulation. Separate intracellular and intercellular contributions to the polarization are identified and in application to infinite chains, the traditional sawtooth definition is found to be missing the latter. For a finite open chain the dipole moment depends upon how the chain is terminated, but the intracellular and intercellular polarization do not. All these results are illustrated through calculations with a simple H\"uckel-like model.   

Session L30: Polymers-Inorganic Composites I

Selective Metallization of Block Copolymers Using Supercritical Carbon Dioxide

In this research phase-separated structures of styrene-vinylpyridine block copolymers are used as templates for macromolecule-metal nanocomposites. Under optimal conditions, templates are prepared as thin films or in bulk and metallized without disturbing the ordered structure. We have developed a procedure that deposits metal within the polymer using supercritical carbon dioxide-soluble metal precursors. The use of supercritical carbon dioxide allows for selective metallization of the polymer at or below the glass transition, without disrupting the morphology. In addition, similar procedures have been investigated using metal salts and acids. Using these techniques, metals and metal-sulfides including silver, gold, platinum and zinc sulfide have been deposited.   

Holographically phase separated gold/nanoparticle films

Holographic Polymer Dispersed Liquid Crystals (HPDLCs) show a shift in the reflected wavelength due to a change in the fringe separation of the Bragg gratings when pressure is applied perpendicular to the surface plane. Typically urethane based monomer forms the polymer layer via photopolymerisation by holographic laser exposure in conjunction with liquid crystals. In this paper we study the effect of step variation in pressure in the range of 0-20psi on the peak wavelength reflection of the holographic polymer dispersed gold nanoparticles. The high refractive index mismatches between gold and polymer composition produces good quality Bragg diffraction gratings. We investigate the use of Polydimethylsiloxane (PDMS), a siloxane based oligomer with high elasticity as the polymer for the gold/ polymer grating formation. The high elasticity of the PDMS along with sensitivity of the film to applied pressure enhances the pressure response. We present the application of the polymer /gold nanoparticle films for pressure sensors, in which a linear change in the wavelength of the reflected light corresponds to the pressure variations on the films.   

Controlling Self-Assembly of Gold Nanoparticles in Block Copolymer Templates

Self assembly of inorganic nanoparticles within a block copolymer offers a way to produce materials with unique optical, electronic and magnetic properties. To reveal some of the fundamentals of the self assembly we have investigated the system consisting of symmetric polystyrene-poly(2-vinylpyridine) (PS-P2VP) whose total molecular weight (M$_{n})$ is 197 kg/mol and polymer-coated gold nanoparticles. Gold nanoparticles are stabilized by carrying out their synthesis in mixtures of thiol terminated PS (PS-SH) and P2VP (P2VP-SH) chains whose M$_{n}$ is 1.3 kg/mol. The surface of these particles is tailored by changing the ratio of PS-SH to P2VP-SH on their surface. While PS coated gold nanoparticles are observed to locate near the center of PS domain, gold nanoparticles coated with a 1:1 mol ratio of PS-SH to P2VP-SH are segregated at the intermaterial dividing surface (IMDS) of PS-P2VP. The particle location (the center of PS domain, the IMDS of PS-P2VP chains, the center of P2VP domain) depends on the ratio of PS to P2VP on the gold surface. The range of ratios of the PS to P2VP where gold particles segregate to the IMDS is extremely broad, raising interesting questions about the spatial distribution of PS and P2VP on the gold surface.   

Synthesis of Ordered Fe$_{2}$O$_{3}$ Nanoparticles within Norbornene Methanol/Norbornene Dicarboxylic Acid Diblock Copolymers

Norbornene methanol-deuterated norbornene dicarboxylic acid diblock copolymers were synthesized by ring-opening metathesis polymerization. Iron oxide nanoparticles were formed in the microdomains of the diblock through a solution doping mechanism. Polymer samples with and without iron oxide particles were solution cast under a solvent saturated atmosphere. The morphology of the diblock copolymer and the dispersion of iron oxide nanoparticles within the copolymer were examined with transmission electron microscopy and the two morphologies were compared. Spherical iron oxide particles of 3-5 nm in size showed ferrimagnetic behavior at 5 K with high magnetization values (40 emu/g polymer or 1300 emu/cm$^{3})$ at 5 Tesla. It was shown that the magnetic properties and morphology of the iron oxide nanoparticles can be tailored by the in-situ synthesis within the domains of the copolymer. By controlling the size and dispersion of the Fe$_{2}$O$_{3}$ naoparticles, the magnetization moments were found to be higher than that of magnetic particles in other polymer systems.   

Microphase Segregation in Organic-Inorganic Randomly Grafted Copolymers

We have studied the microphase ordering of Polybutadiene-Polyhedral Oligomeric Silsesquioxane (PBD-POSS) randomly grafted copolymers using High Temperature Small Angle X-Ray Scattering and TEM. The polybutadiene forms the backbone of these copolymers whereas POSS cubic nanoparticles are grafted randomly along the backbone. In the bulk morphology, these molecules self-assemble due to the aggregation of the POSS nanoparticles and form randomly orientated two-dimensional lamellar sheets of POSS in the polybutadiene matrix. Our results show that the wave vector of the lamellar phase formed by these randomly grafted copolymers, has a temperature dependence different from either linear random copolymers or block copolymers. Our results are in consistent with the recently established randomly grafted copolymer mean-field theory and experiments.   

Electrospun Fibers from Self-assembling Polystrene-b-Polyisoprene Block Copolymers

Formation of submicron scale fibers with various domain shapes via electrospinning poly(styrene-\textit{block}-isoprene) (PS-$b$-PI) has been investigated. Monodisperse PS-$b$-PI block copolymers with a range of compositions were synthesized using anionic polymerization and were dissolved in THF. Solutions of 10 to 40 wt{\%} of PS-$b$-PI in THF were electrospun, and fibers with average diameters from 100 nm to 5 $\mu $m were obtained. Transmission electron micrographs taken from a microtomed thin electrospun fiber of a PS-$b$-PI block copolymer show PI cylinders aligned along the fiber axis, whereas TEM images of thicker electrospun fibers reveal that the skin and core regions can exhibit different domain structures. This skin-core differentiation in the fiber is possibly due to concentration gradient during solvent evaporation and short residence time involved in the electrospinning process. The TEM and SAXS studies show more uniform domain structures in the fibers after the removal of the residual solvent.   

Nanometer scale patterning using di-block copolymer

Di-block copolymer thin films of PS-PI, PS-PB and PS-PMMA are investigated on Si substrates. The morphology evolution with polymer thickness is studied using optical microscopy. As-coated polymer exhibits a very smooth surface. After annealing polymer over glass-transition temperature, the polymer exhibits a smooth surface only at certain thickness L$_{0}$. Transmission electron microscopy is used to study the microphase separation in polymer at different stages. Clear phase separation is observed in the polymer after staining with osmium tetraoxide (OsO$_{4})$. Long time annealing increases the long-range ordering. After treatment with ozone, disappearance of dark dots due to staining of Os and appearance of white hole indicate that the polymer with double-bond is removed from copolymer film. Scanning electron microscopy shows that polymer films after reactive ion etching give a regular hole pattern which can serve as mask for nanometer scale patterning.   

Nanofibers And Related Structures Formed By Polymerization

Nanofibers of cyanoacrylate were obtained by polymerization from the monomer vapor at a temperature near room temperature. The nanofibers had diameters ranging from 20 nm to 100 nm and lengths of up to several millimeters. Water molecules present on the substrate initiated the living anionic polymerization. As growth continued, the living ends were carried on the tip of each growing nanofiber. These nanofibers formed on glass, metal, plastic, electrospun nanofibers of other polymers, and other surfaces. Some fibers were tapered, some were branched, and some were bent. The number of fibers was varied by controlling the exposure of the substrate to water vapor. Under different conditions the monomer vapor was collected as droplets along electrospun nanofibers, or as droplets at the points where two electrospun nanofibers crossed. The addition of the initiator caused the droplets to polymerize, forming permanent beads on the fibers, and strong mechanical connections at the cross points. This phenomenon provides new ways to construct nanofiber structures engineered on nanometer scales. For example, filters constructed from an open structure of fibers can be coated with nanofibers polymerized from a vapor of nanometer scale droplets flowing through the structure, to improve the capture of molecules or particles.   

Session L31: Organic Photovoltaic and Electrochromic Devices

Organic Semiconductor Photovoltaics

Recent developments on organic photovoltaic elements are reviewed. Semiconducting conjugated polymers and molecules as well as nanocrystalline inorganic semiconductors are used in composite thin films. The photophysics of such photoactive devices is based on the photoinduced charge transfer from donor type semiconducting molecules onto acceptor type molecules such as Buckminsterfullerene, C$_{60}$ and/or nanoparticles. Similar to the first steps in natural photosynthesis, this photoinduced electron transfer leads to a number of potentially interesting applications which include sensitization of the photoconductivity and photovoltaic phenomena. Examples of photovoltaic architectures are discussed with their potential in terrestrial solar energy conversion. Several materials are introduced and discussed for their photovoltaic activities. Furthermore, nanomorphology has been investigated with AFM, SEM and TEM. The morphology/property relationship for a given photoactive system is found to be a major effect.   

Numerical Simulations of Layered and Blended Organic Photovoltaic Cells

We present results obtained from numerical simulations of organic photovaltaic cells as the donor-acceptor morphology evolves from sharply defined layers, to partial blends and finally homogeneous blends. We have employed a simple model that describes exciton dissociation and charge transport in continuously changing material concentrations. As the mixing percentage increases, the exciton dissociation increases and the diffusion counter-current decreases, resulting in substantially greater short circuit currents but reduced open circuit voltages. Blended structures are more sensitive to mobility than layers due to recombination throughout the bulk. Our model indicates that solar power efficiencies greater than 10\% can be achieved when the zero-field charge mobilities approach $10^{-3}$ cm$^2$/Vs for partially blended structures.   

Efficiency of Organic Conjugated Polymer/C$_{60}$ Bulk Heterojunction Photovoltaic Devices

We investigate the influence of thin-film morphology on the efficiency of organic conjugated polymer/C$_{60}$ bulk heterojunction polymer photovoltaic devices. Blends of soluble derivatives of fullerenes (PCBM- C$_{60}$) as electron acceptors and MEH-PPV or [MEH-PPV]-biphenylene-vinylene copolymer as donors are used in the fabrication the ITO/PEDOT:PSS/Polymer Blend/LiF/Al photocells. Thermal annealing effects on the phase segregation within the active layer are probed by scanning electron and atomic force microscopies. The micro/nano-structure morphologies are systematically correlated with the electrical and optical properties of the devices by current-voltage, capacitance- voltage, photocurrent, and electroabsorption spectroscopies. The implications of these results on the optimization of the open-circuit voltage, short-circuit current, and efficiency of the polymer photovoltaic devices will be explored.   

High Efficiency Regio-Regular-P3HT/PCBM Flexible Solar Cells

Solar cells (SCs) employing freshly synthesized home-made regio-regular poly(3-hexylthiophene) (RR-P3HT) and PCBM yielded nearly two fold increase of short circuit current, compared with the devices consisting of commercial RR-P3HT. The filling factor (FF) on the other hand decreased significantly resulting in the overall efficiency of 4{\%} for the device with commercial PHT from ADS. However, improving the serial resistance of the device can lead to much higher efficiency. The ideal homogenization of P3HT/PCBM solution and the optimal device heat-treatment [1] were used with the fresh home-made polymer to achieve the good phase separation of PCBM and RR-P3HT into a bi-continuous network structure. The best concentration of PCBM was found rather low; only 54 wt{\%} with respect to RR-P3HT in Toluene contrary to reported high 400 wt{\%} to PPV in 1,2 Dichlorobenzene.   

Time-resolved Photoluminescence Studies of Various Polymer Heterojunction Films for Photovoltaics

Polymer photovoltaics provide a promising avenue for low-cost photovoltaics and other optoelectronics devices, but they are plagued by poor efficiencies. Photogenerated excitons (bound electron-hole pairs) must be separated in order to extract that charge as current. The exciton diffusion range is very short (about 20nm), leading to high recombination. Because excitons may be separated at a junction between an electron- and hole-accepting material, a reliable method of increasing device efficiencies is to create blended or layered heterojunction structures with mixing on the order of 20nm We create blended and layered heterostructures of a hole-transporting polymer (M3EH-PPV) with a variety of canonical electron-transporting materials: an electron-transporting polymer (CN-ether-PPV); PCBM; ITO; and TiO2 solgel and nanoparticles. Using time resolved photoluminescence, we are able to search for new excited state species as well as charge and energy transfer pathways which compete efficiently with charge recombination. Along with traditional device characterization, we thus achieve a rich understanding of how different electron-transporting materials affect exciton dynamics and recombination and thus device performance.   

Temperature dependence of polymer hybrid solar cells

This presentation focuses on understanding the temperature dependent behaviors of polymer hybrid photovoltaic (PV) devices. The PV devices in this study consisted of a thin layer of PPV- based semiconducting polymer (M3EH-PPV) sandwiched between PEDOT and Al. Device architectures were modified by blending electron accepting CN- ether-PPV or PCBM in the photoactive layer, and evaporating LiF prior to Al (bulk heterojunction cells). Comparison will also be made between bulk and interfacial heterojunction structures, which consisted an additional layer of n-type semiconductor. Current-voltage characteristics were measured in a temperature- controlled cryostat to study the temperature dependence of PV parameters. Short circuit current (I$_{sc})$ and open circuit voltage (V$_{oc})$ were measured between 150 K and 400 K. The results showed Isc were more strongly affected by mobility in bulk heterojunction cells than in interfacial heterojunction structures. V$_{oc}$(T) were predominantly determined by device architecture than the mobility. Difference in underlying mechanism for polymer hybrid solar cells will be discussed.   

Photoconductivity of Hybrid Organic/Inorganic Quantum Dot Composite

We report the study of photoconductivity of hybrid organic/inorganic quantum dot composite in sandwich geometry. The uniform films of hybrid composite have been fabricated using conjugated polymers: either regio-regular poly (3-hexyl thiophene) or MEH-PPV and infrared PbSe quantum dots (QD) from Evident Technology Inc. We have observed the significant photoluminescence quenching (more than 30 times when excited by 400nm light) in MEH-PPV/QD composite with increasing concentration of quantum dots, photoluminescence spectrum profile shows obvious change with variation of excitation energy, with enhanced UV part luminescence as excitation moves to the blue side. Comparing with pure MEH-PPV, the main photoluminescence peak shows red-shift (up to 15nm). The enhanced photoresponse of sandwich device in comparison with pure polymer has been observed in broad spectral range from UV to NIR. The results demonstrate the efficient photoinduced charge transfer between polymer and QD and lead to possibilities of application of the polymer/IR QD in hybrid solar cells and photodetectors.   

Solid State Electrochromic Devices Based on PPV Polymers

We present a solid state electrochromic device structure employing a PPV-based light-emitting polymer more commonly used in devices such as LED's and photovoltaics. We explore device performance as a function of salt type, salt concentration and polymer layer thickness. These devices display high reversibility, dramatic optical contrasts, and low operating voltages comparable to state of the art conducting polymer electrochromic devices. We found that salts employing organic anions display slightly improved optical contrasts. Also, thicker devices, higher voltages and higher salt concentrations produce higher optical contrasts at the cost of slowed switching speeds. Apart from novel electrochromic applications these devices also provide insight into the fundamental process of doping in PPV-based polymer solid-state devices, crucial knowledge for the development of applications of polymer light emitting electrochemical cells (LECs), actuators and sensors. We explore the dependence of PL efficiency on doping level and discuss possible implications for the doping of PPV polymers in a solid state device configuration.   

In-situ spectroscopic investigation of infrared transmissive/absorptive electrochromic devices.

Novel transmissive/absorptive electrochromic (EC) devices have been assembled using conjugated polymers on infrared-transparent electrodes made of single-wall carbon nanotubes (SWNTs). We will present results on the design, fabrication and characterization of sandwich type EC devices using dioxythiophene-based conjugated polymers (PXDOT). The polymers were prepared on the SWNT films using a potentiostatic electropolymerization method. The transmittance of the samples was measured over the infrared through visible energy range. To extract the optical constants of the polymer, we modeled all layers of this multilayer thin film structure using a Drude-Lorentz model. From the parameters obtained, we compute optical constants which yield information about the electronic structure of the neutral and doped states of the polymer. Evidence for polaron states at low doping and bipolaron states at maximum doping will be discussed.   

Solid-state electrochromic device for 8-12 $\mu $m based on Poly(3,4-ethylenedioxythiophene)

Poly(3,4-ethylenedioxythiophene) (or PEDOT) was used as the electrochromic element for solid-state devices operating in the 8-12 $\mu $m range. The reflection-mode devices used anti-reflection coated germanium windows as the substrate in order to minimize surface reflection and increase the contrast ratio of the device. Upon doping the polymer using a gel electrolyte, there was a substantial change in the refractive index of PEDOT which induced a large index mismatch with the substrate and produced high reflection from the substrate/PEDOT interface. The reflection modulation between the doped and undoped states was approximated using Fresnel's equation and estimates of the refractive index mismatch. The calculated values were in reasonable agreement with the experimental results. When PEDOT was fully doped, the device exhibited its maximum reflection of 50-60{\%} in the 8- 12 $\mu $m regime, while in the undoped state, the device had 10-20{\%} reflection. The maximum contrast ratio observed for the device, $\sim $ 5.5, occurred at 8.25 $\mu $m. The use of rapid scans of the FT-IR enabled us to monitor, in real time, the infrared switching dynamics. Switching times on the order of 1 to 2 seconds were observed.   

Session L32: Focus Session: Superconductivity: Superconductivity (Mostly Electron-Phonon)

Using first-principles calculations for understanding MgB$_{2}$

Electronic structure methods have been the key to understand the peculiar properties of the multi-band superconductor MgB$_2$. After briefly reviewing the current state of our understanding of the material we will discuss some of the more recent observations on Al or C doped MgB$_2$.\\ We will demonstrate, based on first-principles band structure calculations and Eliashberg theory, that the experimentally observed decrease of the critical temperature $T_c$ of Al and C doped MgB$_2$ samples can be understood mainly in terms of a band filling effect due to the electron doping by Al and C. A simple scaling of the electron-phonon coupling constant $\lambda$ by the variation of the density of states (DOS) as function of electron doping is sufficient to capture qualitatively the observed behavior. Using the virtual crystal approximation to account for the electron doping we calculate the change of the DOS due to doping. The expected hardening of the $E_{2g}$ phonon frequency at $\Gamma$ is also well reproduced by our calculations using this approximation. These two contributions together, the change of DOS and phonon frequency, reproduce the observed dependence of $T_c$ on doping concentration astonishingly well not using any free parameter to fit the data.\\ Further, we also explain the long standing open question of the experimental observation of a nearly constant $\pi$ gap as function of doping by a compensation of the effect of band filling and interband scattering. Both effects together generate a nearly constant $\pi$ gap and shift the merging point of both gaps to higher doping concentrations, resolving the discrepancy between experiment and theoretical predictions based on interband scattering only.\\ The author acknowledges many fruitful collaborations and discussions on this exciting topic with O.K. Andersen, I.I. Mazin, A.A. Golubov, O.V. Dolgov, O. Jepsen and R.K. Kremer over the past years.   

Zero-point band gap renormalization and superconductivity in diamond

During the past decade the zero-point renormalizations of band gaps have been determined both experimentally and theoretically for many semiconductors. Tetrahedral materials containing elements of the second row of the periodic table C,N,O) have been shown to have fundamental gap renormalizations considerably larger (up to one order of magnitude) than those containing only other elements. Diamond, for instance, has a renormalization of $\sim $ 500meV as compared with 60 meV for silicon and germanium. This effect has been attributed to the lack of d-electrons in the core of the carbon atoms [1]. Superconductivity, with $T_{c}$ up to $\sim $9K, has been recently discovered in heavily boron doped diamond [2,3]. It will be conjectured that this phenomenon is related to the large coupling between the optical phonons and the holes at the top of the valence band, which is also related to the large band gap renormalization$. $An estimate of $T_{c}^{ }$based on available values of the hole-phonon deformation potential $d_{o }$and of the hole effective masses will be shown to explain the experimental values found for diamond. The corresponding $T_{c }$for Si and Ge should be either very small or nonexistent. \begin{enumerate} \item M.Cardona Solid State Commun. \textbf{133}, 3 (2004) \item E.A. Ekimov et al., Nature \textbf{428}, 542 (2004) \item Y.Takano et al., Appl. Phys. Lett. \textbf{85}, 2851 (2004) \end{enumerate}   

Origin of Superconductivity in Boron-doped Diamond

Superconductivity of heavily boron-doped diamond having B concentration of 5$\times 10^{21}$ cm$^{-3}$, reported at T$_c$=4 K by Ekimov {\it et} {\it al}.(April, 2004), is investigated exploiting its electronic and vibrational analogies to MgB$_2$. The deformation potential of the hole states arising from the C-C bond stretch mode is 60\% larger than the corresponding quantity in MgB$_2$ that drives its high T$_c$, leading to very large electron-phonon matrix elements. The calculated coupling strength $\lambda \approx$ 0.5 leads to T$_c$ in the 5-10 K range and makes phonon coupling the likely mechanism. Higher doping should increase T$_c$ somewhat, but effects of three dimensionality primarily on the density of states keep doped diamond from having a T$_c$ closer to that of MgB$_2$.   

Electron-phonon coupling and superconductivity in MgB$_2$ under hydrostatic pressure.

We have studied the dynamics and coupling of the $E_{2g}$ phonon mode with the $\sigma$-band in MgB$_2$ under pressure using the Frozen Phonon Approximation. The results were obtained by means of first-principles total-energy calculations using the full potential Linearized Augmented Plane Wave (LAPW) method and the Generalized Gradient Approximation (GGA) for the exchange-correlation potential. We present results for the evolution of the anharmonicity and phonon frequency of the $E_ {2g}$ mode, the electron-phonon coupling constant, and $T_c$ as a function of hydrostatic pressure in the range 0-40 GPa. We find that the phonon frequency increases monotonically with pressure, but the the anharmonicity, the electron-phonon coupling and $T_c$ decreases with pressure. We have obtained a very good agreement between the calculated $T_c(P)$ and the experimental data available in the literature, in particular with the experimental data corresponding to monocystalline samples. This work was supported by Consejo Nacional de Ciencia y Tecnolog{\'\i}a (CONACYT, M{\'e}xico) under Grant No. 43830-F.   

Searching for higher superconducting transition temperature in strained MgB2 using first principles calculations

Since the discovery of the amazing high superconducting transition temperature (T$_{c})$ in non-oxide MgB$_{2}$, great efforts have been made on the search for higher T$_{c}$ in MgB$_{2}$ and related materials by either chemical substitutions or by applying pressures to modify the lattice of MgB$_{2}$. Little success has been achieved so far due to the fact that the T$_{c}$ is always suppressed using either method. To tailor the T$_{c}$ in MgB$_{2}$, the full atomic-level understanding of underlying mechanism of chemical and lattice effects is required. According to the McMillan-Allen-Dynes analysis, T$_{c}$ in MgB$_{2}$ is controlled by the collective contributions from phonon frequency, electron-phonon coupling and Coulomb repulsion. Consideration of one single parameter alone cannot guarantee the improvement of T$_{c}$ because of the strong coupling and competing of other parameters. Here we present a detailed first-principles density functional analysis of the effects of lattice strains to superconducting properties of MgB$_{2.}$ On the basis of our results for structural, electronic, vibrational and superconducting properties of strained MgB$_{2}$, we show how higher superconducting transition temperature might be achieved in strained MgB$_{2}$ superconductor.   

Electronic structure of scandium-doped MgB$_2$

Recently has been reported the synthesis of a new superconducting alloy based on MgB$_2$, where Mg is partially substituted with Sc. In order to analyze the effect of Sc doping on the structural and superconducting properties of Mg$_ {1-x}$Sc$_x$B$_2$, we have performed a detailed study of the electronic structure for this new diboride. The calculations have been done using the first-principles LAPW method, within the supercell approach for modeling the doping. In this work we report results for the electronic band structure, Fermi surface, and density of states. The effect of the Sc-$d$ orbitals on the structural and electronic properties of Mg$_{1-x}$Sc$_x$B$_2$ is analyzed. Increasing the Sc concentration ($x$) the $\sigma$-band is gradually filled, because Sc have one valence electron more than Mg. Interestingly, the analysis of the band structure shows that even for ScB$_2$ the top of the $\sigma$-band remain above the Fermi level, nevertheless the $\sigma$-band presents high dispersion and has an important contribution of $d$ states. In this way, in addition to the band filling effect, Sc doping gradually reduces the two-dimensional character of the $\sigma$- band in Mg$_{1-x}$Sc$_x$B$_2$ as a result of increasing the $sp$(B)-$d$(Sc) hybridization. This research was partially supported by Consejo Nacional de Ciencia y Tecnolog{\'\i}a (CONACYT, M{\'e}xico) under Grant. No. 43830-F   

Nonlocal screening, electron-phonon coupling, and phonon renormalization in metals

A new method for calculating the phonon self-energy in metals arising from the coupling between phonons and electrons near the Fermi surface is developed. The essence of this scheme is the separation of the inter-and intra-band parts of the electron polarizability. The intra-band contribution provides extra screening and is closely related to the electron-phonon coupling and phonon softening in metals. Applications of this scheme to phonons in MgB$_2$ give excellent results when compared with experiments and previous theoretical work. In addition, both electron and hole dopings are found to reduce the renormalization effect of the $E_{2g}$ phonon mode. This indicates weakened electron-phonon couplings in the doped systems. This is consistent with the experimental observations that the superconducting transition temperature of MgB$_2$ drops upon substituting Mg with either Al or Li.   

Phonon renormalization and anharmonicity in Al-doped MgB$_2$

We have studied the evolution of the $E_{2g}$ phonon mode dynamics in Mg$_{1-x}$Al$_x$B$_2$ as a function of doping using the Frozen Phonon Approximation (FPA). The doping was modeled in the ab-initio Virtual Crystal Approximation (VCA). The results were obtained by means of first-principles total-energy calculations using the full potential Linearized Augmented Plane Wave (LAPW) method and the Generalized Gradient Approximation (GGA) for the exchange-correlation potential. We present results for the evolution of the phonon frequency and anharmonicity of the $E_{2g}$ mode as a function of Al concentration ($x$). From a comparison of the experimental data with the calculated $E_{2g}$ phonon frequency we show that the VCA-FPA reproduces the observed phonon renormalization in the whole range of Al concentrations. More interestingly, we find that the anharmonicity gradually decreases with Al doping and vanishes for $x$(Al)$>$0.5, that behaviour correlates with the evolution of the measured Raman linewidth in Al-doped MgB$_2$. The significance of these results are discussed in the light of the experimentally observed loss of superconductivity in Mg$_{1- x}$Al$_x$B$_2$.This work was supported by Consejo Nacional de Ciencia y Tecnolog{\'\i}a (CONACYT, M\'exico) under Grant. No. 43830-F.   

Electrons and phonons in YbC$_6$

Electronic structure and selected zone center phonons, as well as the electron-phonon coupling are calculated for a novel intercalated graphite supercoductor, YbC$_6$, using LDA+U method (fully localized version). The only stable solution either in LDA or in LDA+U is with zero spin and orbital polarization and the 4f band fully occupied. We show that Yb d states are present at the Fermi level and assess a hypothesis that superconductivity may arise from Yb phonons.   

Electronic structure properties and superconductivity of the $\beta$-pyrochlore Os oxides, $A$Os$_2$O$_6$ ($A$=alkali metal)

The recently discovered\footnote{T. Muramatsu {\it et al.} J. Phys. Soc. Jpn. {\bf 73}, 10 (2004).} family of superconducting $\beta$-pyrochlores {\it A}Os$_2$O$_6$ ($A$=alkali metal) represents a particularly interesting example of the interplay between superconductivity and orbital and crystal structure degrees of freedom. Indeed, the pyrochlore lattice formed by the Os-O staggered chains appears to lead to very high Sommerfeld coefficients, increasing of $T_c$ under positive pressure, and other intriguing properties. We present results of a first-principles study of the electronic structure and superconducting properties of these materials ($A$=Na, K, Rb, and Cs) using the highly precise full-potential linearized augmented plane wave (FLAPW) method.\footnote{Wimmer, Krakauer, Weinert, Freeman, Phys. Rev. B {\bf 24}, 864 (1981).} We show that the observed increase of $T_c$ with decreasing mass of $A$ as well as under positive hydrostatic pressure can both be well understood within a conventional phonon-mediated pairing picture. Furthermore, the density of states at $E_F$ depends critically on spin-orbit coupling, due to a van Hove singularity near $E_F$, with a direct effect on $T_c$; the Fermi surface shows strong nesting, which is reflected in the dynamic susceptibility and thus indicates that spin fluctuations may play an important role in these materials.   

Anomalous Dynamics of K Ion in $\beta$-Pyrochlore

Among the recently discovered $\beta$-pyrochlore superconductors AOs$_2$O$_6$ (A=K, Rb, and Cs) KOs$_2$O$_6$ exhibits several anomalous features (electrical resistivity, 1/T$_1$ NMR decay). We have studied electronic structure of AOs$_2$O$_6$ using the density functional theory based methods. We have found a moderate Stoner enhancement of the Pauli susceptibility of 2.15 and a sizable thermal mass enhancement of 2.9 in KOs$_2$O$_6$. While the electronic structure of the three systems is very similar the structural stability of the alkali atom site is rather different. In particular the K ion potential well has a flat bottom, beyond small anharmonic corrections, allowing for large excursions from the symmetric position. We suggest that this feature is behind the striking differences in behavior of otherwise similar AOs$_2$O$_6$ compounds.   

Interface Electronic Structure and Possible Superconductivity in CuCl/Si(111)

To investigate a possible interfacial superconductivity \footnote{B.L. Mattes, Physica C {\bf 162},554 (1989); B.L. Mattes and C. L. Foiles, Physica 135B, 139 (1985)} in CuCl/Si(111), we carried out electronic structure calculations using the highly precise FLAPW \footnote{Wimmer, Krakauer, Weinert, and Freeman, Phys.Rev.B, {\bf 24}, 864 (1981)} method. As a result of charge transfer between CuCl and Si layers, two-dimensional (2D) metallic states are found to be formed at the interface. From the geometry relaxation, it is shown that the ionic bonding of CuCl is weakened and there is mixed metallic and covalent bonding at the interface. The 2D conduction bands at the interface, sandwiched by the highly polarizable dielectric layers, resemble the 2D Cu-O dp$\sigma$ bands of the Cu-oxide superconductors, which are considered to be responsible for high $T_c$ superconductivity. To obtain $T_c$ of the CuCl/Si interface based on the conventional electron-phonon(e-p) interactions, we calculated the e-p coupling constant, $\lambda$, within the rigid ion approximation \footnote{G. D. Gaspari and B. L. Gyorffy, Phys. Rev. Lett. {\bf 28} 801 (1972)}. The results indicate that a strong e-p coupling is present at the interface layers but is not enough to explain the previously reported high transition temperature$^{2}$.   

Non-empirical Calculations of the upper-critical field $H_{c2}$ for Nb, NbSe$_{2}$, and MgB$_{2}$

Detailed Fermi-surface structures are essential to describe the upper critical field $H_{c2}$ in type-II superconductors, as first noticed by Hohenberg and Werthamer [Phys.\ Rev.\ {\bf 153},\ 493 (1967)] and shown explicitly by Butler for high- purity cubic Niobium [Phys.\ Rev.\ Lett.\ {\bf 44},\ 1516 (1980)]. However, most of $H_{c2}$ calculations performed so far have used simplified model Fermi surfaces and/or phenomenological fitting parameters. Due to this lack of {\em ab}-{\em initio}-type calculations, our understanding on $H_{c2}$ remains at a rather unsatisfactory level. With these observations, we have derived an $H_{c2}$ equation for classic type-II superconductors which is applicable to systems with anisotropic Fermi surfaces and/or energy gaps under arbitrary field directions. Based on the formalism, we have calculated $H_{c2}$ curves for clean type-II superconductors Nb, NbSe$_{2}$, and MgB$_{2}$ using Fermi surfaces from {\em ab initio} electronic structure calculations. The results for Nb and NbSe$_2$ excellently reproduce both temperature and directional dependences of measured $H_{c2}$ curves, including marked upward curvature of NbSe$_{2}$ near $T_{c}$. As for MgB$_2$, a good fit is obtained for a $\pi$/$\sigma$ gap ratio of $\sim\! 0.3$. Our results indicate essential importance of Fermi surface anisotropy for describing $H_{c2}$.   

Session L33: Quantum Fluids and Solids II

Simulation of Helium-4 in Aerogel

The superfluid 4He transition in highly porous silica glasses, like aerogel and xerogel, have been studied experimentally [1]. The experiments obtain critical exponents that deviate from the bulk exponents for low porosity of the silica glass, and obtain evidence for violations of hyperscaling. We study this transition as a function of porosity within fractal diluted 3DXY models. We use the Wolff collective update method and go to larger system sizes than in previous simulation studies of the problem. We obtain results for the critical properties of the transition and compare with experiments. \\ 1. J. Yoon et al., Phys. Rev. Lett. 80, 1461 (1998)   

Finite-size effects on the thermal conductivity of $^4$He confined in rectangular channels.

We report results for the thermal conductivity $\lambda(t)$ of $^4$He confined in glass capillary arrays with rectangular channels of size $1\times10\times1000~\mu{\rm m}^3$ near the bulk super-fluid transition temperature $T_\lambda$ as a function of the reduced temperature $t \equiv T/T_{\lambda} - 1$. Even close to $T_\lambda$ we found that $\lambda(t)$ differs very little from similar measurements for cylindrical channels of radius $1~\mu$m, \footnote{D. Murphy, E. Genio, G. Ahlers, F.C. Liu, and Y. Liu, Phys. Rev. Lett. {\bf 90}, 025301 (2003).} indicating similar scaling functions for the two geometries. This differs from the finite-size effects on thermodynamic properties, which have very different scaling functions near the transition for parallel-plate and cylindrical geometries.\footnote{M. O. Kimball, K. P. Mooney, and F. M. Gasparini, Phys. Rev. Lett. {\bf 92}, 115301 (2004).}   

Scaling of thermal resistivity of $^4$He in restricted geometries

The thermal resistivity and its scaling function in quasi-2D $^4 $He systems are studied by Monte Carlo and spin-dynamics simulation of the classical 3D XY model on $L \times L \times H$ lattices with $L \gg H$. Open boundary conditions are applied along the $H$ direction and periodic boundary conditions along the $L$ directions. A hybrid Monte Carlo algorithm is adopted to efficiently deal with the critical slowing down \footnote{M. Krech and D. P. Landau, Phys. Rev. B {\bf 60}, 3375 (1999)}. Fourth-order Suzuki-Trotter decomposition of exponential operators is used to solve numerically the coupled equation of motion for each spin. The thermal conductivity is calculated by a dynamic current- current correlation function. Our results are consistent with a universal scaling function $F (X)=(L/ \xi_0)^{\pi/ \nu} ( \rho / \rho_0)$, $X=(L/ \xi_0)^{1/ \nu}t$ using known values of the critical exponents $\pi$ and $\nu$ $(\rho = \rho_0 t^{- \pi} $ is the thermal resistivity , and $\xi = \xi_0 t^{- \nu}$ is the correlation length). The thermal resistivity scaling function agrees well with the available experimental results \footnote{Experimental data provided by G. Ahlers, S. Jerebers, Y. Liu and F. C. Liu} for slabs using the temperature scale and thermal resistivity scale as free fitting parameters. \\ \\ $^*$Research supported by NASA   

Continuous-Wave Measurement of the Upward-Going Temperature Wave in the Helium-4 Self-Organized-Critical State Near the Lambda Transition

We describe the first continuous-wave (CW) measurements of the upward-going temperature wave in the self-organized-critical (SOC) state which forms in $^4$He under conditions of downward heat flow near $T_{\lambda}$ under gravity. The CW technique permits measurements with extremely low ($<1$ nK) excitation amplitudes, allows continuous measurement of the wave velocity as the SOC state grows, and has yielded the first quantitative measurements of the attenuation. The measured attenuation disagrees with predictions, and this new technique may help address the question of whether the SOC state is occurring in the helium-I or helium-II state. Some intriguing new qualitative features are also described.   

Temperature Gradient of Hydrodynamic Origin in Vertically Counterflowing Helium-II Near the Lambda Transition Under Gravity

We describe a calculation of the temperature gradient occurring in helium-II near $T_{\lambda}$ when it transports a uniform energy flux density vertically upward or downward under gravity. The calculation is performed within the dissipationless two- fluid model and assumes 1D and steady- state. An exact solution is obtained which indicates a temperature gradient of hydrodynamic origin in which both the gravitational hydrostatic pressure head and the suppression of the superfluid component density $\rho_s$ by the counterflow velocity $w^2=(v_n-v_s)^2$ play essential roles. The temperature gradient is very small for temperatures well below $T_{\lambda}$ and rises toward a limiting value of $dT_ {\lambda}/dz$ as temperature is increased. It is quadratic in the component fluid flow velocities and thus its sign, remarkably, is independent of the sign of the heat flow. The predicted gradient occurs in a narrow temperature range and it is not clear if it will be directly observable. However, its existence may provide some insight into the recently-discovered up-down heat flow asymmetry in the ``thermal resistance'' of helium-II near $T_{\lambda}$.   

Precise equation of state measurements of $^4$He near the $\lambda$-point, using dual-mode Superconducting Cavity Stabilized Oscillators

We report on progress towards precise equation of state measurements of $^4$He saturated vapor near the $\lambda$-point using a Superconducting Cavity Stabilized Oscillator (SCSO) system. By operating the SCSO in a dual-mode phase-locked loop configuration we will be able to measure the dielectric constant of $^4$He to parts in $10^{15}$ precision and comparable accuracy. The dielectric constant in turn implies a value of the density to parts in $10^{10}$. Other measured parameters include the temperature to sub-nK precision using paramagnetic salt high-resolution thermometry (HRT) and pressure to parts in $10^9$ using a Straty-Adams type diaphragm gauge. These substantially improved resolutions relative to existing data are expected to provide new insights into the interactions of helium atoms near Bose-Einstein condensation. Numerous error reduction techniques will be discussed, along with other applications of SCSO to precision metrology.   

1-D Numerical Simulation of Heat Transfer near the Liquid-Vapor Critical Point

Near a liquid-vapor critical point, a constant volume fluid undergoes both a fast adiabatic and a slow diffusive heat transfer when heated at the boundary. Earlier numerical studies of this equation revealed the main features of the solution. This numerical technique had many limitations, such as large errors for a highly nonlinear system very close to the critical point, or for a fast varying boundary condition. In this talk, we will present a newly developed numerical solution of the heat transfer equation that utilizes the full implicit method simultaneously for both the differentiation and spatial integration of temperature. The new numerical solution is applicable to the cases of both the single phase above and liquid-vapor coexisting phases below the critical temperature. This new solution is valid for any boundary conditions. In this talk, we demonstrate several case studies for ground-based and microgravity conditions. The special case of an amplified temperature response in the vapor phase when the liquid boundary is subject to an AC temperature oscillation will also be presented.   

Crossover equation of state for the liquid-vapor critical point

The principles and implementation of a new crossover equation of state for liquid vapor systems will be presented. The equation of state incorporates crossover that is correct in both the mean field and asymptotic critical regimes. It is also consistent with both leading order and correction-to-scaling amplitude ratios. We discuss the comparison between this equation of state and the results of recent measurements of thermodynamic properties of $^3$He in the vicinity of its liquid-vapor critical point.   

Kelvon--Phonon Interaction

Kelvin waves (kelvons)---the distortion waves on quantized vortex lines---play the key part in the zero- temperature relaxation of superfluid turbulence. The relaxation scenario implies a Kelvin wave cascade, cut off by sound emission. We derive the kelvon--phonon Hamiltonian, thereby reducing the problem of interaction of Kelvin waves with sound to elementary excitation scattering. On the basis of this formalism, we revisit the problem of sound emission by superfluid turbulence.   

Smoothness and Vortex-Wall Interactions in Superfluid Helium

We study two aspects of the interaction between a surface and a single vortex line terminating on the surface. One is pinning, when a moving vortex becomes caught at some point on the wall. The second is the energy dissipation as the vortex moves, which appears to be dominated by a vortex-surface interaction. When we reduce the surface roughness through mechanical polishing, we find that the energy loss decreases, as expected if the dissipation comes from a ``friction" force which is weaker for smoother walls. The change is small, about a factor of 3 for several orders of magnitude difference in surface roughness. This is consistent with the very small vortex core size in ${}^4$He since even our highly polished surfaces are ``rough" on the scale of the vortex core. A more surprising finding is that the vortex is {\em more} likely to pin on the smooher walls, suggesting that this vortex-surface interaction is stronger for smoother walls. We will discuss how a mesh of small vortex lengths pinned along the container surface may contribute to these observations.   

Josephson oscillations in superfluid $^{4}$He

We will describe observations of superfluid oscillations between two samples of $^{4}$He joined by an array of submicron-sized apertures. The fluid oscillates at the Josephson frequency, $f_{j} = \Delta\mu/h$, where $h$ is Plank’s constant and $\Delta\mu$ is the full chemical potential difference, containing both temperature and pressure differences. The oscillations are observed at temperatures sufficiently below the superfluid transition temperature $T_{\lambda}$ that the current phase relation is linear, ie. not sine-like. Evidently the oscillations are the signature of coherent $2\pi$ phase slippage in the array. Work supported in part by grants from the NSF and NASA.   

Frequency Dependence of Hydrodynamic Inductance in 4He Helmholtz Resonators

The oscillatory motion of helium near the superfluid transition temperature changes from superfluid-only to two-component solid body when viscous length becomes smaller then the flow channel size. In Helmholtz oscillators commonly used to study Josephson effect and related phenomena in helium, the size of the sub-micron apertures is typically smaller than the viscous length, while the size of the macroscopic flow path can be smaller or larger than the viscous length, depending on the frequency used. This opens the possibility that the hydrodynamic inductance ratio of the flow path and the aperture array can be varied. We discuss the implications of this behavior for superfluid SQUID mechanical and thermodynamic stability.   

Spin Relaxation in Superfluid $^3$He

The spin relaxation time in superfluid $^3$He A$_1$ phase is studied using magnetic fountain pressure techniques. Measurements have been made previously as functions of temperature and pressure. Preliminary measurements will be reported on the dependence of the relaxation time (0.5 $\sim$ 1.5 s) on applied magnetic field (H$_a$) and $^4$He coverage. At low field range of 0.5 $<$ H$_a$ $<$ 1 tesla, the spin relaxation time increases linearly with H$_a$ as expected. Unexpectedly, in the 2 $<$ H$_a$ $<$ 8 tesla range, the relaxation shows little variation. When the interior wall surfaces (including those of heat exchanger) are covered with five layers of $^4$He, surprisingly, the measured relaxation time decreases.   

Session L34: Dynamics in Condensed Phase I

Refinement and Application of Single Molecule Resonance Energy Transfer

Single molecule fluorescence measurement of resonance energy transfer (RET) is revealing a new level of detailed information about biomolecular systems that exhibit conformational change. This technique has been particularly successful when applied to processes that involve large, binary intramolecular movements, for example rearrangement of enzymes and the folding of two-state proteins. Despite many advances in the application of single molecule RET, most researchers are still reluctant to convert measured energy transfer efficiencies to distances. A number of experiments inspired by Stryer and Haugland's 1967 ``spectroscopic ruler'' paper have been carried out at the single molecule level using DNA and polyproline, but the results are not nearly so clear as they were in that original work. I will discuss progress that has been made toward understanding the results of these experiments and the deviations they show from F\"orster's theory.   

In vitro translation study using single molecule fluorescence resonance energy transfer

Single molecule fluorescence resonance energy transfer (FRET) reveals the mechanism of tRNA selection by ribosome. In biological protein synthesis, or translation, a ribosome selects correct tRNAs according to the genetic code written on an mRNA with an unusually low error frequency (1/10,000). Single molecule study showed that a correct tRNA has slightly stronger interaction with the mRNA than an incorrect tRNA, resulting in tighter binding to the ribosome. Results further suggest that a small difference in the initial tRNA-mRNA interaction induces a significant difference in the fluctuation of the system to proceed to the next step. Only correct tRNA-mRNA interaction often leads the system to the correct pathway to the next step with a high energy barrier, i.e. GTP hydrolysis. Such induction process overcoming a high energy barrier is found to be critical in explaining the abnormally low translation error.   

First-principles study of the excited-state properties of Tryptophan in water

Tryptophan (Trp) is an optically-active amino acid that is highly sensitive to its local environment and responsible for much of the UV fluorescence in proteins. The spectral properties of Trp are primarily associated with the two low-lying excited states, $^1\rm L_a$ and $^1\rm L_b$, of its indole side chain. These states possess strong dipole moments and give rise to a complex excited-state relaxation dynamics in polar solvents like water. Here we apply first-principles calculations to examine the impact of the surrounding solution on the indole excited states responsible for UV fluorescence. Results of excited-state calculations of molecular indole using the GW-Bethe Salpeter equation (GW-BSE) formalism will be presented and compared with existing quantum-chemical calculations. Results of ground- and excited-state molecular dynamics simulations will also be presented.   

Single-Molecule Studies of Enzymatic Structure-Function Dynamics

Many biological macromolecules rely on some form of conformational flexibility to perform their designated tasks. This flexibility can be a factor in such important effects as cooperativity and allostery, or in providing the basis for entropic control. The structural origins and functional manifestations of enzymatic flexibility, however, are still poorly understood. F\"{o}rster Resonance Energy Transfer (FRET) mediated measurements of intra-molecular distance in single molecules can help to bridge this gap. In this way, the 3D structural motions of individual AKe molecules were projected on the 1D coordinate, $q$, defined by the placement of FRET probes. Here, we report the application of single-molecule time-resolved FRET to measuring the conformational fluctuation of adenylate kinase from \textit{E. coli} (AKe) under reactive conditions. The velocity-position time traces on the $\left( {\dot {q},q} \right)$ configuration space were acquired from single AKe molecules. Information about reaction dynamics was extracted photon-by-photon using the recently developed maximum-information and change-point methods. The structure-function dynamics will be discussed from the perspective of such configuration-space trajectories.   

Onsets of Anharmonicity in Protein Dynamics

Dynamics of protein lysozyme at various hydration levels is studied using neutron scattering spectroscopy and molecular dynamic simulations. Two onsets of anharmonicity are observed in the temperature variations of the mean-squared displacements of atoms $<$x$^{2}>$. One at $T\sim $100K appears in all samples regardless of hydration level. Based on analysis of experimental and simulations data, we ascribe the onset primarily to methyl group rotation. The second, the well-known dynamical transition at $T\sim $200--230K, is only observed at a hydration level $h$ greater than $\sim $0.2 and is ascribed to the activation of an additional relaxation process. We demonstrate that its variation with hydration correlates well with variation of catalytic activity suggesting that the relaxation process is directly related to the activation of modes required for protein function. Microscopic nature of this relaxation process is discussed.   

Power-Law Dynamics in DNA

Measurements of solvation dynamics in the interior of DNA have been extended to cover a six decade time range from 40 fs to 40 ns. The solvation dynamics are reported by the Stokes shift of a coumarin group that is covalently bound within an oligonucleotide in place of a native base pair. Results from three techniques: time-correlated single photon counting, fluorescence up-conversion and transient absorption; are combined to cover the entire time range. Over this range, the dynamics follow a simple power law with a small exponent of 0.15. If the coumarin group is placed near the end of the oligonucleotide, a process with a 10 ps time constant occurs, in addition to the power-law dynamics. The power-law dynamics change if the counterion changes, and they change in a manner that correlates with the hydrodynamic radius of the counterion. Results on single-stranded DNA and on a DNA:protein complex are also reported. These experiments provide a variety of unexplained results that provide a challenge to the theory of DNA dynamics on fast time scales.   

Polymer translocation through a nanopore studied by Langevin dynamics

Polymer translocation through a nanopore has gained considerable attention in recent years, due to its potential application in DNA-sequencing. The design of a corresponding device requires a full understanding of the translocation dynamics. The scaling of polymer translocation time $\tau$ with polymer chain length $N$ is an important measure of the underlying dynamics. A recent experiment\footnote{A. J. Storm \emph{et al.}, arXiv q-bio/0404041 (2004).} has uncovered a scaling behavior $\tau \propto N^{1.26}$ that differs from the linear law observed in other experiments. To explain this newly-observed scaling behavior, we have employed Langevin dynamics simulations. Using a bead--spring model for the polymer chain and a membrane composed of one layer of hard-sphere particles, we have studied a wide range of chain lengths $20 \leq N \leq640$, for different friction coefficients $\xi$. A crossover scaling behavior was found for $\tau$, which is controlled by both $N$ and $\xi$. We explain the measured scaling behavior from the chain conformations and instantaneous translocation velocities.   

Rotational motions in the dynamics of proteins and biological macromolecules

By changing its shape while conserving angular momentum, a polyatomic molecule can return to its initial shape with a different orientation (as a ``falling cat'' or a diver can do). Examples where this phenomenon has been observed include the dynamics of protein molecules and the dynamics of a rotary molecular motor. Computational biophysicists have observed the overall rotation of a protein molecule at zero total angular momentum due to the molecule's flexibility. A counter-rotary motion has been observed in the rotary ${\rm F}_o$ motor of ATP synthase. Using geometric mechanics, the net angle of overall rotation is described in terms of coordinates. The net angle of overall rotation is the sum of a dynamic phase and a geometric phase; the latter is also described in terms of a gauge potential. This is an extension of a result for smaller polyatomic systems, where the geometric phase contribution is also described explicitly in terms of moments of inertia and, alternatively, molecular rotational constants. Potential applications of this result include computational molecular dynamics studies.   

Session L35: Nanophotonic Materials, Nonlinear Optics and Spectroscopy II

Three-dimensional micro/nano fabrication with photopolymer for the production of functional microdevices

Three-dimensional (3D) two-photon microfabrication with 100nm resolution is based on pinpoint solidification of two-photon-absorbed polymerization, which is stimulated by focusing a femtosecond pulsed laser beam inside a photopolymer [1]. The 3D scanning of the laser beam permits the fabrication of any 3D microstructures. This technique has been widely applied to create functional micro/nano devices such as photonic crystals [2] and micromechanical components [3, 4]. Currently the development of 3D photonic crystals is one of the most promising applications. On the other hand, we intend to develop micromachines driven by optical radiation pressure to create novel microtools for biotechnology. We demonstrated that a microgear could be rotated around an attached shaft by means of a laser-scanning manipulation technique [3]. Nanotweezers with submicron probes were also developed [4]. The nanotweezers can be driven with femtonewton order force control. Such optically driven micromachines, including micro pumps and manipulators, will be useful for micro total analysis systems and biotechnology. [1] Opt. Lett. \textbf{22}, 132 (1997). [2] Nature \textbf{398}, 51 (1999). [3] J. of Microelectromech. Syst. \textbf{12}, 533 (2003). [4] Appl. Phys. Lett. \textbf{82}, 133 (2003).   

Laser Direct Writing for Wiring of Nanodevices

Making electrical contacts to nanorods or nanowires generally requires the use of electron-beam lithography, which can be a time-consuming and complex process. We demonstrate the use of multiphoton laser direct writing to make electrical contacts to metallic nanorods. The entire process takes a matter of hours and can be accomplished on the bench top. Four-probe conductivity measurements show that the contacts to the nanorods are of high quality. We will also discuss applications of these nanodevices.   

Two-Photon Absorbing Materials for 3D Microfabrication, Sensing and Imaging

The design of conjugated organic chromophores with large two-photon cross sections and how these chromophores can be used to develop highly efficient materials for two-photon 3D microfabrication, sensing and imaging applications will be described. Two-photon excitation of materials with focused laser beams allows for free-form patterning of materials in three dimensions with nanoscale ($<$ 200 nm) resolution. We have been developing efficient photoactive precursor materials for two-photon 3D patterning of polymers, in both positive and negative patterning processes. We have also developed a class of photoactive nanocomposites containing metal nanoparticles that allow for direct patterning of continuous metal features using lasers or electron beams. We are utilizing these two-photon materials and fabrication processes to prepare 3D microstructures that are of interest for photonic, microfluidic, and micromechanical applications, as well as others. An overview of our efforts to develop two-photon 3D microfabrication will be presented and the wide range of 3D microstructures that can be fabricated with this method will be highlighted. The coupling of two-photon chromophores to ligand receptors for sensing of metal ions, the assembly of two photon dyes on metal nanoparticles to make ultrabright nanobeacons, and the strong enhancement of two-photon excited fluorescence by coupling of chromophores to clusters of metal nanoparticles will also be discussed.   

Toward Three-Dimensional MEMS Fabricated by Multiphoton Absorption Polymerization

The use of multiphoton absorption polymerization (MAP) to produce three-dimensional structures with sub-micrometer resolution has garnered attention recently. While many of these polymeric systems are quite impressive, the functionality of a purely polymeric structure is quite limited. We have developed a method by which metal can be deposited selectively onto an acrylic polymer structure that has been fabricated \textit{via} MAP. Postpolymerization chemical modification of the polymer provides moieties that have the ability to reduce metal ions or bind metal ions and/or metal nanoparticles. Additional electroless enhancement of the metal produces a conductive structure. This method provides a means to produce three-dimensional micro-electro-mechanical systems by way of MAP.   

Generation and focusing of radially polarized beams

Light beams with spatially homogeneous state of polarization have been extensively studied in the past. However, if one can spatially arrange the polarization of light beam purposefully and carefully, new effects that can expand the functionality and enhance the capability of the optical system are expected. One such example that has gained increasing interest recently is laser beams with radial polarization symmetry. In this talk, methods of generating and manipulating radially polarized light in free space using radial analyzer and spiral phase element are described. Realization of optical fiber modes with radial and azimuthal polarization symmetry will be presented. Simple methods of manipulating these radially polarized beams have also been developed. Radially polarized beam has very unique focusing properties when it is focused by high numerical aperture objective lens. For example, a radially polarized beam can be focused into a much tighter focus than linearly or circularly polarized light due to an extremely strong longitudinal field component. Meanwhile, this strong longitudinal field component does not contribute to the Poynting vector along the optical axis. The focusing properties of radially polarized beams can be used for three dimensional shaping of the optical focal field. With proper combination of radial polarization and azimuthal polarization, optical focal field with flattop profile can be obtained. Combined with diffractive optical phase element, it is possible to obtain optical focus with maximally homogenized field profile in both longitudinal and transversal directions. Optical bubble with dark hollow center as well as chain-like optical focal field can be generated. Finally, the applications of radially polarized beams in optical tweezers, high resolution optical microscopy and materials characterization will be discussed.   

Optically Active Sum-Frequency Generation from Solution of Molecules with a Chiral Center

Optically active sum frequency generation (OA-SFG) is being developed as a novel probe for investigation of molecular chirality. We report here the first attempt of OA–SFG to study chirality of molecules with a chiral center but an intrinsically achiral chromophore in isotropic solution. We used amino acids in 4M NaOH solution as the model systems, and found that similar to circular dicroism (CD), OA-SFG near electronic resonance appears to originate from the extrachromophoric chiral perturbation on the carboxyl chromophore. The difference between CD and OA-SFG, however, is in the details of the perturbations pertinent to the two effects, giving rise to different relative strengths of OA-SFG and CD among different amino acids. A general theoretical formulation for OA-SFG from molecules with chiral centers will be presented.   

Optical Lattice Microscopy

New classes of two- and three-dimensional optical lattices are described that yield excitation maxima of controllable polarization and periodicity relative to the excitation wavelength, confined to near the diffraction limit in all directions. Methods for their generation are proposed, as are methods for the simultaneous, independent detection of luminescence from numerous maxima across multiple lattice planes when such lattices are applied to dynamic live cell imaging or massively parallel single molecule spectroscopy. Performance metrics are also introduced that favorably compare lattice microscopy to widefield, confocal, and 4pi microscopy in terms of speed, resolution, photobleaching, and molecular sensitivity. Finally, the possible adaptation of lattice microscopy to superresolution methods such as total internal reflection microscopy and stimulated emission depletion microscopy is discussed.   

Fundamental limitation of the spatial resolution of a perfect nanolens

We have established a fundamental limitation on the ultimate spatial resolution of the perfect lens (thin metal slab) in the near field. This limitation stems from the spatial dispersion of the dielectric response of the Fermi liquid of interacting electrons in the nanolens material. Such dispersion leads to the aberrations of this lens in the wave vector space in the plane of the slab. This principally limits the resolution of this lens making it imperfect on the scale below five nanometers. This effect is different and independent from the known source of the lens imperfection due to the absorption (optical losses) in the metal and temporal dispersion that is related to the absorption by Kramers-Kronig relations. Even if the absorption is reduced or compensated by the optical gain the spatial dispersion will remain and limit the resolution. We reveal the link between this limitations and dispersion of surface plasmons. We discuss possible applications in nanoimaging, nano- photolithography, and nanospectroscopy.   

Localized Plasmons of Nanometric Holes in Thin Gold Films

We have investigated the optical properties of sub-wavelength (60-200 nm) holes in 20 nm thin Au films using extinction and elastic scattering spectroscopy [1]. The samples are prepared by colloidal lithography on glass and consist of either spatially isolated holes or disordered hole arrays with varying density. We show that single holes exhibit a well-defined optical resonance in the visible to near-infrared spectral region, which we assign to a localized surface plasmon (LSP) excitation. The hole LSP red-shifts with increasing hole size or with increasing refractive index of the surrounding medium, in analogy with LSP's in metal nanoparticles, but exhibit a pronounced blue-shift with decreasing hole density, possibly due an enhanced hole-hole coupling mediated via surface plasmon polaritons. Similar to particle plasmons or flat metal surfaces, the hole LSP can be used for biochemical sensing based on refractive index contrast. New results on single hole sensing [2] and biofunctionalization of holes using lipid vesicles [3] will be discussed. [1] J. Prikulis et al, Nano Letters 4, 1003-1007 (2004); [2] T. Rindzevicius et al., submitted ms.; [3] A. Dahlin et al., submitted ms.   

Session L36: Focus Session: Granular Gases and Liquids I

Velocity statistics of a uniformly heated granular fluid

We report results from an experimental investigation of a uniformly heated granular fluid. We vertically vibrate an ensemble of spheres (diameter $D$) confined by two horizontal glass plates. The top and bottom plates are separated by $1.6D$ which ensures a quasi-two dimensional configuration. We show that the use a rough, instead of flat, bottom plate has the considerable advantage of a more effective transfer of momentum from the vertical mode of the cell's vibration onto the motion of individual spheres in the horizontal plane. This allows a greater range of cell's filling fraction, $\phi$, to be explored. We study the single particle velocity distributions, $f(c)$, as a function of $\phi$ and vibration parameters; frequency and amplitude. In agreement with previous studies, we find a consistent overpopulation in the distribution's high energy tails, of the form $\log f \sim -c^{3/2}$. Moreover, we calculate the deviations from a Mawellian, $\Delta(c)=f(c)/f_{MB}(c)-1$, where $f_{MB}\sim\exp(-c^2)$. We find $\Delta(c)$ to be well described by a 4th-order polynomial which, however, is not the Sonine polynomial commonly used in the solution of the Enskog-Boltzmann equation for inelastic hard spheres driven by a stochastic thermostat.   

Testing Kinetic Theory in a Driven Granular Gas Experiment for a Mechanically Fluidized Bed

Robust Gaussian velocity statistics[*] and uncorrelated particle-particle velocities indicative of Molecular Chaos[**], are exhibited in a novel two-layer experiment in which a vertically shaken horizontal plate drives a layer of heavy coupled grains that, in turn, drive a layer of lighter monomer grains. While the experiment is clearly driven far from equilibrium, the dynamics are well-described by an analogy to equilibrium kinetic theory, providing a testbed for phenomena not easily observed in real molecular gases. Recent measurements have sought to test theoretical assertions concerning the relationship between pressure and temperature in granular gases as well as the conditions under which kinetic theory fails to describe inelastic hard sphere dynamics. The novel design also allows a variety of results from hard sphere molecular dynamics simulations to be tested in a real experiment including the relationships between temperature, pressure, and volume effects. [*] G. W. Baxter and J. S. Olafsen, Nature, 425, 680 (2003). [**] G. W. Baxter and J. S. Olafsen, submitted to Physical Review Letters.   

Equipartition of energy for a gas-fluidized grain.

The dynamics of a sphere rolling in a nearly-levitating upflow of air are described perfectly by the Langevin equation [1]. Surprisingly, statistical mechanics is applicable and can be exploited to infer the nature of the forces at play in this driven mechanical system. To probe the flexibility of statistical mechanics we perturb the original experiment in three ways: first, we break the circular symmetry of the confining potential by using a stadium-shaped trap and observe if the velocity distributions remain circularly symmetric; second, we fluidize multiple grains of different density to check if each has the same effective temperature; and third, we fluidize two grains of different size and check to see whether statistical mechanics remains applicable. It is found that the velocity distributions are unresponsive to asymmetry in the trapping potential and that the effective temperature is independent of grain mass-density, so that statistical mechanics remains applicable. When grains differ in size beyond a critical ratio, however, statistical mechanics breaks down. [1] Ojha et. al., Nature 427, 521 (2004).   

Experiments on long-time effective temperatures in granular fluids

Studies of effective temperatures to describe the state of fluidization of a granular medium have emphasized kinetic granular temperatures determined from the instantaneous motions of grains. In this talk, I will focus on experiments that study effective temperatures derived from long-time grain dynamics. One formulation to extract long-time effective temperatures is via the fluctuation-dissipation relation, which is valid for linear response to small perturbations to near-equilibrium states; I will report briefly on experiments that study the applicability of this relation to vibrated and flowing granular media. I will then discuss in greater detail an effective temperature that emerges from consideration of the statistics of the power input to maintain the granular fluid in its nonequilibrium steady state. The analysis is done in the context of recent Fluctuation theorems\footnote{ G. Gallavotti, E.G.D. Cohen, Phys. Rev. Lett. \textbf{74}, 269 (1995).} that are proven for dynamical steady-states arbitrarily far from equilibrium. We have performed experiments\footnote{ K. Feitosa, N. Menon, Phys. Rev. Lett. \textbf{92}, 164301 (2004)} and simulations which show that power fluxes in our system satisfy the Fluctuation relation and that the pertinent effective temperature is an intensive variable. In the dilute, nearly-elastic regime, this effective temperature and the kinetic temperature follow each other as experimental parameters are varied. Beyond this regime, these temperature scales depart from each other. We speculate that the effective temperature remains a useful variable even in the regime where kinetic approaches to granular fluids no longer apply.   

Shear banding of slowly sheared granular packings in an annular geometry

We investigate experimentally a quasi-static flow of glass beads packed and sheared in an annular channel under a constant normal load. The experiments utilize techniques of refractive-index-matched fluorescent imaging to determine the motion of individual particles inside the sheared packing.[1] The measured steady-state velocity fields have a dynamical range of five decades; parameters such as packing size and particle size are varied systematically. We demonstrate that crystalline ordering has a significant impact on the spatial gradient of grain velocity. Changing particle size does not influence the gradient of particle velocity significantly; the characteristic length for velocity decay does not show a direct scaling with particle size. Instead, the characteristic length for velocity decay decreases as the channel width is narrowed. By analyzing the measurements in this and other experimental systems of granular shear flows, we argue that the spatial scale for the decay of grain velocity should be geometry-specific; a heuristic model is proposed to explain the shear banding in this geometry. Supported by NSF-DMR-0405187 [1] Phys.Rev.E 70, 031303 (2004)   

MRI Study of Granular Flow in a Split-Bottomed Couette Cell

Recent studies of dense granular flow in a split-bottomed Couette geometry have brought new insights into the concept of shear bands in granular systems [1]. However, to date experimental results have primarily focused on the flow at the top surface of the system. Here we present a study of the 3- dimensional structure of shear band formed in such a geometry using magnetic resonance imaging (MRI). We show that the angular velocity profiles in horizontal plane follow an error function as observed at the top surface. By measuring the center and the width of the shear band at the different heights in the bulk, we map out the 3-D shape of the shear band and investigate the behavior of the shear band as a function of the total filling height. We find that when the top of the shear band detaches from the surface of the bulk, its shape changes dramatically, similar to a first order transition as has been proposed by theory [2]. [1] D. Fenistein, J. W. van de Meent, and M. van Hecke, PRL 92, 094301 (2004). [2] T. Unger, J. Torok, J. Kertesz, D. E. Wolf, PRL 92, 214301 (2004).   

Simultaion of Dense Granular Flows in a Modified Couette Cell

Dense granular flows often exhibit thin, localized regions of particle motion, shear bands, separating largely solid-like regions. Recent experiments using a split-bottom Couette cell found that the width of the shear zone grew as the pack height increased and the azimuthal velocities when rescaled fall on a universal curve regardless of the particle properties. Here we present large-scale Discrete Element simulations of a similar system for packs of varying height up to 180,000 monodisperse spheres. We find a similar universal scaling relation for the azimuthal velocities both at the surface as well as in the bulk of the pack. However, we find the rescaled velocity profiles are asymmetric for smaller diameter systems. We observe a quasi-static inner core which changes shape with increasing pack height and undergoes a transition from a curved cylinder intersecting the surface of the pile to a closed surface within the bulk as predicted by theory. The mean-squared velocity fluctuations are found not to follow a simple scaling form with the shear rate as observed in traditional Couette cells and the velocity fluctuations in the cross-coordinate radial-azimuthal and azimuthal-vertical directions differ significantly from the other directions suggesting secondary flows. \\ \\ Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy's NNSA under contract No. DE-AC04-94AL75000.   

Secondary Granular Flow a Split-Bottomed Couette Cell

Granular materials and ordinary fluids react differently to shear stresses. The former develop shear bands rather than deform uniformly. Thus, shear regions of large particle motion are present in the surrounding of the essentially rigid adjacent zones. The extent of these shear bands may be increased when the system is sheared from the bottom of the container, thus taking advantage of the gravity [1]. In this talk, a new effect appearing in slit-bottom geometry is shown. A secondary flow in the vertical and radial directions becomes apparent and strongly depends on the height of grains in the system. Video tracking from the top free surface and MRI videos from the bulk of the system show this effect. [1] D. Fenistein, J. W. van de Meent, and M. van Hecke, PRL 92, 094301 (2004)   

Axial transport of bidisperse granular mixtures in a rotating drum

Bidisperse granular mixtures rapidly size segregate when tumbled in a partially filled, horizontal drum. The smaller component moves radially toward the axis of rotation and forms a buried core. On a longer time scale, axial modulations of the core may develop and grow into a series of bands along the drum, which become visible upon breaking the surface. Using a narrow pulse of the smaller component as the intitial condition, we observe that the axial transport of the radial core is a subdiffusive front advancement process. The front motion is subdiffusive in the sense that the radially integrated concentration forms a self-similar, compact axial pulse whose width grows as $t^{\alpha}$, with $\alpha \sim 1/3 < 1/2$, and hence it spreads much more slowly than by diffusion in a mixture which does not exhibit axial banding. By coloring some of the larger grains, we find that the mixing and axial transport of the larger grains is similarly subdiffusive. We report on the effects of changing relative grain size and drum diameter on the axial transport of grains. We find that mixing occurs in the radial core, and axial band formation is enhaced in these cases.   

Simulating and Shaking a Rotating Drum of Beads

It is well known that different sized particles will segregate when rotated in a horizontal cylinder, but the mechanism--for axial segregation in particular--is not well understood. We use a combination of high-speed imaging and perturbation experiments to elucidate flow properties during axial segregation in bi- and tri- disperse mixtures in a rotating drum. In addition, we use molecular dynamics (MD) simulations to investigate the motion of particles in the bulk and to measure internal stresses and dissipation. Experimental results indicate slow convective flow on timescales comparable to the band formation time. We use MD simulation to thoroughly investigate this possibility in three dimensions. The MD simulations also highlight the crucial role of sidewalls in the segregation phenomena. To further investigate the stability properties of the flowing layer, we shake the rotating drum horizontally--perpendicular to the surface flow direction. This allows us to estimate a `viscosity' of the flowing layer based on amplitude and phase lag of the oscillation at the surface of the flow.   

Numerical Tests of Kinetic Theory for Sheared Granular Flows

The Revised Enskog Theory generalizes the kinetic theory of dense gases to include granular materials by incorporating inelasticity. It still relies, however, on the assumption that grains interact only through binary collisions. We explore the validity and breakdown of kinetic theory for granular materials utilizing Contact Dynamics simulations of two-dimensional sheared granular materials. Using an analytical expression for the contribution of binary collisions to the stress tensor, we directly measure the ``collisional'' stress and the total (Kirkwood) stress for a wide range of densities and restitution coefficients. Our measurements demonstrate that the ``collisional'' stress becomes negligible at high densities and low restitution coefficients. This suggests that the whole structure of kinetic theory breaks down in these regimes and emphasizes that jamming results from variations of the frictional stress, but not from a crossover between a collision-dominated and friction-dominated regime.   

Friction coefficients in 2D granular Couette flow

Within geologic fault zones the internal friction in a material is expected to produce a large amount of heat. However, far less heat than expected is generated, giving rise to what is known as the heat flow paradox in geophysics. One possible explanation is that a fraction of the stress is not released by sliding friction but rolling of particles. We address this issue by studying a dense two-dimensional granular system under shear. In a Couette cell the overall torque is measured on the inner wheel for various packing fractions. This is compared to stress measured within the system using photoelastic particles. The relation between torque and mean shear force is interpreted as an effective friction coefficient $\mu$. The goal of this work is to determine $\mu$ as a function of the mean applied load.   

Session L37: Microfluidic Physics I: Applications, Separations, Polymers

Biological Large Scale Integration

The integrated circuit revolution changed our lives by automating computational tasks on a grand scale. My group has been asking whether a similar revolution could be enabled by automating biological tasks. To that end, we have developed a method of fabricating very small plumbing devices chips with small channels and valves that manipulate fluids containing biological molecules and cells, instead of the more familiar chips with wires and transistors that manipulate electrons. Using this technology, we have fabricated chips that have thousands of valves in an area of one square inch. We are using these chips in applications ranging from screening to structural genomics to ultrasensitive genetic analysis. However, there is also a substantial amount of basic physics to explore with these systems the properties of fluids change dramatically as the working volume is scaled from milliliters to nanoliters!   

Multistream Laminar Flow: From a challenge in mixing to membraneless fuel cells and microreactors for cofactor regeneration.

Over the last decade a wide variety of research efforts in microchemical systems, in which fluid flow is laminar, has developed. The original challenge of mixing in the absence of turbulence in this laminar flow regime has been overcome through various technical approaches including zig-zag or serpentine-shaped channels (Branebjerg, Beebe \textit{et al.}), lamination (e.g. Manz, Jensen \textit{et al.}), hydrodynamic focusing (Austin \textit{et al}.), and integrated herringbone features (Stroock \textit{et al.}). Others have grasped the opportunity to utilize multistream laminar flow for example for a T-sensor for blood analysis (Weigl \textit{et al.}) and in microfabrication or cell studies (Whitesides \textit{et al.}). This presentation will highlight the development of (i) a membraneless fuel cell, and (ii) a microreactor for cofactor regeneration that utilize multistream laminar flow. Various performance-determining characteristics and engineering improvements will be discussed.   

Continuous separation of human blood components through deterministic

Using a microfluidic device, the separation of red and white blood cells from their native blood plasma has been demonstrated. The device takes advantage of the asymmetric bifurcation of laminar flow around obstacles. This asymmetry creates a size dependent deterministic path through the device which depends on particle size.. All components of a given size follow equivalent migration paths, leading to high resolution. One-micron diameter fluorescent polystyrene beads were added to a mixture of blood to mark the flow patterns The blood WAS sorted into three distinct stream, consisting of 1 micron beads, red blood cells and white blood cells, respectively.   

DNA Dynamics in a Microfluidic Device

We present a general analysis and experimental study of DNA deformation in complex electric field gradients created inside microfluidic devices. Double-stranded DNA, free of its native proteins, is a ``model polyelectrolyte'' because reliably monodisperse samples can be attained with a uniform negative charge. Furthermore, the molecules can be stained with fluorescent probes and their motion directly observed by single-molecule microscopy. Large DNA coils move at a size-independent velocity in uniform electric fields and can significantly deform in electric field gradients. Because the electrophoretic velocity field is a potential field, local deformation of DNA in any electric field gradient is pure elongation, quantified by a strain rate and axes of extension and compression. From this analysis we construct the electrophoretic Deborah number, the non-dimensionless parameter that governs deformation of a polyelectrolyte in an electric field gradient. Over a range of Deborah numbers, we experimentally study single-molecule DNA deformation dynamics in model geometries such as a sharp contraction and a non-conducting cylinder. Analogous to deformation in hydrodynamic velocity gradients, we report both critical coil-stretch behavior and a strong dependence on the zero-strain DNA configuration. Furthermore, we apply these results to DNA mapping and DNA sequencing innovations.   

DNA transport in magnetic arrays

We present a experimental/theoretical study of the microfluidic electrophoresis of long DNA in an innovative matrix consisting of a hexagonal array of magnetic bead columns. Using videomicroscopy, we examined the motion of long T4 DNA (169 kbp) under a wide range of array densities and electric fields. By tracking the motion of many individual DNA molecules, we computed (i) the distribution of collision times; (ii) the collision probability; (iii) and the mean passage time through the viewing area. Based on our single molecule results, we will present a model focusing upon the non-Markovian characteristic of the transport in the array. DNA transport is represented by a Scher-Lax walk, where each molecule undergoes cycles of collisions. The model qualitatively and quantitatively captures the main features on separation in microarrays. As this work represents the first systematic theoretical study of dispersion in these devices, it is a significant step towards a detailed understanding of realistic miniaturized separation systems.   

Shear Flow Induced Chain Migration in Nanochannels

Flow induced migration of polymer chains in nanochannels has potential applications for DNA separation and sequencing operations. In this work, we use molecular dynamics (MD) simulation to investigate the effect of shear flow on chain migration effects. In particular, flow behavior of a dilute polymer solution is studied using a bead-spring model of the polymer chain and a coarse grained model of the solvent. A purely repulsive Lennard-Jones potential is used to represent all of the intermolecular interactions in the system. The diffusion and hydrodynamic effects are determined by the intermolecular interactions in such a model system. Our results are used to identify the flow conditions that cause chain migration both towards and away from the channel walls. The MD simulation results are compared with the experimental data and the predictions from recent Brownian dynamics simulation and kinetic theory. Our results are used to assess the relative importance of the wall hydrodynamics and chain diffusion on the chain migration effects.   

The interplay of fluid inertia and fluid viscoelasticity in complex microfluidic flows

The interplay of inertia and elasticity is explored by investigating the extensional flow behavior of dilute solutions of high molecular weight polyethylene oxide (PEO) flowing through micro-fabricated contractions. The small length scales and high deformation rates inherent to microfluidic devices make it possible to induce strong non-Newtonian effects even in dilute polymer solutions. By preparing solutions with varying solvent viscosities, it is possible to tune the ratio of elasticity to inertia at the same polymer concentration. The kinematics upstream of the contraction are characterized in terms of the strong viscoelastic enhancement of the corner vortex, which is accompanied by an increase in the pressure drop across the contraction plane that reaches a value in excess of 6 times the equivalent Newtonian pressure drop. The shape and maximum of the pressure drop/flowrate curve is found to be strongly dependent on the elasticity number, which is only a function of fluid properties and the size of the channel. This work provides valuable insight into the behavior of macromolecular fluids that are common in lab-on-a-chip processes, such as those containing proteins or dilute solutions of DNA.   

Dynamics of DNA Molecules in Slit Microchannels

Microfluidic devices used for high accuracy/throughput bio- chemical analysis could potentially revolutionize genome analysis. As an example, DNA adsorption on microfluidic channel walls has been fruitfully exploited for single DNA restriction mapping. Brownian dynamics simulations accounting for full hydrodynamic interactions (including perturbations from the wall) are employed to investigate the dynamics of a single DNA molecule undergoing pressure-driven shear flow in rectangular channels of height comparable to and less than the radius of gyration of the molecule. Good agreement is found between theory predictions, simulation results, and experimental measurements. Further investigations explore how shear flow may be used to manipulate the chain conformation and transport properties. We also examine how predictions of the chain dynamics and conformation in confined environments are affected by the degree of coarse-graining in the simulated DNA molecule, and propose a novel approach to dynamically optimize the resolution of our models.   

Brownian Dynamics Simulations of Polyelectrolyte Adsorption in Shear Flow

The adsorption of polyelectrolytes onto charged surfaces often occurs in microfludic devices and can influence their operation. We employ Brownian dynamics simulations to investigate the effect of a simple shear flow on the adsorption of an isolated polyelectrolyte molecule onto an oppositely charged surface. The polyelectrolyte is modeled as a freely-jointed bead-rod chain where the total charge is distributed uniformly among all the beads, and the beads are allowed to interact with one another and the charged surface through screened Coulombic interactions. The simulations are performed by placing the chain some distance above the surface, and the adsorption behavior is studied as a function of the screening length. Specifically, we look at the components of the radius of gyration, normal and parallel to the adsorbing surface, as functions of the screening length, both in the absence and presence of the flow. We find that in the absence of flow, the chain lies flat and stretched on the adsorbing surface in the limit of weak screening, but attains free solution behavior in the limit of strong screening. In the presence of a shear flow, the chain orientation in the direction of the flow increases with increasing Weissenberg number over the entire range of screening lengths studied. We also find that increasing the strength of the shear flow leads to an increased contact of the chain with the surface compared to the case when no flow is present.   

Flow-modulated ssDNA reaction in microchannels

We model the recombination of very-short-strand DNA in microchannels with the assumption that two complementary strands will combine only if they are close together in position as well as alignment. We describe this as a reaction-advection-diffusion system in position-orientation space with avenues for control in the form of velocity fields and external potentials. We prove that, without reaction, chaotic (uniformly and non-uniformly hyperbolic) flows can lead to simultaneous mixing of the strands and their alignment. We develop numerical simulation of the process showing that reaction is enhanced by chaotic mixing. We analyze the dynamics in a flow caused by an active (shear superposition) micro-mixer.   

Electrophoresis of a Bead-rod Chain through a Narrow Slit: A Brownian Dynamics Study

We use two-dimensional Brownian dynamics simulations to study the electrophoresis of a bead-rod chain through a narrow slit. A constant electric field is assumed to act inside and outside the slit, and the total charge on the chain is distributed equally among all beads. We study the dependence of the polymer transit velocity on the chain length, slit dimensions (width-to-length ratio), and electric field strength. We find that for sufficiently narrow slits, the transit velocity increases non-linearly with the applied field for low field strengths, whereas it increases linearly for high field strengths. In the low-field-strength region and for sufficiently narrow slits, the transit velocity decreases rapidly for small chain lengths and then decreases slowly beyond a critical chain length. With increasing width of the slit, the decrease in velocity is observed to be more continuous, and becomes independent of chain length for large slits. These results show the sensitivity of the transit velocity vs. chain length relationship to the slit dimensions and electric field strength, and could be useful for microfluidic separations.   

Electrokinetic Concentration of Charged Macromolecules In Nanoporous Media

We present a technique for concentrating proteins and other charged macromolecules in microfabricated systems through the manipulation of molecular electromigration at the interface between bulk flow in microchannels and flow through micro/nanoporous material with a bimodal pore distribution. The presence of a bimodal pore structure allows for the creation of metastable electrokinetic regions in areas of double layer overlap without the attendant electrokinetic pressure generation required for continuity in transitions between open microchannels and unimodal nanoporous structures. Directed experiments on the dependence of the observed phenomena on pH, macromolecule charge state, chemi- and electrosorption are supportive of a relatively simple model that defines the criteria that must be satisfied to observe macromolecule concentration. This technique has been applied to concentrate proteins by 2-3 orders of magnitude before pressure elution for analysis, and has been implemented with both polymeric and silicate materials in capillaries and in microfabricated glass microfluidic devices.   

Continuous microfluidic immunomagnetic cell separation

We present a continuous-flow microfluidic device that enables cell by cell separation of cells selectively tagged with magnetic nanoparticles. The cells flow over an array of microfabricated magnetic stripes, which create a series of high magnetic field gradients that trap the magnetically labeled cells and alter their flow direction. The process was observed in real-time using a low power microscope. The device has been demonstrated by the continuous flow separation of leukocytes from whole human blood. The dependence of the process on the flow rates and direction of flow with respect to the magnetic stripes will be described.   

Session L38: Phase Transitions in Model Magnets

Magnetic Field Induced Shifts of the Spin Rotation Phase Transition of GdFe$_3$(BO$_3$)$_4$

GdFe$_3$(BO$_3$)$_4$ exhibits a structural phase transition at 156 K, antiferromagnetic order of the Fe moments at 36 K followed by a spin reorientation transition at T$_{SR}$ = 9 K. At the lower transition the dielectric constant of GdFe$_3$(BO$_3$)$_4$ shows a distinct peak indicating an interesting coupling between the magnetic order and the dielectric properties. We study thoroughly this lower temperature phase transition through dielectric, magnetic and thermodynamic measurements under the application of external magnetic fields up to 1 Tesla. The spin reorientation transition is shown to split into two phase transitions under external magnetic fields. The dielectric constant at low temperature changes with the applied field and a magneto- dielectric effect of up to 1\% is observed at 8 K and 0.7 Tesla.   

Neutron Scattering Studies of the Random Field Domain State in CsCo$_{0.83}$Mg$_{0.17}$Br$_3$

We have extended previous neutron scattering measurements on CsCo$_{0.83}$Mg$_{0.17}$Br$_3$[1], a dilute stacked triangular lattice Ising antiferromagnet. Pure CsCoBr$_3$ exhibits three magnetic phase transitions[2]. At $T_{n1}$=28.3 K a 3 sublattice Neel state forms, in which two sublattices are ordered up and down, while the third remains disordered. This disordered sublattice itself orders below $T_{n2} \sim 16 K$, forming ferrimagnetic sheets which stack antiferromagnetically. The non-magnetic dopants induce a random field domain state at $T_{n1}$ in CsCo$_{0.83}$Mg$_{0.17}$Br$_3$, as the magnetic vacancies couple to the disordered sublattice as a random field. We resent new high resolution results showing the evolution of this random field state in both zero magnetic field and for the case where a magnetic field is applied along the Ising moment direction, the c-axis. [1] J. van Duijn et al. Phys. Rev. Lett. 92, 077202 (2004) [2] M. Mao et al. Phys. Rev. B 66, 184432 (2002)   

Field-Induced ordering in NiCl2-4SC(NH2)2

The compound NiCl$_2\cdot$4SC(NH$_2$)$_2$ (DTN) is a potential candidate for Bose Einstein Condensation (BEC) of spins. The S = 1 Ni spin triplet ground state is split by the atomic anisotropy into a lower S = 0 state and a higher S$ = \pm 1$ state. Applied magnetic fields parallel to the tetragonal axis reduce the splitting until a level crossing occurs at H = 2 T. Between H = 2 T and H = 12 T, field-induced long range order has been observed below T = 1 K. In other similar compounds, this long range order has been interpreted in terms of a BEC of triplet spins. Unlike many previous BEC candidates, DTN exhibits a strong single axis anisotropy with rotational symmetry in the ab plane, which allows for the conservation of magnons. We will present the first measurements of specific heat and the magnetocaloric effect of single crystalline DTN in high fields, and investigate the high field phase diagram, long range order, and possible BEC of magnons. This work was supported by the NSF through the National High Magnetic Field Laboratory, the State of Florida and the Department of Energy.   

Magnetic Transition in Antiferromagnetic Spin-$\frac{1}{2}$ Chains with Staggered Long-Range Interactions

Antiferromagnetic spin-$\frac{1}{2}$ chains with non-frustrated long-range couplings are studied using the powerful Quantum Monte Carlo algorithm based on a Stochastic Series Expansion of the partition function [1]. The case of power-law decaying interaction $J(r)=-(-1)^r r^{-\alpha}$ is investigated for the general one-dimensionnal XXZ Hamiltonian $$ {\mathcal{H}}=\sum_{i,j}J(|i-j|)\left(S_{i}^{x}S_{j}^{x}+S_{i}^{y}S_{j}^{y} +\Delta S_{i}^{z}S_{j}^{z}\right).$$ Very large scale numerical results obtained on systems up to $L=8000$ spins are compared and discussed through bosonization and spin-waves predictions [2] for the onset of antiferromagnetic ordering in the ground-state in function of $\beta$.\\ $[1]$ O. F. Sylju{\aa}sen and A. W. Sandvik, Phys. Rev. E {\bf 66}, 046701 (2002).\\ $[2]$ E. Yusuf, A. Joshi, and Kun Yang, Phys. Rev. B {\bf 69}, 144412 (2004).   

Spin Peierls transitions within the 1/4-filled Peierls extended Hubbard model

We investigate the Peierls transition within the one-dimensional Peierls extended Hubbard model at 1/4 filling. We treat both electron-electron and bond-coupled electron-phonon interactions (finite frequency) exactly using the Stochastic Series Expansion Quantum Monte Carlo method. As previously found for the 1/2-filled band, Peierls distortion for finite-frequency phonons only occurs for electron-phonon coupling above a critical value. Unlike the 1/2-filled band, at 1/4 filling the Peierls transition must be preceded at a higher temperature by another structural or charge order transition. We compare the resulting Peierls transition for two possible high temperature states, either (a) dimerization or (b) $4k_{F}$ ..1010.. charge order at high temperature.   

Criticality in 2-D frustrated quantum triangular antiferromagnets

We revisit 2-D frustrated quantum magnetism from a new perspective, with the aim of exploring new critical points and exotic critical phases. We study easy-plane s = 1 and s = 1/2 triangular antiferromagnets using a dual vortex approach, fermionizing the vortices with a Chern-Simons field. This enables us to formulate a low-energy QED3 critical theory with emergent SU(2) and SU(4) flavor symmetry in the s = 1 and s = 1/2 cases, respectively. We conjecture that the SU(2) theory describes a multicritical point separating the quantum paramagnet and two magnetically-ordered states, while the SU(4) theory describes a new stable gapless spin-liquid phase.   

Quantum Griffiths effects in itinerant Heisenberg magnets

We study the influence of quenched disorder on quantum phase transitions in itinerant magnets with Heisenberg spin symmetry, paying particular attention to rare disorder fluctuations. In contrast to the Ising case where overdamping suppresses the tunneling of rare regions, the XY- and Heisenberg system displays strong power-law quantum Griffiths singularities in the vicinity of the quantum critical point. We discuss these phenomena based on general scaling arguments, and we illustrate them by an explicit calculation for O(N) spin symmetry in the large-N limit. We also discuss broad implications for the classification of quantum phase transitions in the presence of quenched disorder.   

On Spin 1/2 Excitations and Quantum Criticality in Two Dimensional $O(3)$ Antiferromagnets

We utilize the $2+1$ $O(3)$ nonlinear sigma model for antiferromagnets to study the suggestion that there are corrections to quantum criticality due to low energy degrees of freedom intrinsic to the quantum critical point. The N\`eel ordered ground state, besides the gapless Goldstone excitations, has gapped skyrmion and antiskyrmion topological configurations. These are responsible for the system being disordered at all finite temperatures, as they gain energy by becoming arbitrarily large and thus lead to finite correlation length no matter how few of them are present. We map the skyrmions and antiskyrmions to $SU(2)$ spin $1/2$ objects and further show that they superpose in exactly the same way as spin $1/2$ objects. Therefore the N\`eel ground state has gapped spin $1/2$ excitations, i.e. spinons. This conclusion is not due to a Hopf term and it is independent of whether the microscopic spins are integral or half integral. We write an effective low energy field theory that correctly takes into account the spinon and Goldstone excitations, and their interactions.   

On Fractionalized Quantum Criticality in Two Dimensional $O(3)$ Antiferromagnets

We consider a recently propose field theory which inculdes magnetizationa and gapped skyrmion and antiskyrmion excitations in 2 +1 dimensional antiferromagnets. The skyrmions and antiskyrmions as spin 1/2 objects, i.e. spinons.From this field theory we show how the spinon fluctuations change the renormalization of the coupling constant, thus changing the critical coupling at which N\`eel order is lost. We also show that spinon fluctuations will lead to corrections to critical exponents as they renormalize the magnetization propagators beyond the usual renormalizations due to order parameter fluctuations. Since the spinon gap is inversely proportional to the coupling constant, and the renormalized inverse coupling constant, or spin stiffness, vanishes at the quantum critical point, the onset of paramagnetism is identified with spinon gap collapse. Because of this we conclude that essentially free skyrmions and antiskyrmions are the low energy degrees of freedom intrinsic to the quantum critical point as there are no Goldstone eigenstates at criticality due to lack of N\`eel order.   

U(1) spin liquids and valence bond solids in a large-N three-dimensional Heisenberg model

We study possible quantum ground states of the Sp($N$) generalized Heisenberg model on a cubic lattice with nearest-neighbor and next-nearest-neighbor exchange interactions. The phase diagram is obtained in the large-$N$ limit and fluctuation effects are considered via appropriate gauge theories. We find three U(1) spin liquid phases with different short-range magnetic correlations. These phases are characterized by deconfined gapped spinons, gapped monopoles, and gapless ``photons.'' As $N$ becomes smaller, a confinement transition from these phases to valence bond solids (VBS) may occur. This transition is studied by using duality and analyzing the resulting theory of monopoles coupled to a non-compact dual gauge field; the condensation of the monopoles leads to VBS phases. We determine the resulting VBS phases emerging from two of the three spin liquid states. On the other hand, the spin liquid state near $J_1 \approx J_2$ appears to be more stable against monopole condensation and could be a promising candidate for a spin liquid state in real systems.   

Evolution of the single-hole spectral function across a quantum phase transition in the anisotropic-triangular-lattice antiferromagnet

We study the evolution of the single-hole spectral function when the ground state of the anisotropic-triangular-lattice antiferromagnet changes from the incommensurate magnetically-ordered phase to the spin-liquid state. In order to describe both of the ground states on equal footing, we use the large-$N$ approach where the transition between these two phases can be obtained by controlling the quantum fluctuations via an `effective' spin magnitude. Adding a hole into these ground states is described by a $t$-$J$ type model in the slave-fermion representation. Implications of our results to possible future ARPES experiments on insulating frustrated magnets, especially Cs$_2$CuCl$_4$, are discussed.   

Ferromagnetic Instability in Disordered Systems: A Hartree-Fock Approach

It was realized two decades ago that two dimensional diffusive Fermi liquid is unstable against arbitrarily weak electronic interactions. Recently, using the nonlinear sigma model developed by Finkelstein, several authors showed the instability gives rise to a ferromagnetic state. In this work, we consider electrons moving in a random potential with the following interaction: $-J\vec{S(x)}\cdot\vec{S(x')}$. We calculate the electron self energy and find that in two dimensions, the total energy is always minimized by ferromagnetic phase, while in three dimensions, ferromagnetism occurs only if $J$ exceeds a critical value proportional to the conductivity. Although the model and the calculation method are apparently different from the ones used before, the results are in qualitative agreement, which shows the robustness of the ferromagnetic instability in interacting disordered systems.   

Superfluid--Solid Quantum Phase Transitions and Landau-Ginzburg-Wilson Paradigm

We study superfluid (SF)--solid zero-temperature transitions in 2d lattice boson/spin models by Worm-Algorithm Monte Carlo simulations. The SF -- Valence Bond Solid (VBS) transition was recently argued to be generically of II order in violation of the Ginzburg-Landau- Wilson (GLW) paradigm [1]. We simulate the J-current model on lattices up to 64x64x64, and observe that SF- columnar VBS and SF-checkerboard solid transitions are typically weak I-order ones and in small systems they may be confused with the continuous or high-symmetry points [2]. Thus, in the simulated model, the SF-VBS transition proceeds in agreement with the GLW paradigm. We explain this by dominance of standard particle and hole excitations, as opposed to fractionalized (spinon) excitations [1]. We developed a technique based on tunneling events (instantons) in the insulating phase which reveals charges of the revelant long-wave modes. While in 1d systems spinons are clearly seen in tunneling events, in two spatial dimensions tunneling is solely controlled by particles and holes in our system. This work is supported by NSF grant ITR-405460001 and PSC-CUNY- 665560035. [1] T. Senthil, A. Vishwanath, L. Balents, S. Sachdev, and M.P.A. Fisher, Science {\bf 303}, 1490 (2004); [2] A.B. Kuklov, N.V. Prokof'ev, B.V. Svistunov, condmat/0406061; PRL, to be published.   

Broken Symmetries and Gapless Excitations of SU(N) Antiferromagnets Investigated With Variational Wavefunctions

We use Gutzwiller-projected wavefunctions to investigate variationally the phase diagrams of SU(N) quantum antiferromagnets in the self-conjugate representation. The method is first tested against the known phase diagram of a one-dimensional SU(4) bilinear-biquadratic spin chain which has a quantum-critical point separating a dimerized phase from a phase with spontaneously broken charge-conjugation symmetry\footnote{I. Affleck {\it et al.}, Nucl. Phys. B {\bf 366}, 467 (1991).}. In the case of two-dimensional SU(N) antiferromagnets, recent analytical\footnote{M. Hermele {\it et al.}, \urllink{cond-mat/0404751}{http://arxiv.org/abs/cond-mat/0404751}.} and numerical\footnote{F. F. Assaad, \urllink{cond-mat/0406074}{http://arxiv.org/abs/cond-mat/0406074}.} work suggests the existence of a gapless spin-liquid phase with no broken symmetries. Such a phase would be consistent with a recent generalization of the Lieb-Schultz-Mattis theorem\footnote{M. B. Hastings, Phys. Rev. B{\bf 69}, 104431 (2004); \urllink{cond-mat/0411094}{http://arxiv.org/abs/cond-mat/0411094}.} to more than one spatial dimension. We examine the stability of the $\pi$-flux phase against tendencies to spin-order, crystallize into various valence-bond solids, or break charge-conjugation symmetry.   

Current Carrying Ground State in a Bi-layer Model

Strongly interacting systems have been conjectured to spontaneously develop current carrying ground states under certain conditions. We conclusively demonstrate the existence of a commensurate staggered interlayer current phase in a bi-layer model by using the recently discovered quantum Monte-Carlo algorithm without the sign problem. A pseudospin SU (2) algebra and the corresponding anisotropic spin-1 Heisenberg model are constructed to show the competition among the staggered interlayer current, rung singlet and charge density wave phases.   

Form of the exact partition function for the generalized Ising model

The problem of N interacting spins on a lattice is equivalent to one of N identical clusters linked in a specific manner. The energy of any configuration can be expressed in terms of the energy levels of this cluster. A new expression is obtained for the probability of occurrence of any configuration. A closed form expression is obtained for the exact partition function per spin in terms of the energy levels of this cluster and the degeneracies are functions of temperature. This form represents an alternate and equivalent (sum over energy levels) framework to determine the partition function. The partition functions of all Ising-like models have a common form. This raises a new possibility that the partition function may be determined as a sum of finite number of terms, which may not sum to a single term expression. Seven functions need to be determined to describe the exact partition function of the 3D Ising model. The key to understanding phase transitions and critical phenomena lies in the temperature dependence of degeneracies. It is necessary to develop new techniques to determine the partition function that account for this temperature dependence.   

Valence bond orders in spin-1/2 antiferromagnets in three dimensions

We discuss possible valence bond orders in spin-1/2 quantum antiferromagnets on a 3D cubic lattice and expose their relation to a possible fractionalized Coulomb spin liquid state in the vicinity of the collinear Neel order. The analysis is an extension to (3+1)D of techniques previously developed in two spatial dimensions. In three dimensions, the identification of such confining phases with broken translational symmetry is formulated as a problem of monopole condensation patterns for monopoles hopping on the dual lattice and accumulating nontrivial Berry phases specific for the spin-1/2 system. Columnar and "box" valence bond states appear as natural candidates.   

Session L39: Focus Session: Intrinsic Inhomogeneity in Multiferroic Materials

Nanoscale Structural Correlations in Magnetoresistive Manganites

The colossal magnetoresistance (CMR) effect in perovskite manganites is a magnetic-field-induced transition form a paramagnetic insulating (PI) to a ferromagnetic metallic state. The large electrical resistivity of the PI state lies at the heart of the CMR effect. This enhanced resistivity stems, in part, from strong electron-lattice coupling and the associated local lattice distortions. Both uncorrelated local distortions (Jahn-Teller polarons) and correlated distortions are present in the PI state. The latter are believed to signal the presence of nanoscale orbital correlations. In this talk, we describe recent x-ray and neutron scattering studies of the orbital correlations in pseudocubic manganites Ln$_{1-x}$B$_x$MnO$_3$. Possible microscopic structures giving rise to these correlations are discussed. Dynamical properties of the correlated and uncorrelated distortions are presented. It is found that the correlations are ubiquitous in the orthorhombic PI phase of hole-doped manganites, and that their properties are defined by a single parameter - the doping level $x$. The correlations, however, are absent in the other paramagnetic phase exhibited by the manganites - the rhombohedral phase. The latter phase is metallic in the doping range in which the CMR effect is observed. Since the uncorrelated lattice distortions are present in the both of these phases, the insulating character of the PI state in CMR manganites results from the presence of the correlated lattice distortions. The orbital correlations, therefore, play the key role in the CMR effect.   

Electrical and structural investigations, and ferroelectric domains in nanoscale structures

Generally speaking material properties are expected to change as the characteristic dimension of a system approaches at the nanometer scale. In the case of ferroelectric materials fundamental problems such as the super-paraelectric limit, influence of the free surface and/or of the interface and bulk defects on ferroelectric switching, etc. arise when scaling the systems into the sub-100 nm range. In order to study these size effects, fabrication methods of high quality nanoscale ferroelectric crystals as well as AFM-based investigations methods have been developed in the last few years. The present talk will briefly review self-patterning and self- assembly fabrication methods, including chemical routes, morphological instability of ultrathin films, and self-assembly lift-off, employed up to the date to fabricate ferroelectric nanoscale structures with lateral size in the range of few tens of nanometers. Moreover, in depth structural and electrical investigations of interfaces performed to differentiate between intrinsic and extrinsic size effects will be also presented.   

Magneto-electric phase diagrams in Tb$_1-x$Gd$_x$MnO$_3$

{\it R}MnO$_3$ with distorted perovskite structure has the ferroelectric (FE) ground state with long-period antiferromagnetic spin order for {\it R}=Tb and Dy [1], while showing the A-type (layered antiferromagnetic) paraelectric (PE) state for R=Gd. The A-type PE state in GdMnO$_3$ is associated with the appreciable ferromagnetic component (~0.2$\mu$B) along the c axis due to the Dyaloshinskii-Moriya interaction. When an external field of about 7T is applied along the c axis, the FE state of TbMnO$_3$ is observed to turn into PE. This is likely because the A-type PE state with canted spin component is induced by the magnetic field via Zeeman coupling. In this study, we have prepared single crystals of solid solutions Tb$_1-x$Gd$_x$MnO$_3$, whose both end materials are the PE and FE with different magnetic orders. Due to the exchange interaction between the Heisenberg like Gd moments and Mn spins, the series of compounds show the rich phase diagrams as functions of Gd composition x and magnetic field H. Enhanced magnetic-field response of the ferroelectric phase is demonstrated in the intermediate x-region. [1] T. Goto et al. Phys. Rev. Lett.92,257201 (2004).   

Observation of a Griffiths phase in paramagnetic La$_{1-x}$Sr$_x$MnO$_3$

We report on the discovery of a new phase boundary above the magnetic ordering temperature in low doped La$_{1-x} $Sr$_x$MnO$_3$ by means of ESR and susceptibility measurements. The observed triangular phase regime in the paramagnetic state is identified as a realization of a Griffiths phase, where disorder in the ferromagnetic bonds leads to the existence of a temperature scale above $T_C$. The influence of quenched disorder to allow for the occurrence of a Griffiths phase becomes evident by its appearance within the Jahn-Teller distorted orthorhombic structure.   

Search for uniform periodicity around the chemical window in manganites

We have recently shown that the low temperature superstructure in La$_{1-x}$Ca$_{x}$MnO$_{3}$ ($x\ge $0.5) possesses a uniform modulation that is not consistent with the charge-ordered stripe picture [cond-mat/0308581]. Here we investigate whether this phenomenon is confined to (La,Ca)MnO$_{3}$ or whether it arises in other manganites where the tendency to charge localisation is stronger.   

Complex Phenomena in Nanostructured Transition Metal Oxides

When the spatial dimension of a material becomes comparable or even smaller than the characteristic length scale of the relevant cooperative (collective) phenomena, it is expected that all related physical properties including phase transitions of this material will be dramatically changed. In this work, we focus on the discovery, understanding, and design of low-dimensional 3d transition metal oxides (TMO). We use both physical and chemical methods including laser MBE growth and hydrothermal synthesis to grow TMO thin films and nanostructures. The electronic and magnetic properties of the TMO films have been investigated by in-situ scanning tunneling microscopy/spectroscopy and ex-situ SQUID magnetometer. We have observed both large-scale (large than a few tens nanometers) and nano-scale electronic phase separation (PS) in epitaxially grown thin films of (La$_{5/8-0.3}$Pr$_{0.3})$Ca$_{3/8}$MnO$_{3}$. While the large PS domains are present only below the Curie temperature (T$_{c})$, the nano-scale PS clusters exist at temperatures both below and above T$_{c}$, which implies that the small clusters may originate from doping-related disorder.   

Spin and Orbital Ordering of Y$_{1-x}$La$_x$VO$_3$ (0 $\leq$ x $\leq$ 1)

At lowest temperatures, LaVO$_{3}$ exhibits G-type orbital ordering (OO) and C-type spin ordering (SO), while YVO$_{3}$ has C-type OO and G-type SO. To study the transition of different ground states with the variation of the size of the A-site cation, single-phase Y$_{1-x}$La$_{x}$VO$_{3}$ (0 \textbf{$\le $ }x \textbf{$\le $ }1) samples were melt-grown with the aid of an image furnace. The structural change accompanying the spin and orbital ordering was studied by high-energy, high-resolution x-ray powder diffraction. The temperature dependence of magnetic susceptibility and thermal conductivity was also studied. The results show that C-type OO is stabilized over a wider temperature range as the La content increases in the compositional range 0 \textbf{$\le $ }x \textbf{$\le $ }0.18; Too drops from 200 K for YVO$_{3}$ to 145 K for Y$_{0.82}$La$_{0.18}$VO$_{3}$, while T$_{N}$ decreases slightly from 116 K to 112 K. An abrupt change of the ground state from C-type OO to G-type OO was observed at a critical composition x = 0.20. At x $\ge $ 0.20, all compositions show G-type OO as the ground state and no long-range OO above T$_{N}$ was observed. *Y. Ren, Experimental Facilities Division, Advanced Photon Source, Argonne National Laboratory, Argonne, IL 60439   

Thermal expansion measurements in multiferroic HoMn$_{2}$O$_{5}$

Thermal expansion measurements were done on HoMn$_{2}$O$_{5}$ along the a,b,and c axes at zero applied magnetic field. Distinctive anomalies in the linear expansivities along the principal axes were seen at T$_{N}$=44K,T$_{C}$=39K,T$_{N\mbox{'}}$=20K and T$_{C\mbox{'}}$=15K with a notable negative c-axis thermal expansivity below 100K.All three axes were observed to shrink at T$_{N}$ and T$_{N\mbox{'}}$ while a and b expand as c shrinks when it passes through T$_{C}$ and T$_{C\mbox{'}}$ upon cooling. These anomalies are intimately correlated with anomalies in the dielectric constant and the specific heat at the phase transition temperatures. Our observations suggest that the coupling of the magnetic orders with the dielectric properties are mediated by strong magnetoelastic effects and the lattice anomalies play a crucial role in understanding the ferroelectricity in the compound. The anomalies associated with the ferroelectric transitions at T$_{C}$ and T$_{C\mbox{'}}$ show a thermal hysteresis revealing the first order nature of the transitions.   

Strains and rotations in cubic-tetragonal ferroelastics

Many ferroelastic materials are also ferroelectric or ferromagnetic; a classic example is barium titanate which transforms from cubic to tetragonal with decreasing temperature. The various low-temperature variants in ferroelastics are described by different values of the order-parameter (deviatoric) strain(s). In order to maintain a coherent interface free of disclinations, dislocations and other defects, the variants are rotated as well as separated by domain walls; the rotation might be observable by birefringence imaging. Non-order-parameter strains (dilatational and shear) are generally present in walls but are usually small there. In regions where walls collide however, the rotations are confused, generating dilatational and shear strains of order of the deviatoric strains. This talk describes various kinds of collision regions observed in simulations of cubic-tetragonal systems.   

Session L40: Dillon Medal Symposium

Surface engineering with soft matter

In my presentation, I will outline several novel strategies facilitating the generation of functional polymeric surfaces. In particular, I will present and discuss simple methodologies leading to the formation of complex surface assemblies of surface-tethered polymers with continuous variation of physico- chemical properties (e.g., wettability, molecular weight, grafting density, composition). I will illustrate how these grafted “gradient” surfaces can be utilized to control the spatial distribution of adsorbates, such as nanoparticles and proteins, and administer the proliferation of living cells on the surfaces. Furthermore, I will illustrate how flexible elastomeric networks can be utilized to tailor the grafting density of oligomers or polymers, create responsive (``smart'') surfaces, and generate topographically corrugated surfaces comprising multidimensional cascades of wrinkles. Application of these wrinkled surfaces for material assembly will also be demonstrated.   

Grafting reactions between end-functional polymers at polymer interfaces

Reactions to produce graft copolymers at polymer interfaces during extruder mixing are important for controlling dispersed phase size by retarding droplet coalescence and reducing interfacial tension while providing interface reinforcement. We investigate such reactions at various temperatures in a model bilayer film system consisting of amine end-functional deuterated polystyrene (dPS-NH$_{2})$ in PS and anhydride end-functional poly(2-vinylpyridine) (P2VP-anh) in P2VP as a function of molecular weight M and initial volume fraction $\phi _{0}$ of the end functional chains. After various times of reaction the interfacial excess z* of block copolymer formed at the interface is determined by detecting the $^{2}$H$^{- }$ion using dynamic SIMS depth profiling. At low $\phi _{0}$ ($\sim $ 0.01) of dPS-NH$_{2}$ and P2VP-anh, such that the normalized interface excess z*/R$_{g} \quad <$ 1 and the blocks are unstretched, the forward reaction rate constant k$^{+}$ decreases as M$^{-0.68}$ in rough agreement with theoretical predictions (k$^{+ }\sim $ M$^{-0.55})$ for this regime. The rate constant is thermally activated with an activation enthalpy 165 kJ/mol that is independent of M.   

Design and Realization of Temperature-Responsive Polymers with Tunable Onset of Response

Temperature responsive polymers offer high potential for applications involving chemical sensing and/or stimuli-driven actuation, but their proliferation has been hampered by the inability to tailor by design the onset-point of their response. A systematic series of temperature-responsive polymers were designed, synthesized, and studied, and the onset of their T-response was tailored by design of their monomer. Their T-response was studied both for their water solutions, and when they were end-tethered on a surface. Thermodynamic considerations for the monomer design, afford the possibility to fine-tune the lower critical solution temperature (LCST) point, at values ranging from 5 to 70$^{\circ}$C, in water. Solubility studies and phase diagrams will be presented for their aqueous solutions, whereas water contact angle, ellipsometry, and atomic force microscopy will be shown for end- grafted polymers.   

Structure and Phase Behavior of End-Tethered Weak Polyelectrolytes

The behavior of weak polyelectrolytes tethered at one of their ends to a surface and/or interface is studied using a molecular theory that explicitly accounts for the inhomogeneous acid-base equilibrium of the polymer segments. The predictions for the thickness of polyacrylic acid as a function of salt concentration are in excellent quantitative agreement with the experimental observations from the Genzer group. The local degree of dissociation of the polymer segments varies with bulk salt concentration and the local pH can change by two units in the interior of the polymer layer. In general, water is a relatively poor solvent for the polymer segments, which are soluble due to the presence of the charged groups. Therefore, one expects phase separation as the quality of the solvent decreases, depending on the degree of charging of the polymer. We will show how the phase diagrams of mobile weak polyelectrolytes tethered layers depend upon the salt concentration and bulk pH, together with the structure of the polymer layers along the coexistence curves. The two coexisting phases show very different degree of charge and as a result different structural properties. The possibility of microphase separation in the case of polymers end-grafted to the surface will be discussed. The implications of the results for practical application of weak polyelectrolyte layers will be presented.   

How do grafting points influence the structure formation in binary and one-component polymer brushes?

Grafting of incompatible polymers onto a substrate one prevents macroscopic phase separation. Theory predicts a rich phase diagram of laterally periodic morphologies of brushes consisting of incompatible polymers or one-component brushes in a bad solvent, however, structures observed in experiments lack long-range periodic order.\\ We employ MC simulations of a coarse-grained model to investigate the influence of spatial correlations of the grafting points (patterns) onto the morphology of one component brushes in a bad solvent and binary brushes. Comparing morphologies on identical sets of grafting point we observe a pronounced correlation between the average morphology of the brush and density fluctuations of the grafting points. These fluctuations prevent long-range ordering. Rather than a sharp thermodynamic transition, we observe a gradual building up of local structure upon increasing the incompatibility. The structure formation occurs at smaller incompatibility and the length scale is slightly larger than in case of grafting on a regular lattice. Different morphologies as a function of composition give rise to very similar structure factors but can be well distinguished by their Euler-characteristics.   

Dendronized polymer is a Single Molecule Glass

The molecular architecture of dendronized polymers can be tuned to obtain nanoscale objects with desired properties. In this work, we bring together experiments and computer simulations to study the thermodynamic and dynamic properties of a single dendronized polymer chain. We find that, upon changing certain architectural features, dynamic correlations characterizing backbone conformational fluctuations of a dendronized polymer exhibit dynamical arrest akin to glass-forming bulk liquids. Thus, a dendronized polymer chain is a novel macromolecule that is a single molecule glass. The range of conditions that leads to dynamic arrest does not, however, correspond to any thermodynamic singularities. Therefore, a dendronized polymer provides the first example of an experimental system that can directly test theories of constrained dynamics. We also show that defect densities characteristic of typical synthesis conditions do not alter the material properties of dendronized polymers. The self-assembly of the chains studied using the results of the single chain yields different phases from lamellar to gyroid phases and nematic phases depending the relative volume fractions of the backbone and the dendron units and the flexibility of the backbone.   

Understanding the Assembly of Pi-Conjugated Dithiol Molecules on Metal and Semiconductor Surfaces

We examined the assembly of terphenyl- (TPDT) and quaterphenyl-dithiol (QPDT) molecules on gold and gallium arsenide (GaAs) surfaces from ethanol (EtOH), tetrahydrofuran (THF), and mixtures of the two solvents using a combination of x-ray photoelectron spectroscopy, synchrotron-based near-edge x-ray absorption fine structure spectroscopy, and Fourier transform infra-red spectroscopy. While the molecular assembly on gold is solvent independent, our experimental results suggest that the assembly of both TPDT and QPDT on GaAs is extremely solvent dependent. Specifically, TPDT and QPDT form highly oriented monolayers with excellent surface coverage on gold substrates regardless of the solvent from which assembly occurred. When the molecules are assembled on GaAs, however, the surface coverage degrades with increasing THF fraction. Correspondingly, the molecules also become progressively less ordered. When assembled from pure THF, the molecules on GaAs are completely disordered and exhibit poor surface coverage. The origin of this dramatic solvent effect is currently under investigation.   

Adhesive Transfer of Thin Viscoelastic Films

Micellar suspensions of acrylic diblock copolymers are excellent model materials for studying the adhesive transfer of viscoelastic solids. The micellar structure is maintained in films with a variety of thicknesses, giving films with a well-defined structure and viscoelastic character. Thin films were cast onto elastomeric silicone substrates from micellar suspensions in butanol, and the adhesive interactions between these coated elastomeric substrates and a rigid indenter were quantified. By controlling the adhesive properties of the film/indenter and film/substrate interfaces we were able to obtain very clean transfer of the film from the substrate to the portion of the glass indenter with which the film was in contact. Adhesive failure at the film/substrate begins with the nucleation of a cavity at the film/substrate interface, followed by complete delamination of this interface. The final stage in the transfer process involves the failure of the film that bridges the indenter and the elastomeric substrate at the periphery of the contact area. This film is remarkably robust, and is extended to three times its original length prior to failure. Failure of this film occurs at the periphery of the indenter, giving a transferred film that conforms to the original contact area between the indenter and the coated substrate.   

Adhesion Induced Instability in Thin Polymer Films

Geometric confinement strongly influences the adhesion and fracture of soft elastic films. Aided by external force, confined elastic films develop undulations almost instantaneously and instability patterns appear in the form of cavitations and fingers at the interface. The characteristic wavelength of the instability is remarkably insensitive to the materials properties of the system except the thickness of the film. Geometric confinement and its relief via pattern formation shed light on the mechanism of adhesion of various thin film adhesives. In particular, when discontinuities are generated on the films via incisions, the length scale of the instability dictate to which the incised segments communicate with each other via shear field. This information is useful in the design of the adhesives that mimic the behavior of those found in biological world. .   

Symmetry, Equivalence and Molecular Self-Or\-ganization

Molecular self-organization at equilibrium is central to the formation of many biological structures and the emulation of this process through the creation of synthetic counterparts offers great promise for nanofabrication. The central problems in this field area are an understanding of how the symmetry of the interacting particles encodes the geometry of the organized structure and the nature of the thermodynamic transitions involved. Our approach is inspired by the self-organization of actin, tubulin and the icosahedral self-organization of clathrin and spherical viruses and proceeds from the general observation that biological macromolecules and synthetic molecules exhibiting supermolecular self-organization often exhibit large dipolar or other highly directional interactions, in addition to short range interactions responsible for phase separation. Correspondingly, we find chain-like, membrane-like, tubular and icosahedral particle self-organization using `equivalent' particles exhibiting an interplay between directional (dipolar and multipolar) interactions and short-range (van der Waals) interactions. Specifically, a dipolar potential having a continuous rotational symmetry gives rise to chain formation while potentials having discrete rotational symmetries [e.g., square quadrupole or a triangular ring of dipoles (`hexapole')] led to the self-organization of sheets, hollow tubes and icosahedral structures with resemblance to biological and synthetic structures.   

Direct Comparison of Surface and Bulk Relaxation of PS - A Temperature Dependent Study

Near-edge X-ray absorption fine structure (NEXAFS) spectroscopy was used to measure simultaneously the relaxation rates of polystyrene (PS) molecules at the free surface and in the bulk. The samples were uniaxially oriented at room temperature via a modified cold rolling process. The density of the oriented samples as determined by liquid immersion technique is identical to that of bulk PS. At temperatures below its bulk glass transition temperature the rate of surface and bulk chain relaxation was monitored by measuring the partial-electron yield (PEY) and the fluorescence NEXAFS yields (FS), respectively, both parallel and perpendicular to the stretching direction. The decay rate of the dichroic ratios from both PEY and FY at various temperatures was taken as a measure of the relaxation rate of surface and bulk molecules respectively. In addition, the decay rate of the optical birefringence was also measured to provide an independent measure of the bulk relaxation. Relaxation of PS chains was found to occur faster on the surface relative to the bulk. The magnitude of the surface glass transition temperature suppression over the bulk was estimated to be 18 \r { }C based on the measured temperature dependence of the relaxation rates.   

Does Coarsening Begin During the Initial Stages of Spinodal Decomposition?

We have studied the early stages of spinodal decomposition for a critical 50/50 binary blend of high molecular weight rubbery polymers by time-resolved small angle neutron scattering. Many aspects of the data are described by the well-established linearized theory of Cahn, Hilliard and Cook. In this theory, the time-dependent scattering profiles are given by three time-independent but wave vector ($q)$ dependent functions: the initial structure factor [$S_{0}(q)$], the terminal structure [$S_{t}(q)$], and a kinetic parameter [R(q)]. Phase separation leads to a periodic bicontinuous structure with a well-defined lower cut-off. This lower cut-off is characterized by a pole in $S_{t}(q)$ and $R(q)$=0. The linearized theory also predicts a wave vector ($q_{peak})$ corresponding to a maximum in $R(q)$. Our experiments do not support this prediction. Instead, the scattering peak decreases linearly with timer indicating that coarsening occurs throughout the initial stages of spinodal decomposition.   

Temperature-Dependent Conformational Changes of PNIPAM Grafted Chains in Water: Effects of Molecular Weight and Grafting Density

Poly(N-isopropyl acrylamide) (PNIPAM) is perhaps the most well known member of the class of responsive polymers. Free PNIPAM chains have a lower critical solution temperature (LCST) in water at about 31$^{\circ}$ C. This very sharp transition (about 5$^{\circ}$ C) is attributed to alterations in the hydrogen bonding interactions of the amide groups. Grafted chains of PNIPAM have shown promise for creating responsive surfaces. Conformational changes of the polymer are likely to play a role in some of these applications, in addition to changes in local interactions. In this work we investigated the temperature-dependent conformational changes of grafted PNIPAM chains in D2O over a range of surface density and molecular weight using neutron reflection and AFM. The molecular weight and surface density of the PNIPAM brushes were controlled using atom transfer radical polymerization (ATRP). We discovered a strong effect of surface density and molecular weight. Large conformational changes were observed for intermediate grafting densities and high molecular weights.   

Session L41: Correlated Electrons: Hall Effect and Kondo Physics

Hall effect in CoO2 Layer with Hexagonal Structure

The Hall effect in layered cobalt oxides with hexagonal structure is examined. We have pointed out that the large thermopower in the cobalt oxides is caused by the degeneracy of $t_{2g}$ orbitals in Co ions. Here, we show that the orbital degeneracy brings about a Kagomé lattice electronic structure hidden in the CoO$_{2}$ triangular crystal lattice. This is because the electron hopping occurs between Co ions via neighboring oxygens by exchanging the orbitals in the triangular lattice. The importance of $t_{2g}$ orbital degeneracy on the transport properties of the cobalt oxides under the magnetic field at high temperatures is discussed in light of the theory.   

The anomalous Hall heat current and Nernst effect in the ferromagnetic spinel $\rm CuCr_2Se_{4-x}Br_x$

In a ferromagnet, the anomalous Hall current has been demonstrated to be independent of the electron lifetime $\tau$ [1], which provides a strong support for Karplus and Luttinger's anomalous-velocity theory based on the intrinsic spin-orbit interaction. Inspired by the fact that a heat flow accompanies a charge current, we studied the anomalous Nernst effect (ANE) in the ferromagnetic spinel $\rm CuCr_2Se_{4-x}Br_x$ with x ranging from 0 to 1.0. Combining with other transport parameters measured on the same sample, at low temperatures, we uncovered a simple relation for the off-diagonal Peltier conductivity tensor $\rm \alpha_{xy}$, which is a quantity directly relating to the anomalous Hall heat current. We found that $\rm \alpha_{xy}=C {\it N}_F T$ [2], where C is a constant independent of $\tau$, ${\it N}_F$ is the D.O.S. at the Fermi energy, and T is the temperature. This simple relation turns out to be consistent with the prediction based on the anomalous-velocity theory. Therefore, our observation provides another strong support for the anomalous-velocity theory in a ferromagnet. The AHE and ANE can then be understood in a unified picture. * Supported by funds from the U.S. National Science Foundation under grant DMR 0213706. [1]Lee, {\it et al}., Science 303, 1647 (2004). [2]Lee, {\it et al}., PRL 93, 226601 (2004).   

Unusual magnetization and Hall effect in the spinel chalcogenide (Fe$_{0.5}$Cu$_{0.5}$)Cr$_2$S$_4$

We have measured the magnetization M and Hall resistivity $\rho_{xy}$ in single-crystal (Fe$_{0.5}$Cu$_{0.5}$)Cr$_2$S$_4$, a spinel chalcogenide compound. At the ferromagnetic transition (T$_c$ $\sim$ 280 K), M at weak field (at 100 gauss) displays an unusually abrupt onset vs. temperature T. Below $\sim$120 K, we observe the appearance of a second contribution to the magnetization, which appears as a kink in the field dependence of both $\rho_{xy}$ and M, characterized by a linearly rising segment and a final saturation in high field. We will discuss how these features relate to a possible second ordering arising from the two different sublattices (Fe,Cu) and Cr.   

Hall effect in NdB$_6$

We report results of electrical resistivity, Hall effect and magnetization measurements in a NdB$_6$ single crystal, in a temperature range from 2 to 300 K and in magnetic fields of up to 7 T. We took care to use low magnetic fields ($H_{appl} < 1$ T) in our Hall effect experiments in order to avoid magnetic field smearing out anomalies at the critical points. The Hall resistivity $\rho_H$, which is electronlike, shows a maximum at $T\simeq T_N$. In the paramagnetic (PM) region, we find a linear dependence of $\rho_H/H_{appl}$ on effective susceptibility. This enables us to separate two contributions to the Hall effect, an ordinary and an anomalous one. The estimated anomalous Hall coefficient $R_s$ (holelike) is much larger than the ordinary one in the PM state and is independent of temperature. As the antiferromagnetic (AF) order sets in below $\sim 8$ K, $R_s$ decreases sharply with decreasing temperature. In the AF state, $R_s\propto\rho$, where $\rho$ is the total resistivity. We do not find any significant variations of the anomalous Hall coefficient near the critical point. Both magnetization and Hall voltage show an amomaly in low magnetic fields, which may be related to domain rotations.   

Studies on Single Crystals CePdGa$_{6}$ and Ce$_{2}$PdGa$_{12}$

Single crystals of tetragonal CePdGa$_{6}$ and Ce$_{2}$PdGa$_{12}$ are found to be antiferromagnetic below a T$_{N}$ of 5 and 12 K respectively, with ordering along the $c$-axis. The electronic specific heat coefficients at T$_{N }$are approximately 350 and 140 (mJ/mole-Ce-K$^{2})$ for CePdGa$_{6}$ and Ce$_{2}$PdGa$_{12}$ respectively, suggesting strong Kondo competition. In addition, a metamagnetic transition can be driven by relatively small magnetic fields applied along the $c$-axis in both systems. Field dependent thermodynamic properties and magnetization data will be presented and the origin of the complex ground states will be discussed. This work was supported by NSF DMR-0433560.   

A new mean field theory for the emergence of magnetism in the Kondo lattice

We present a new mean-field theory of the Kondo lattice model, which is exact in the large $N$ limit, that is able to span the heavy magnetic quantum critical point of the Kondo lattice. 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. One of the interesting aspects of the phase diagram in this theory, is the appearance of a new paramagnetic region in which short-range magnetic correlations grow in response to a pairing of the of the Schwinger bosons. We will discuss the evolution of the specific heat and the quasiparticle effective mass in the approach to the heavy electron quantum critical point, and the fascinating possibility that the critical theory involves the interplay of gapless critical excitations both bosonic and fermionic in character.   

Toulouse point of a non-fermi liquid impurity

We present a mapping between a two-channel mixed-valece impurity model and a resonant-level Hamiltonian, that for particular values of the parameters becomes non-interacting. This property is analogous to the Toulouse point of the single-channel Kondo problem or the Kotliar-Si point of the infinite-U Anderson model, but displaying characteristics of non-Fermi-liquid physics like in the Emery-Kivelson mapping for the two-channel Kondo problem. We will discuss briefly some of the quantities that can be calculated by exploiting our mapping.   

Studies of the ferromagnetic Kondo lattice system of single crystal CeZnSb$_{2}$

We have grown single crystals of the ferromagnetic Kondo lattice system CeZnSb$_{2}$. The ferromagnetic ground state was confirmed by neutron scattering. In addition, a small hysterisis in magnetization data indicates soft ferromagnetism. The large heat capacity coefficient (350 mJ/mol-Ce K$^{2})$ below the transition temperature, on the other hand, suggests a heavy mass state even below the magnetically ordered state. Thermodynamic, magnetic, and transport properties along with pressure experiments will be presented. The origin of such unusual magnetism will be discussed. This work was supported by NSF DMR-0433560.   

Two-fluid behavior of the Kondo lattice in the $1/N$ slave boson approach.

It has been recently shown by Nakatsuji, Pines, and Fisk [S. Nakatsuji, D. Pines, and Z. Fisk, Phys. Rev. Lett. \textbf {92}, 016401 (2004)] from the phenomenological analysis of experiments in Ce$_{1-x}$La$_x$CoIn$_5$ and CeIrIn$_5$ that thermodynamic and transport properties of Kondo lattices below coherence temperature can be very successfully described in terms of a two-fluid model, with ``Kondo gas" (interacting magnetic moments) and ``Kondo liquid" (heavy electron Fermi liquid) contributions. We analyze thermodynamic properties of Kondo lattices using $1/N$ slave boson treatment of the periodic Anderson model and show that these two contributions indeed appear below the coherence temperature. We find that the ``Kondo gas" contribution to thermodynamics corresponds to thermal excitations into the flat portion of the energy spectrum. As a result, the relative fraction of ``Kondo gas" depends exponentially on temperature, rather than linearly in experiment.   

Optical study of rival interactions in a d-electron ferromagnetic Kondo system

We report on a combined optical, transport and thermodynamic study of the Zintl compound Yb$_{14}$MnSb$_{11}$, demonstrating that it is the first ferromagnetic Kondo lattice in the underscreened limit. We suggest a scenerio whereby the combination of Kondo and Jahn-Teller effects provides a consistent explanation of both transport and optical data.   

The Exact Solution for the Physical Properties of a Heinsenberg-Kondo Spin Glass, Including the Superconductor Properties of this System

We have carried out an exact solution for many physical problems of a Heisenberg-Kondo spin glass. Results will be reported over a wide range of system parameters of 1) the tunneling density of states $N_T \left( {eV} \right)$, the arpes cross section $d\sigma /d\omega d\Omega $, the $I.R_\lambda $and optical conductivities, the susceptibility $X_{\alpha \beta } \left( {Q,\omega } \right)$ the Roman cross section $R_{\alpha \beta } \gamma \left( {\omega ,v} \right)$, the specific heat $C_v \left( t \right)$, thermal conductivity $K_{Th} \left( T \right)$, etc. The influence of phonons on the system will be discussed, including the structure of the spin-phonon-polaron and it's propagation.   

Decoupling methods for the solution of the dynamical mean field theory impurity problem

The use of equation of motion decoupling methods as impurity solvers to be used in conjunction with the dynamical mean field self-consistency condition for the solution of lattice models is explored. By comparing the impurity solver to exact diagonalization results and applying it to lattice models like Hubbard model and Periodic Anderson model it is shown that the method could be a cheap alternative to computationally demanding methods like quantum Monte Carlo. The method works in a large range of parameters and promises to be useful in combination with density functional theory for the study of strongly correlated materials.   

Magnetic and Kondo Behavior of CeCoGe$_{2}$

There is great interest in exotic states among Ce-Kondo lattices. Recently, specific heat and magnetic susceptibility of CeCoGe$_2$ have been accounted by the S=5/2 Kondo model; i.e., no crystalline electric field (CEF) effects have been observed.$^1$ This result is surprising since related $^2$ CeNiGe$_2$, belonging to this crystal structure, exhibits strong anisotropy in magnetic and transport properties due to CEF’s. Our magnetic susceptibility and specific heat results on (Ce,La)CoGe$_2$ suggest that exotic properties of this system are due to magnetic interactions in a quasi-two-dimensional Ce-lattice. This work was supported by NSF, grant DMR-0104240. \\ \\ $^1$ E. D. Mun et al., Phys. Rev. B {\bf 69}, 85113 (2004).\\ $^2$ Y. Okada et al., J. Phys. Soc. Jpn. {\bf 72}, 2692 (2003).   

Session L42: Focus Session: Magnetic Nanoparticles, Nanostructures & Heterostructures V

Intergranular Exchange in Magnetic Nanostructures

Exchange interactions determine not only atomic-scale properties such as the Curie temperature but are also paramount to the realization of mesoscopic magnetism. Nanoscale exchange reflect the relativistic origin of magnetism. On an atomic scale, interatomic exchange tends to be much stronger than magnetic interactions, but the quadratic wave-vector dependence of the exchange energy makes magnetic interactions competitive on a nanoscale. The corresponding characteristic length scale is $a_{o}$/\textit{$\alpha $} = 7.252 nm, where $a_{o}$ is the Bohr radius and \textit{$\alpha $} = 1/137 is Sommerfeld's fine structure constant. In homogeneous solids, the competing relativistic and nonrelativistic interactions determine, for example, the thickness of domain walls. In nanostructures, the situation is more complex, because mesoscopic and atomic exchange effects interfere with structural length scales. This is important in many areas of magnetism, such as permanent magnetism, soft magnetism, spin electronics, and magnetic recording. (For a recent review, see Skomski, J. Phys. CM, vol. 15, 2003, p. R841.) From an atomic point of view, local magnetic moments embedded in an itinerant electron gas are coupled by RKKY-type interactions, whose oscillatory period is determined by the Fermi wave vector $k_{F}$. First, RKKY interaction between embedded clusters or particles do not average to zero but actually \textit{increase} with particle size. Second, the low carrier densities of semimetals and semiconductors yield small Fermi wave vectors and nanoscale oscillation periodicities. From a mesoscopic point of view, traditional random-anisotropy scaling amounts to a dimensionless coupling constant $A$/$K_{1}R^{2}$, but this expression fails to account for important real-structure features. For example, grain boundaries with reduced interatomic exchange give rise to a quasi-discontinuity of the magnetization, create a magnetization perturbation that extends far into the bulk, and modify scaling relations for the coercivity and other quantities. Additional complexity is due to finite-temperature excitations. Nanostructuring has little or no effect on the Curie temperature, but strongly affects the temperature dependence of the coercivity and the magnetization dynamics.   

Classifying magnetic interactions in nanoparticle assemblies using Cole-Cole plots

Measuring the magnetic ac susceptibility, $\chi = \chi' - i \chi''$, of nanoparticle assemblies leads to important conclusions about the role of inter-particle interactions. We demonstrate that the Cole-Cole plot, $\chi''$ vs. $\chi'$, can be used as a tool for classifying the magnetic behavior of interacting or non-interacting nanoparticles. Distinguishing non-interacting superparamagnetic from superspin glass (SSG) or superferromagnetic (SFM) behavior is easily possible. We performed measurements of the magnetization hysteresis and ac susceptibility on both SSG and SFM granular multilayers [Co$_{80}$Fe$_{20}$/Al$_{2}$O$_{3}$] $_n$. The SFM results are successfully compared to simulations of a driven domain wall in a random medium. \noindent Work supported by the DFG, Alexander von Humboldt Foundation and DAAD.   

Transport spectroscopy of Kondo quantum dots coupled by RKKY interaction

We develop the theory of conductance of a quantum dot which carries a spin and is coupled via RKKY interaction to another spin-carrying quantum dot. The found dependence of the differential conductance on bias and magnetic field at fixed RKKY interaction strength may allow one to distinguish between the possible ground states of the system. Transitions between the ground states are achieved by tuning the RKKY interaction, and the nature of these transitions can be extracted from the temperature dependence of the linear conductance.   

Infrared Reflectivity of Metal Transition Granular Films

We present infrared reflectivity of 550 nm granular films made of transition metals embedded in a SiO$_{2}$ amorphous matrix. TM$_{x}$(SO$_{2}$)$_{1-x}$(TM=Fe, Ni, Co),(0.25$ \leq $x$ \leq $0.85) display giant magnetoresistance and giant Hall effect slightly above the percolation threshold.Our samples yield spectra typical of conducting oxides where carrier localization, depending on concentration, is triggered by the nanoparticles roughness and SiO$_{2}$. They have a distinct Drude component raising in intensity as the concentration and/or size of TM nanocrystallites increases Thus, while granular films of Fe and Ni, x$\sim$ 0.85, above the percolation threshold, have a reflectivity with a tail extending beyond 0.8 eV, characteristic of carrier hopping conductivity (small polaron localization), reducing the size and amount of the transition metal crystallites, as for x$\sim$0.35, the spectra have mid-infrared bands known for a localized polaron scenario with a very strong longitudinal optical-electron interaction. Our results support a model involving transition metal d-orbitals hybridizing oxygen p orbitals, yielding, as the crystallites get closer, the crossover between semiconducting and metal-like behavior within the context of a mixed phase environment.   

Extraordinary Hall Effect in (Ni80Fe20)x(SiO2)1-x Thin Films

The extraordinary Hall effect (EHE) in ferromagnetic samples is generally attributed to scatterings of iterant electrons in the presence of spin-orbit interactions. In this work, study on the thickness dependence of the EHE in the $(\mbox{Ni}_{\mbox{80}} \mbox{Fe}_{\mbox{20}} )_x (\mbox{SiO}_{\mbox{2}})_{1-x} $ system showed the spontaneous Hall resistivity, $\rho _{sy}^S$, to be a constant while the Hall coefficient, $R_S (\equiv \rho _{sy}^S /M_S$ where $M_S $ is the saturated magnetization) increased monotonically owing to a depression in $M_S $. We propose the constancy of $\rho _{sy}^S $ with reducing thickness to arise from the sample morphological structure becoming two-dimensional with decreasing film thickness, expected from classical percolation theory. We also find in this system with varying $x$ that $\rho _{sy}^S \propto \rho _{xx}^\gamma $, with $\gamma =0.53$ to 1 in disagreement with the value 2 frequently attributed to the side jump effect, but explainable in terms of the more general form $\rho _{sy}^S =\rho _{xx} \Delta y_e /\Lambda _{SO} $, where $\Delta y_e $ is the side jump displacement and $\Lambda _{SO} $ the spin-orbit mean-free-path.   

Interaction effects in heterostructures of nanoscale magnetic particles and magnetic thin films

Elongated magnetic nanoparticles attract continuing attention both because of potential technological applications such as high-density information storage. Particles of 5 - 15 nm in diameter may be grown by STM assisted CVD, an advantageous technique for exact positioning of individual Fe particles on different substrate materials. A first step towards an ultimate application of these small and local magnetic flux sources to intentionally influence and investigate other materials is to study heterostructures of magnetic particles and an underlying magnetic film. Growing arrays or individual particles onto a magnetic thin film strongly enhances interactions between adjacent particles. Also, the particles alter the magnetic domain structure of the magnetic thin film making the transport properties of the latter sensitive to the magnetization state of the particles grown on top. We will present magnetization measurements of magnetic nanoparticles/thin-film heterostructures with permalloy and the concentrated magnetic semiconductor EuS as film material. These results will be compared with measurements of non-interacting particles grown onto a non-magnetic gold film. We also studied the effect of the particle's magnetization state on the transport properties of a magnetic permalloy film. We find a distinct (negative) switching effect in the magnetoresistance that persists up to room temperature.   

Magnetic multilayers on nanospheres

Nanoparticle media using arrays of monodisperse nanoparticles with high magnetic anisotropy are assumed to be the ideal future magnetic recording media [1]. However, key requirements like control of the magnetic anisotropy orientation along with magnetic domain isolation have not been achieved so far. Here, we report on a combination of a two-dimensional topographic pattern formed of self-assembled polystyrene particles [2] with sizes as small as 20 nm and magnetic film deposition. The so formed nanostructures on top of a sphere are monodisperse and reveal a uniform magnetic anisotropy which can be tailored by changing the stack of a Co/Pd multilayer film and the deposition angle. Magnetic exchange isolation depends strongly on the total film thickness and the particle size as observed by MFM imaging and MOKE studies. Moreover, results on the switching mechanism as a function of nanostructure size will be presented. [1] M. Albrecht et al., Physik Journal, 10 (2003) [2] F. Burmeister et al., Appl. Surf. Sci. 144-145, 461 (1999)   

Observation of an anisotropy-induced antiparallel-parallel switching at the interface of Fe3O4/Mn3O4 superlattice on MgO(011)

An anisotropy-induced magnetic phase transition is first time observed from magnetization vs. field measurement in an antiferromagnetic coupled Fe3O4/Mn3O4 superlattice on MgO(011). Relative to a twisted phase transition previously found in isotropic layer systems, the present transition only occurs along the easy axis in the plane. An abrupt increased magnetization associated with the on-set of the transition corresponds to a direct switching of spin from an antiparallel state to a parallel state at the interface. Large magnetic hysteresis associated with the spin switching are observed on H greater than 0 and H smaller than 0 and thus 4 stable magnetization stages exist in the present system. The critical external field provides a direct estimate of the anisotropy energy of the superlattice. Magnetic hysteresis curves measured at various temperatures further provide a quantitative understanding of the interface coupling of Fe3O4/Mn3O4 superlattices.   

Kondo physics with organic molecules

The presence of a magnetic impurity on a nonmagnetic metal substrate is known to give rise to a Kondo resonance in the local density of states (LDOS) of the sample. We have carried out a low temperature STM study on a Co-porphyrin molecule, 5, 10, 15, 20-Tetrakis-(4-bromophenyl)-porphyrin-Co (II) 98{\%}, adsorbed on a Cu(111) substrate. Single molecules as well as self-assembled molecular layers (SAM) were studied by scanning tunneling spectroscopy at 12 K. A suppression of the LDOS at the Fermi energy was observed and is explained in terms of the Kondo resonance. Electronic properties of molecules are crucial for the design of new molecular electronic devices. This work is financially supported by the US-DOE grant: DE-FG02-02ER46012.   

Low-Temperature Expansion of a Model Describing Helical Magnetic Phases in Rare-Earth Heterostructures

The variety of magnetic phases observed in rare-earth heterostructures at low temperatures, such as Ho/Y, may be elucidated by an ANNNI-like model Hamiltonian. In previous work modeling bulk Ho, such a Hamiltonian with a one-dimensional parameter space (possibly pressure) produced a single multicritical point, in consequence of which there was no long-range order at zero temperature. In contrast, the parameter space of the heterostructure model is three-dimensional, and instead of an isolated multicritical point, we find two-dimensional multicritical regions. In an example of Villain's ``order from disorder,'' an infinitesimal temperature breaks the ground-state degeneracy. In first order of a low-temperature expansion, we find that the degeneracy is broken everywhere in a multicritical region except on a line. In higher orders, the line may give way to a set of isolated multicritical points, or it may vanish entirely, or it may remain a multicritical line. We present exact computational results on the fate of a multicritical region in this model.   

Effect of Dipolar Interactions on the Magnetization of Single-Molecule Magnets in a cubic lattice

Since the one-body tunnel picture of single-molecule magnets (SMM) is not always sufficient to explain the fine structure of experimental hysteresis loops, the effect of intermolecular dipolar interactions has been investigated on an ensemble of 100 3D-systems of 5X5X4 particles, each with spin S = 5, arranged in a cubic lattice. We have solved the Landau-Lifshitz-Gilbert equation for several values of the damping constant, the field sweep rate and the lattice constant. We find that the smaller the damping constant is, the stronger the maximum field needs to be to produce hysteresis. Furthermore, the shape of the hysteresis loops also depends on the damping constant. We also find that the system magnetizes and demagnetizes faster with decreasing sweep rates, resulting in smaller hysteresis loops. Variations of the lattice constant within realistic values (1.5nm and 2.5nm) show that the dipolar interaction plays an important role in magnetic hysteresis by controlling the relaxation process. Examination of temperature dependencies (0.1K and 0.7K) of the above will be presented and compared with recent experimental data on SMM.   

Dispersion of magnetic nanoparticles in polymer films

Magnetic nanoparticles embedded in polymer matrices have excellent potential for EMI shielding and biomedical applications. However, uniform dispersion of particles in polymers without agglomeration is quite challenging. We have fabricated PMMA/polypyrrole bilayer structures embedded with Fe$_{3}$O$_{4}$ magnetic nanoparticles (mean size $\sim $ 12 nm) synthesized using wet chemical methods. The magnetic polymer nanocomposites were spin-coated on various substrates. Agglomeration-free dispersion of nanoparticles was achieved by coating the particles with surfactants and by dissolving both the particles and PMMA in cholorobenzene. Structural characterization was done using XRD and TEM. Magnetic properties of the bilayer structures indicated that the superparamagnetic and ferromagnetic response of the polymer nanocomposites including parameters such as the coercivity, remanence and saturation magnetization could be systematically varied with controlled amounts of nanoparticle dispersions in the polymer media. The RF impedance up to frequencies of 3 \textit{GHz }measured using a vector network analyzer will also be presented. Overall, we demonstrate that magnetic polymer nanocomposite films are excellent candidates for EMI suppression applications. Work supported by NSF through Grant No. ECS 0140047   

Chromium Oxide Clusters as Building Blocks to Nanoscale Materials

It is shown that by varying the formation conditions, two distinct families of stable chromium oxide nanoparticles can be generated, each with unique electronic and magnetic properties. The clusters are found to demonstrate remarkable stability, acting as building blocks and providing the framework to form extended structures. By use of gradient corrected density-functional theory and mass spectra data, we demonstrate that different classes of stable oxygen-passivated chromium oxide clusters have class-specific electric and magnetic properties. More specifically, irrespective of cluster size, the Cr$_{n}$O$_{2n+2}$ cages are ferromagnetic, and the saturated rings of Cr$_{n}$O$_{3n}$ are nonmagnetic. The rings are characterized by high electron affinities in addition to their stability and an investigation into the formation of ionic molecules upon combination with alkali atoms is addressed.   

Session L43: Focus Session: Phase Complexity and Enhanced Functionality in Magnetic Oxides I

Evidence of magnon phonon interaction in La$_{0.70}$Ca$_{0.30}$Mn$O_3$

One of the unresolved issues in the understanding of the spin dynamics of the CMR manganites is the anomalous softening and damping of the magnons near the zone boundary observed in A$_{1-x}$B$_{x}$MnO$_{3}$ (A=La, Pr, Nd, etc; B=Ca, Sr, etc. x $\sim $ 0.30). [1], [2]. While many theoretical explanations have been formulated to explain this phenomena we proposed that these anomalous features are related to a strong magnon-phonon coupling expected from the strong magnetoelastic interactions in the CMR manganites.[2] In this talk we present neutron scattering evidence for the coupling of the magnons and a severely damped branch of optical phonons in La$_{0.70}$Ca$_{0.30}$MnO$_{3}$. ORNL is managed by UT-Battelle, LLC, for the U.S. Dept. of Energy under contract DE-AC05-00OR22725. [1] H. Y. Hwang et al, Phys. Rev. Lett 80, 1316 (1998) [2] P. Dai et al, Phys. Rev. B 61, 9553 (2000)   

Electron-phonon coupling of La2-2xSr1+2xMn2O7 revealed by ARPES

Angle-resolved photoemission experiments were performed on Bi-layer manganite La$_{2-2x}$Sr$_{1+2x}$Mn$_{2}$O$_{7}$ in the doping regions where strong CMR effects are observed. The low temperature electronic band structure near the (pi, 0) point shows a clear kink structure with an energy scale near 65 meV. We attribute this kink structure to electrons coupling to the LO bond-stretching phonons with a coupling constant $\sim $ 1. Moving along the straight nested segments of Fermi surface near the zone boundary, we didn't find any change of the kink energy or coupling strength. This indicates the nested Fermi surface is strongly correlated to electron-phonon coupling, which should play an important role in the rich physics of manganites.   

Acoustic phonon instabilities in La$_{1-x}$Ca$_{x}$MnO$_{3}$

We report on time-resolved ultrafast optical measurements on a La$_{1-x}$Ca$_{x}$MnO$_{3}$ (LCMO) single crystal ($x$= 0.3) and thin film ($x$= 0.33). The differential reflectivity shows coherent GHz oscillations due to the excitation of longitudinal acoustic phonons. The wavelength dependence of the period of such oscillations allow us to determine the sound velocity. The measured GHz sound velocity is 30\% - 40\% larger than the sound velocity in the MHz range as determined by resonant ultrasound measurements [Jin et al., PRL 90, 036103 (2003)]. Temperature- dependent data show that the sound velocity anomaly disappears at T \symbol{126}350 K, a value that is close to the temperature for which charge-ordered clusters form.   

Spatially Resolved Photoemission Spectroscopy to Probe Electronic Phase Separation in Manganites and Related Compounds

Manganese oxides that exhibit colossal magnetoresistance (CMR) are often characterised by a competition of different electronic phases that critically influence their properties and leads to the coexistence of spatially separated competing phases. Despite extensive experimentation, characteristic length-scales associated with phase coexistence remains an important open question. While theoretical work has pointed to a nanometric length-scale, experiments have uncovered multiple length-scales ranging from the atomic to the sub-micron, covering many orders of magnitude. The role of chemical inhomogeneity in driving this phenomenon is not well understood. Moreover, these early experiments were carried out on polycrystalline and thin film specimens. Here we use a spatially resolved, direct spectroscopic probe for electronic structure with an additional unique sensitivity to chemical compositions to investigate high quality single crystal sample of La$_{1/4}$Pr$_{3/8}$Ca$_{3/8}$MnO$_{3}$. The formation of distinct electronic domains is observed in absence of any perceptible chemical inhomogeneity, where the relevant length-scale is at least an order of magnitude larger than all previous estimates. The present results, exhibiting memory effects in the domain morphology, suggest that electronic domain formation is intimately connected with long-range strains, often thought to be an important ingredient in the physics of this effect. Additionally, we have also applied this technique to a variety of related materials, such as (LuMnO$_{3})_{0.79}$(La$_{5/8}$Sr$_{3/8}$MnO$_{3})_{0.21}$, and Sr$_{2}$Fe$_{x}$Mo$_{1-x}$O$_{6}$. Our preliminary results in all these cases suggest that the existence of spatially inhomogeneous electronic phases plays important roles in determining many of the interesting properties of such systems. This work is carried out in collaboration with M. Bertolo, G. Cautero, S-W. Cheong, A. Fujimori, T. Y. Koo, S.R. Krishnakumar, U. Manju, S. Ray, S. La Rosa P. A. Sharma and D. Topwal.   

Magnetic and Electronic Properties of Ln$_{1-x}$Sr$_x$CoO$_3$

The magnetic and electronic properties of bulk, polycrystalline, Ln$_{1-x}$Sr$_{x}$CoO$_{3}$ (Ln=La, Nd and Pr, 0$\le $x$\le $0.6) were investigated. The La and Nd systems show a crossover from insulating glassy behavior to metallic ferromagnetism at x=0.18. The only differences between the two systems are the lower transition temperatures for Nd (due to reduced bandwidth) and the antiferromagnetic alignment of Nd and Co, inducing ferrimagnetism. Pr$_{1-x}$Sr$_{x}$CoO$_{3}$ however, is radically different. A similar cluster glass to ferromagnet crossover occurs at low doping but at higher doping the system exhibits a second magnetic transition [1], well below $T_{C}$. Through a systematic investigation as a function of composition we have discovered that this effect is maximized at half-doping, vanishing completely at x$<$0.40 and x$>$0.60. Possible explanations (including ferrimagnetism, spin-state transitions, phase competition, and charge/orbital ordering) are discussed in light of the neutron diffraction, transport and magnetometry results. We acknowledge support from the ACS Petroleum Research Fund. [1] R. Mahendiran and P. Schiffer, Phys. Rev. B. \textbf{68} 024427 (2003).   

Transverse susceptibility as a probe of spin and charge dynamics in LSMO single crystals

AC and RF susceptibility measurements are excellent probes to investigate the charge and spin dynamics in novel materials such as half-metals, exchange bias materials, intermetallic Kondo systems and manganites. We will present temperature and field-dependent transverse susceptibility (TS) measurements on {\$}La$_{1-x}$ Sr$_{x}$MnO$_{3}${\$}; (x=0.1, 0.15, 0.25) single crystals at radio frequency (10 MHz) using a precise resonant tunnel-diode oscillator technique. These dynamic experiments probe the coupled electronic, magnetic and structural transitions in LSMO. The effective magnetic anisotropy fields are directly probed over a wide range in temperature, from the ferromagnetic Curie temperature to low temperatures well into the charge-ordered state in the insulating samples. Our experiments reveal distinct features at characteristic temperatures within the charge-ordered state that indicate a change in magnetic anisotropy associated with the dynamics of charge ordering itself. Overall, we demonstrate the unique advantages offered by RF susceptibility measurements in probing magnetic phase transitions in CMR oxides and other novel materials.   

Optical and magneto-optical studies of ferromagnetic manganese oxides thin films

We report on the optical and magneto-optical (MO) properties of ferromagnetic La$_{0.7}$Sr$_{0.3}$MnO$_3$, La$_{0.7}$Ca$_{0.3} $MnO$_3$, La$_{0.7}$Ce$_{0.3}$MnO$_3$, and La$_{0.7}$MnO$_3$ thin films epitaxially grown on SrTiO$_3$ substrate. The optical reflectance and transmittance of the samples were measured over a broad frequency range (50-52000 cm$^{-1}$) and at temperatures between 20 and 340 K. To extract the optical constants of the films, we analyzed all of the layers of this thin-film structure using a Drude-Lorentz model. The MO polar Kerr spectra of the samples were measured in an applied magnetic field of 1.5 Tesla between 0.74 and 5.6 eV. The off-diagonal components of the dielectric tensor were calculated by analyzing Kerr rotation, ellipticity, and the determined diagonal elements of the dielectric tensor. These data clearly show that accurate values of diagonal and off-diagoanl components of the dielectric tensor are important for the spin-polarized band-structure studies in the ferromagnetic manganese oxides thin films.   

Phase diagram of La1-xCaxMnO3 thin films in 0.40 = x = 0.45

The $x-T$ phase diagram of La$_{1-x}$Ca$_{x}$MnO$_{3}$ films is expected to be very different from that of polycrystalline samples, due to epitaxial strain. Two series of films differing in composition by $x$=0.01 were prepared in the range 0.40 $\le $ x $\le $ 0.45 by pulsed laser deposition on SrTiO$_{3}$ (001) and NdGaO$_{3}$ (001) substrates. Ferromagnetism was found in all samples, with the ferromagnetic fraction decreasing with increasing $x$. Low temperature metallic behaviour was only observed for $x \quad \le $ 0.41.   

TEM Study of Two-Dimensional Incommensurate Modulation in La(2-2x)Ca(1+2x)Mn2O7 (0.6 Leonid A. Bendersky, Ian D. Fawcett, Martha Greenblatt

Ruddlesden-Popper n = 2 compounds La$_{2-2x}$Ca$_{1+2x}$Mn$_{2}$O$_{7}$ with 0.6 x 0.8 were synthesized by a citrate gel technique. Electron diffraction identified the presence of two-dimensional incommensurate modulations. The modulation is sensitive to electron-beam irradiation and rapidly degrades under a focused beam. In-situ heating experiments indicated the existence of possible tetragonal-to-incommensurate phase transition around 350 $^{o} $C. High-resolution imaging proves that the modulation in the real space is truly two-dimensional and incommensurate. Such modulation has never been observed before for La-Ca-Mn-O or any other perovskite system. The best approximation of the incommensurate superstructure is by a commensurate lattice \textbf{a}$_{x}$' = \textbf{a}$_{xt }$+ 2\textbf{a}$_ {yt}$; \textbf{a}$_{y}$' = -2\textbf{a}$_{xt}$ + \textbf{a}$_{yt}$ ; \textbf{c}' = \textbf{c}$_{t}$ (t-tetragonal I4/mmm).$_{ }$ Such superstructure could be satisfied with a 4:1 two-dimensional ordering in a (001) plane. For the studied compositions, such 4:1 ordering is plausibly described by charge ordering between Mn$^{4+}$ and Mn$^{3+ }$ions. Structural models of the charge ordering between Mn$^{4+}$ and Mn$^{3+ }$ions are suggested.    Preview Abstract]

Variable temperature magnetic force microscopy of patterned La$_{0.6}$Ca$_{0.4}$MnO$_3$ devices

Patterned La$_{0.6}$Ca$_{0.4}$MnO$_3$ films have been fabricated into planar devices with the intention of studying the properties of domain walls in colossal magnetoresistive films doped near phase boundaries. The phase balance is very sensitive near phase boundaries and may lead to interesting phenomena associated with domain walls. The field dependent transport data from these devices show features which suggest domain configuration changes that can be imaged by magnetic force microscopy (MFM). We present variable temperature MFM images of these devices in applied fields and correlate the field dependent micromagnetic structure to the field dependent transport data.   

Transport Noise in Ordered Layered LCMO

Both intrinsic alloy disorder and strain effects have proposed as important sources of the mixed-phase colossal magnetoresistive regime in manganites We have measured transport properties in films of La$_{2/3}$Ca$_ {1/3}$MnO$_{3}$ prepared by atomic-layer-by-layer epitaxy on nearly lattice- matched NGO substrate, both as standard random alloys and as nominally ordered multilayers of LaMnO$_{3}$ and Ca$_{1/3}$MnO$_{3}$. The ordered multilayer samples show a similar transition to the alloys, but with a slightly reduced T$_{C}$. The width in T of the coexistence regime, either determined form the standard R(T) or from resistance noise, is actually slightly greater for the ordered samples than for the alloy. In both cases it is much less than found for alloys under tensile strain on STO or films on STO with e-beam damage. The apparent interpretation is that pointlike disorder plays a smaller role than do strain effects and correlated disorder. Other measurements of ac magnetoresistance, non-linear transport, and noise statistics also indicate the importance of strain interactions.   

High magnetic field phase diagram of multiferroic DyMn$_{2}$O$_{5}$ up to 45 T

Strong magnetoelectric coupling in multiferroic crystals such as ReMn$_{2}$O$_{5}$ (Re=rare earth) has provided unprecedented opportunity to manipulate ferroelectric (FE) polarization using magnetic fields. Most of investigations to dates have yet been limited to a rather low magnetic field region. Herein, we present the first high magnetic field (B) versus temperature phase diagram of DyMn$_{2}$O$_{5}$ in magnetic fields up to 45 T and temperatures below 50 K, determined from the dielectric constant, pyroelectric, and magnetoelectric current measurements using various magnets; superconducting magnets up to 17 T, a dc resistive magnet up to 33 T, and a mid-pulse magnet up to 45 T. Our phase diagram reveals that at least 4 different kinds of FE domains, which show strong temperature-and field-history- dependence, develop at low temperatures below 40 K, and exhibit dramatic evolution under B. For example, as B increases at 4 K, FE polarization shows successive flopping at B$\sim$2, 7 T and 22 T, producing large dielectric constant changes, $\Delta$$\epsilon$(B)/$\epsilon$(0 T) $\sim$15, 70, 20\%, respectively. We discuss the complex phase diagram in the context of strong spin-lattice coupling that is linked to the exchange interaction between Dy f- and Mn d-spins.   

Magnetic and charge correlations in the 2D Hubbard model on a triangular lattice

The high temperature superconductors have motivated numerous theoretical studies of strongly correlated many-body systems for nearly two decades. The richness of the phase diagram of these materials belies their relatively simple quasi-two-dimensional structure of stacked CuO$_2$ planes, where copper ions form a square lattice. With the recent discovery of superconductivity in Na$_x$CoO$_2$ $\cdot$ yH$_2$O, the physics community has an experimental system where strong electron correlations in a quasi-two-dimensional environment are further complicated by geometric frustration arising from the triangular lattice structure of the cobalt ions. Motivated by the interesting experimental phase diagram of Na$_x$CoO$_2$ $\cdot$ yH$_2$O, we investigate the 2D Hubbard model on a triangular lattice at different filling fractions. We report preliminary results from a numerical Hartree-Fock calculation for the magnetic and charge correlations in our model. We also discuss the potential application of the constrained path quantum Monte Carlo (CPQMC) method to the study of frustrated 2D Fermi systems.   

Session L44: Panel Discussion with APS Journal Editors

Panel Discussion with APS Journal Editors

All are invited to a panel discussion with the Editors of the American Physical Society journals. The panel will include Editors from Physical Review Letters, Physical Review B, and Physical Review E. They will briefly discuss some current issues facing the journals such as how to express appreciation for good refereeing, possible inclusion of popular abstracts in PRL to make Letters accessible to physicists in all fields, the challenge posed by open access, etc. The Editors look forward to hearing opinions on these and other issues. They will also respond to questions and comments.