Session J1: Vortex Avalanches

Self-organized criticality of vortices in superconducting films

Magnetic flux dynamics in continuous and periodically patterned Nb films is studied using the magneto-optical imaging technique. Slow vortex penetration forming weak flux gradients and smooth flux fronts characterize the magnetization process at elevated temperatures. At low temperatures, microavalanches (irregular jumps of flux bundles with preferential orientation along field gradients) dominate the flux entry and exit and successive redistribution of the vortex density. Thus formed critical state is probed using scaling analysis of the correlation functions, lengths, width, and power spectra of fractal induction profiles in the samples. The resulting Housdorff and roughness exponents correspond to 1+1 dimensional nonlinear flux diffusion in systems with quenched disorder and long range correlations. The power spectra scaling confirms the self-affine character of the dynamically formed critical state. $\backslash $The nature of the matching effects in periodically patterned samples is reexamined. It is established that neither flat vortex distributions nor terraced states are realized at the fields corresponding to the integer number of vortices per hole. Rather, stronger flux gradients are formed at these fields indicating to the increased average pinning at matching conditions. Macroscopic thermo-magnetic avalanches (TMA) resulting in catastrophic magnetization jumps appear at T$<$4.5K. The sample structure is shown to be crucial for the development of thermally assisted flux instabilities, which follow the topography of the strongest pinning centers in the films. These observations will be analyzed using recent theoretical TMA models. This work was supported by the U.S. Department of Energy, Basic Energy Sciences, under Contract No. W-31-109-ENG-38.   

Avalanches on vortex piles

The Bean state of pinned vortex matter very much resembles the slope of a pile of sand, as noted a long time ago by de Gennes. Recently, we discovered that this similarity goes much further: (i) avalanches occur on the slopes of both systems (ii) a close relation exists between the statistical properties of the (vortex) pile surface and those of the avalanches. We find that the punctuated behavior of the avalanches falls in the class of Self-Organized Criticality (SOC). The intriguing relation between the amount of disorder and the onset of SOC behavior was investigated in Niobium thin films, where disorder was introduced by adding interstitial hydrogen atoms, absorbed from the controlled surrounding gas. In Niobium deposited on R-plane sapphire we find that a minimum amount of disorder (created by absorbing hydrogen) is necessary for SOC to occur. In Niobium on A-plane sapphire, huge compact avalanches are observed. The behavior of these avalanches is compared to a recent model by Aranson et al., who e.g. predict a minimum amount of penetration before avalanches can occur, which is corroborated by experiment.   

Dynamics of Magnetic Flux Avalanches in Superconducting Films

Magnetic flux penetration into superconducting films can occur along two different scenarios: either in the form of homogeneously propagating flux fronts, or as a dendritic instability with branch-like flux avalanches propagating into the previously flux-free reagion of the superconductor. Since the relevant time scale for these processes in the case of thin films is in the nanosecond range, we have developed a fast pump-probe technique for magnetooptic imaging. The method is based on nucleating an event (e.g. the formation of a flux avalanche in a superconductor) by means of a femtosecond ``pump'' laser pulse, and taking a magnetooptic snapshot of the developing flux distribution by a delayed ``probe'' beam. The time resolution of this technique is given by the response time of the magnetooptic garnet films used, which in our experiment is about 100ps. Using this technique have investigated the dendritic instability for various film materials (e.g.YBa$_2$Cu$_3$O$_{7-d}$ and MgB$_2$) and have constructed a ``stability diagram'' which separates regions with homogeneous flux penetration from unstable ones. In addition we have studied systematically the influence of relevant parameters like film thickness and external magnetic field on the propagation characteristics of the flux dendrites. The experimental results are compared with a theoretical model for dendrite propagation, and good agreement is found.   

Size distribution of flux avalanches in MgB2 films visualized by magneto-optical imaging

We report on the quantitative and spatially resolved observation of flux avalanches in superconducting films. Magneto-optical imaging was used to visualize the flux penetration in MgB2 films subjected to a slowly varying perpendicular field. Below 10 K, flux avalanches with typical size around 20 microns and regular shape are found to occur at random locations along the flux front. The total number of vortices that participates in one avalanche is varying between 50 and 10000 [1]. An adiabatic model is proposed to calculate the flux jump size for a thin-strip superconductor. The flux density and temperature distributions in the final state after flux jump are calculated. The jump size is found to grow monotonously with applied field and this dependence is in a good agreement with experimental data. At larger applied fields we observe another kind of jumps: a much bigger dendritic and branching avalanches [2,3]. Their dimensions are limited only by the sample size, while their morphology can be described within a linear model based on Maxwell and thermal diffusion equations [4]. References: [1] A. V. Bobyl et al., Physica C 408-410, 508 (2004) [2] T. H. Johansen et al., Europhys. Lett. 59, No. 4, 599-605 (2002) [3] F. L. Barkov et al., Phys. Rev. B 67, 064513 (2003) [4] A. L. Rakhmanov et al., Phys. Rev. B 70, 224502 (2004)   

Vortex nanoliquid in high-temperature superconductors

Vortex matter is commonly considered as a homogenous glassy medium. Correlated disorder in the form of columnar defects (CDs) is shown to result in formation of new heterogeneous phases of vortex matter. We have developed a magneto-optical method that allows visualization of the distribution of small transport currents applied to BSCCO crystals irradiated through patterned masks [1]. When vortices outnumber CDs we identify two distinct populations: vortices residing on CDs are strongly pinned and form a rigid `porous' skeleton, whereas the excess vortices form weakly pinned ordered crystallites caged within the pores of the skeleton [2,3]. The melting process of this porous vortex matter is qualitatively different from melting of a homogenous system. The soft crystallites melt while the rigid skeleton remains in tact, forming a vortex nanoliquid in which intercalated liquid droplets of just few vortices are embedded in a porous solid matrix. The nanoliquid phase possesses unique properties and displays a high degree of correlation along the c-axis but no transverse critical current. The melting of heterogeneous vortex matter occurs in two steps resulting in a ``Y'' shaped phase diagram: first the soft crystallites undergo a melting transition forming a nanoliquid in which localized and delocalized vortices coexist, while a homogeneous liquid is formed at higher temperatures upon a delocalization transition of the skeleton from the CDs [1]. At lower fields the solid melts through a single first-order phase transition. [1] S. S. Banerjee, S. Goldberg, A. Soibel, Y. Myasoedov, M. Rappaport, E. Zeldov, F. de la Cruz, C. J. van der Beek, M. Konczykowski, T. Tamegai, and V. M. Vinokur, PRL 93, 097002 (2004). [2] S. S. Banerjee et al., PRL 90, 087004 (2003). [3] M. Menghini et al., PRL 90, 147001 (2003).   

Session J2: Heterostructures of Correlated Oxide Materials

Dielectric Superlattices with Broken Inversion Symmetry

Dielectric phases with large electronic susceptibilities and high resistivity can be grown by ozone assisted atomic layer by layer molecular beam epitaxy. We have combined different perovskite titanate phases that have different unstrained lattice constants into fully strained superlattices in which inversion symmetry is broken. For example, a supercell consisting of one unit cell each of CaTiO$_{3}$, SrTiO$_{3}$ and BaTiO$_{3}$ is denoted 111+, while the opposite stacking ordering is called 111-. For testing, these superlattices are sandwiched between lattice matched conducting oxide electrodes and patterned into capacitor structures. In such superlattices, each layer is connected to two different phases. The TiO$_{3}$ octahedra in each layer asymmetrically distort because of the different strain each layer experiences. This results in a built-in polarization that persists at high temperatures in the paraelectric phases. The direction of the polarization is controlled by the molecular stacking architecture, ``plus'' or ``minus.'' At low temperatures an unusual ferro-like phase develops with asymmetric polarization states. This work has been done in collaboration with Maitri Warusawithana, Hao Chen, Michael Weissman and Jian-Min Zuo and supported by the US Department of Energy   

Surface versus Bulk Coulomb Correlations in Photoemission Spectra of SrVO$_3$ and CaVO$_3$

Photoemission is a key spectroscopy for the study of the electronic properties of strongly correlated materials. Spectra taken at low photon energies tend to show significantly stronger correlation features than high energy spectra, suggesting that Coulomb correlations near the surface are enhanced compared to the bulk. To investigate these differences the dynamical mean field theory is used in combination with the multi-orbital Quantum Monte Carlo method in order to evaluate quasi-particle spectra which can be compared directly with photoemission distributions. In the case of perovskites like SrVO$_3$ and CaVO$_3$, the planar character of the partially filled $t_{2g}$ bands and the reduced coordination of surface atoms give rise to an effective narrowing of the surface density of states. As a result, the quasi-particle weight near $E_F$ is reduced and the amplitude of the lower and upper Hubbard bands is enhanced, in agreement with experiment [1]. Preliminary results for VO$_2$ surfaces will also be discussed. [1] A. Liebsch, Phys. Rev. Lett. {\bf 90}, 96401 (2003); Eur. Phys. J. B {\bf 32}, 477 (2003).   

Artificial charge-modulation in atomic–scale perovskite titanate superlattices

In research for exploring new phenomena and functioning devices based on oxide heterostructures and artificial superlattices, it is important to understand the interfacial electronic structure, which is quite distinct from the bulk electronic states because of epitaxial strain and charge/polarization discontinuity. This aspect is also related to the charge ordering phenomena seen in bulk perovskites. Thus, the atomic-scale study to measure and design the interfacial electronic structure is highly relevant. We have grown a number of perovskite titanate superlattices by pulsed laser deposition, specifically controlling oxidation, surface crystallization kinetics, surface termination and layer thickness. In an abrupt interface between band insulator, SrTiO$_{3}$, and Mott insulator, LaTiO$_{3}$, we found that the extra electrons on the Ti site distribute on a length scale exceeding Thomas-Fermi screening length, due to a large induced lattice polarization. This results in a metallic interface down to low temperature. We also found a conducting interface in SrTiO$_{3}$/LaAlO$_{3}$ superlattices. In this case, the conductivity can be controlled over a wide range by changing the composition of the interface. We have also grown solid solution films consisting of SrTiO$_{3}$, LaTiO$_{3}$, and LaAlO$_{3}$ within the framework of charge modulation in perovskite titanates. A variety of electronic states was developed and will be discussed in terms of optical absorption spectra and transport properties comparing with those of the superlattices.   

Electronic Reconstruction in Correlated Electron Heterostructures

Understanding of surface/interface properties of correlated electron materials is an important scientific question and is necessary for possible devices utilizing these materials. As a first step, we focus on the ``charge leakage'' between the different materials inspired by a recent experiment by Ohtomo {\it et al}.[1] We present the following theoretical studies of heterostructures comprised of a Mott insulator and a band insulator; (1) Hartree-Fock analysis of a realistic three-band model for a heterostructure between LaTiO$_3$ and SrTiO$_3$, the structure grown and measured by Ohtomo {\it et al}.,[2] (2) Dynamical-mean-field approximation analysis of a simplified single-band Hubbard model heterostructure.[3] In each case, the heterostructure is defined by placing charge +1 at La sites (charge difference between La$^{3+}$ and Sr$^{2+}$ ions), and the long-range Coulomb repulsion between conduction (Ti $d$) electrons is treated by Hartree approximation. We show that spin/orbital orderings in thin heterostructures are generically different from the bulk and that the interface region, $\sim$ 3 unit cell wide, is always metallic. Prediction for photoemission experiments are made to show how the electronic properties change as a function of position through the interface. Optical conductivity measurements are proposed to investigate the nature of orderings and quasiparticle subbands. We also give general discussion of the correlated electron surface/interface problem. This work has been done in collaboration with Andrew J. Millis, and is supported by JSPS, NSF DMR 0338376 and DOE ER46169. [1]Ohtomo et al., Nature \textbf{419}, 378 (2002). [2]Okamoto and Millis, Nature {\bf 428}, 630 (2004), and Phys. Rev. B {\bf 70}, 195120 (2004). [3]Okamoto and Millis, Phys. Rev. B (in press), cond- mat/0407592.   

Enhancing Ferroelectrics using Strain

We have used epitaxy and the misfit strain imposed by an underlying substrate to shift the paraelectric-to-ferroelectric transition temperature ($T_{c})$ by \textit{hundreds} of degrees and to enhance the ferroelectric properties of SrTiO$_{3}$ and BaTiO$_{3}$. Although SrTiO$_{3}$ is normally not ferroelectric at any temperature, predictions based on thermodynamic analysis imply that a biaxial strain of order 1{\%} will shift its $T_{c}$ to the vicinity of room temperature. Such strains are also predicted to elevate the $T_{c}$ of BaTiO$_{3}$ by comparable amounts. In practice, the synthesis of uniformly strained ferroelectric films is challenging. Epitaxial ferroelectric films are often grown to thicknesses greatly exceeding their critical values, resulting in undesirable relaxation toward a zero-strain state by the introduction of dislocations. Dislocation densities of $\sim $10$^{11}$~cm$^{-2}$ are common in epitaxial ferroelectric films grown on lattice-mismatched substrates, and the resulting inhomogeneous strain smears out the ferroelectric phase transition. Our approach to controlling the properties of ferroelectric SrTiO$_{3}$ and BaTiO$_{3}$ films centers on the development of new substrates (DyScO$_{3}$ and GdScO$_{3})$ that enable the growth of uniformly strained films below, or at least far closer to, the critical thickness for relaxation. Our results$^{1,2}$ demonstrate not only the largest strain-induced shift in $T_{c}$ ever achieved, but also manifest a paradigm shift in how to manipulate the properties of ferroelectric thin films. Strain is a viable alternative to the traditional method of chemical substitutions for shifting $T_{c}$ by large amounts. These strained SrTiO$_{3}$ and BaTiO$_{3}$ films have better structural perfection (narrower rocking curve widths) than SrTiO$_{3}$ and BaTiO$_{3}$ single crystals. An unexpected surprise is that the strained SrTiO$_{3}$ films exhibit a frequency dependence of their dielectric constant consistent with \textit{relaxor} ferroelectricity. $^{1 }$J.H. Haeni, P. Irvin, W. Chang, R. Uecker, P. Reiche, Y.L. Li, S. Choudhury, W. Tian, M.E. Hawley, B. Craigo, A.K. Tagantsev, X.Q. Pan, S.K. Streiffer, L.Q. Chen, S.W. Kirchoefer, J. Levy, and D.G. Schlom, \textit{Nature} \textbf{430} (2004) 758-761. $^{2 }$K.J. Choi, M. Biegalski, Y.L. Li, A. Sharan, J. Schubert, R. Uecker, P. Reiche, Y.B. Chen, X.Q. Pan, V. Gopalan, L.-Q. Chen, D.G. Schlom, and C.B. Eom, \textit{Science} \textbf{306} (2004) 1005-1009.   

Session J3: Recent Progess in Quantum Physics and Quantum Information

Superconducting quantum bits

Superconducting qubits are solid state electrical circuits fabricated using techniques adapted from conventional integrated circuits. They are based on the Josephson tunnel junction, the only non-dissipative, strongly non-linear circuit element compatible with low temperature operation. In contrast to microscopic entities such as spins or atoms, these qubits tend to be well coupled to other circuits, an appealing feature for readout and 2-qubit gate implementation. Very recently, new circuit topologies have solved the problem of qubit isolation from unwanted extrinsic electromagnetic noise, yielding coherence quality factors in excess of 10,000. Current experiments are addressing the intrinsic decoherence mechanisms in tunnel junctions circuits and whether the Preskill criterion of 10,000 coherent 1- and 2-qubit gate operations can be met.   

Small quantum algorithms realized in an ion trap array

Atomic ions confined in an array of traps represent a potentially scalable approach to quantum computing. All basic requirements have been experimentally demonstrated in one and two qubit experiments. The remaining task is to scale the system to hundreds and later thousands of qubits while minimizing and correcting errors in the system. While this requires extremely challenging technological improvements, no fundamental roadblocks are currently foreseen. I will give a survey of recent progress in implementing simple two and three-qubit quantum algorithms with ions in trap arrays. In particular, implementations of quantum teleportation,~quantum error correction and the quantum Fourier transform will be discussed. I will also summarize the prospects and challenges of scaling this particular approach towards a large scale computing device.   

Photonic Quantum Communication and One-Way Quantum Computation

A century after Einstein's invention of the photon concept and 80 years after the introduction of entangled states by Einstein-Podolsky-Rosen and by Schrodinger, entangled photon states have become important in quantum communication and quantum computation schemes. Quantum communication with entangled states is approaching large distances and experiments involving even satellite-bases systems become possible. In some schemes like teleportation and entanglement swapping active feed forward of Bell state measurement results is an essential part of the scheme: together with the intrinsic randomness of the individual measurement result a violation of Einstein relativity is avoided that way. Active feed forward then plays a central role in the completely novel concept of one-way quantum computation as proposed by Raussendorf and Briegel. That concept is qualitatively different from all quantum computer concepts where a sequence of one- and two-qubit quantum gates acts on a suitably chosen input state. In contrast the one-way quantum computer scheme starts with a sufficiently complex and general highly entangled initial state, a cluster state. The specific calculation performed is then defined as a specific sequence of measurements performed on that initial state. An important point is that the specific choice of later measurements is defined by the results of earlier measurements. Using such active feed-forward the one-way quantum computer overcomes the problem of the intrinsic randomness of the individual results in quantum measurement. For photons, the one-way quantum computer can be seen as an extension of the linear optics quantum computation proposal by Knill, Laflamme and Milburn. Recently we realized a one-way quantum computer using four- photon entangled cluster states (P. Walther, K. J. Resch,, T. Rudolph, E. Schenk, H. Weinfurter, V. Vedral, M.Aspelmeyer {\&} A. Zeilinger, submitted to Nature). The state was characterized by full four-qubit tomography. Using various types of cluster states a universal set of 1- and 2-qubit operations was demonstrated. Finally, a Grover search algorithm was implemented.   

Probabilities From Envariance: Born's Rule (And More) From Entanglement

I shall discuss consequences of envariance (environment - assisted invariance), a symmetry exhibited by entangled quantum states [1]. I shall focus on the implications of envariance for the understanding of the origins and nature of ignorance, and, hence, for the origin of probabilities in physics. While the derivation of Born's rule for probabilities ($p_k = |\psi_k|^2$) is the principal accomplishment [1-3] of this research, I shall discuss the possibility that essentially all other symptoms of the quantum - classical transition that are now justified using decoherence (e.g. pointer states, einselection, etc.) can be understood as a direct consequence of envariance, without invoking Born's rule explicitly or implicitly (that is, without using ``trace" or reduced density matrices). Thus, envariance appears to supply a new and deep foundation for the origin of quantum probabilities and, more generally, leads to a new understanding of the quantum origins of the classical [5]. [1] W. H. Zurek, PRL 90, 120404 (2003); RMP 75, 715 (2003). [2] H. Barnum, quant-ph/0312150; M. Shlosshauer \& A. Fine, quant-ph/0312058 [3] W. H. Zurek, quant-ph/0405161   

Bell Nonlocality and Retrospective Error Correction for Noisy Channels

Using a channel similar to one side of a Bell inequality experiment, we show how the auxiliary resources of shared sender:receiver entanglement and classical back communication, neither of which increases the forward capacity of any classical channel, can greatly increase both the quantum and classical capacities of some quantum channels. Joint work with Igor Devetak, Peter Shor, and John Smolin.   

Session J4: Conducting Polymers

Fundamental electronic processes in organic photovoltaic cells

In most organic photovoltaic (PV) cells, excitons must diffuse to a donor-acceptor interface where they dissociate by electron transfer. It is therefore critically important to understand exciton diffusion in detail in order to optimize device architectures. We have measured the exciton diffusion length of various organic materials using steady-state photoluminescence quenching in well-defined polymer/titania bilayer heterostructures. We address how processing conditions can affect the exciton diffusion length in the investigated systems and suggest ways to engineer new materials with larger exciton diffusion lengths. To study charge transport of polymers confined in nanometer channels, we have formed films of anodic alumina with arrays of straight nanopores on indium tin oxide electrodes. We filled the pores with conjugated polymers by spin casting them over the pores and then melting the film. Measurements of the transmission and reflectance of polarized light as a function of angle show that the polymer in the nanopores is partially aligned in the direction perpendicular to the substrate as compared to a neat film. Fitting the current-voltage curves of the diodes to a space charge limited current model shows that the mobility can be increased by a factor of 20. By replacing the insulating alumina with a semiconductor, such as titania, we should be able to make efficient ordered bulk heterojunction solar cells.   

Semiconducting block copolymers and their devices: the relationship between electronic properties, morphology and interfaces

For the optimal performance of organic opto-electronic applications, such as light emitting diodes (LEDs) and photovoltaic devices (PVDs), the morphology of the active layer is of crucial importance. One way to control the morphology of organic materials is by making use of the self- assembling properties of block copolymers. Their well-known microphase separation into highly ordered lattices occurs on the length scale of the radius of gyration of the two blocks, which is comparable to the exciton diffusion length. The morphology depends mainly on the block copolymer composition and can thus easily be adapted in order to optimize device performances. This control over morphology therefore allows one to study the relationship between active layer morphology, interfaces and device performance. In order to fully exploit this block copolymer concept, several rod-coil semiconducting diblock copolymers consisting of a conjugated block and a second coil block functionalized with electron transporting and/or accepting materials (such as oxadiazole or C$_{60})$ were synthesized. The conjugated block acting as light absorbing, electron donating and hole transporting material. The donor/acceptor photovoltaic devices, with active layer the above mentioned semiconducting block copolymers, were used to investigate the relation between the photovoltaic cell performance and the thin- film morphology involving the spatial distribution of donor and acceptor phases within the active layer.   

Structural Influences on Conjugated Polymer Optoelectronic Properties

This paper will focus on understanding the influence that structure has on the optoelectronic properties of device relevant conjugated polymer thin films. In particular, information will be provided on the development and utilization of spectroscopic probes to monitor changes in electronic properties for polyfluorene films and devices. Poly(9,9-dioctylfluorene) [PFO] and its copolymer derivatives provide a suitable test bed for such studies: PFO is especially instructive as it possesses a wide variety of morphologies amongst which a thin film can be readily transformed. Optical measurements using absorption, ellipsometry, fluorescence and Raman spectroscopies will be reported that probe phase transition temperatures, optical constants and their anisotropies (birefringence), and the influence of degradation and microscopic environment on the emission characteristics of thin films. In addition, device-related studies concerning charge transport and optical gain will also be reported.   

Water-Soluble Conjugated Polymers: Self-Assembly and Biosensor Applications

Homogeneous assays can be designed which take advantage of the optical amplification of conjugated polymers and the self-assembly characteristic of aqueous polyelectrolytes. For example, a ssDNA sequence sensor comprises an aqueous solution containing a cationic water soluble conjugated polymer such as poly(9,9-bis(trimethylammonium)-hexyl)-fluorene phenylene) with a peptide nucleic acid (PNA) labeled with a dye (PNA-C*). Signal transduction is controlled by hybridization of the neutral PNA-C* probe and the negative ssDNA target, resulting in favorable electrostatic interactions between the hybrid complex and the cationic polymer. Distance requirements for F\"{o}rster energy transfer are thus met \textit{only} when ssDNA of complementary sequence to the PNA-C* probe is present. Signal amplification by the conjugated polymer provides fluorescein emission $>$25 times higher than that of the directly excited dye. Transduction by electrostatic interactions followed by energy transfer is a general strategy. Examples involving other biomolecular recognition events, such as DNA/DNA, RNA/protein and RNA/RNA, will also be provided. The mechanism of biosensing will be discussed, with special attention to the varying contributions of hydrophobic and electrostatic forces, polymer conformation, charge density, local concentration of C*s and tailored defect sites for aggregation-induced optical changes. Finally, the water solubility of these conjugated polymers opens possibilities for spin casting onto organic materials, without dissolving the underlying layers. This property is useful for fabricating multilayer organic optoelectronic devices by simple solution techniques.   

Understanding the Intra- and Interchain Electronic Structure of Conjugated Polymers by Encapsulation in Mesoporous Silica

It is becoming increasingly clear that the morphology of a conjugated polymer sample -- that is, the conformation of the individual polymer chains and the way they pack together -- plays a direct role in the electronic properties conjugated polymer films. In this talk, we describe the results of work that provides a new method for controlling conjugated polymer morphology and hence electronic properties: encapsulation of the polymer chains into the channels of aligned mesoporous silicas. We find, for example, that in small-pore silica samples where only a single polymer chain can fit into each pore, essentially no polarons are formed upon excitation and phenomena such as exciton-exciton annihilation that are prevalent in bulk films do not occur. We also find that energy transfer (exciton migration) occurs much more slowly along the polymer backbone than between polymer chains, suggesting that the predominant mechanism for exciton diffusion in bulk films is interchain Forster energy transfer. In medium-sized pores that contain just a few polymer chains, we find that the chains are still aligned by the encapsulation, and that polarons can be formed upon directly upon excitation. We also see that exciton-exciton annihilation is now possible, but that it occurs predominantly at kinks or defect sites. In still larger pores, the polymer chains exhibit more bulk film-like properties, including a high degree of photogeneration of polarons and exciton-exciton annihilation. We also show that in the intermediate pore size regime, the alignment of the polymer chromophores leads to significantly lower lasing thresholds than bulk films of comparable optical density, even though the physical density of the chromophores encapsulated into the porous silica is much lower than that in bulk films. Taken together, the results allow a new picture of how interactions between polymer chains control the electronic properties of conjugated polymer films.   

Session J5: Teaching Classical Mechanics and Non-Linear Dynamics: Highlights from a Gordon Conference

What Should Be in an Intermediate Mechanics Text?

I shall survey the coverage of several popular intermediate mechanics texts --- texts for a typical junior-level course in classical mechanics. I shall then discuss the difficult choices that must be made by the author of such a book and the teacher using it. Should it include Lagrangian mechanics? (My answer is a definitive ``Yes.'') Should it include Hamiltonian mechanics (``Yes,'' but perhaps a little less emphatically.) and if so when? What about chaos theory? Collision theory? Continuum mechanics? And relativity?   

Laboratory-based nonlinear dynamics course for science and engineering students

We\footnote{Collaborators: N. Sungar, J. P. Sharpe and N. Fleishon (Physics); K. Morrison and J. McDill (Mathematics); R. Schoonover (Chemistry)} describe the implementation of a new laboratory-based interdisciplinary undergraduate course on nonlinear dynamical systems. Geometrical methods and data visualization techniques are especially emphasized. A novel feature of the course is a required laboratory where the students analyze the behavior of a number of dynamical systems. Most of the laboratory experiments can be economically implemented using equipment available in many introductory physics microcomputer-based laboratories. Student response to the course, especially to the laboratory component, has been enthusiastic and positive.   

Nonlinear Dynamics of Soft-Matter: Continuum Mechanics in the Classroom

Recent efforts in soft-condensed matter physics has generated a renewed interest in the fundamental physics of continuum systems. There has been a recognition that a wide variety of systems, from glasses to foams to granular material, exhibit similar behavior with regard to their dynamics. Even under conditions of external driving, these systems are often ``jammed''. In other words, they exhibit a solid like response to the external driving. With sufficient driving force, there is a transition to a flowing state as the system ''unjams''. This flowing state is generally comprised of nonlinear rearrangements of particles within the system. The question has been raised as to whether or not this represents a general new state of matter, or if the details of each individual system is relevant. At the same time, the interest in the response of complex fluids, such as foams and granular matter, that are composed of mesoscopic, or even macrosopic, sized ``particles'' (such as sand grains), has raised interesting questions concerning the application of continuum mechanics to these systems. Both the nonlinear response of these materials and the application of continuum mechanics raise fundamental physics questions that are generally not covered in typical undergraduate (or even graduate) curricula. This talk will not only review some of the important questions in this field, but also present suggestions as to its integration into the undergraduate curriculum.   

Challenges in enhancing student learning in intermediate mechanics: Identifying the need for a tutorial approach to instruction

One area of ongoing physics education research at Grand Valley State University is to probe the conceptual understanding and reasoning skills of advanced undergraduates as they make the transition from a traditional sequence in introductory calculus-based physics to their first course in upper-level mechanics. [1] The results thus far are consistent with findings from other investigations in upper division courses, which indicate that persistent difficulties with fundamental concepts can hinder meaningful learning of advanced topics. To address this problem, the tutorial approach developed at the University of Washington [2] is being adapted and incorporated into the intermediate mechanics course. Evidence from ungraded quizzes (pretests) and course exams will be presented to illustrate the presence of specific difficulties and the effectiveness of the modified instructional approach. [1] B.S. Ambrose, Am. J. Phys. \textbf{72} (4), 453 -- 459 (2004). [2] L.C. McDermott, P.S. Shaffer, and the Physics Education Group at the University of Washington, \textit{Tutorials in Introductory Physics} (Prentice-Hall, Upper Saddle River, NJ, 2002).   

Variational Mechanics in One and Two Dimensions

We develop heuristic derivations of two alternative principles of least action to be introduced early in the undergraduate physics curriculum. A particle moving in one dimension can reverse direction at will if energy conservation is the only criterion. Such arbitrary changes in direction of motion are eliminated by demanding that \textit{abbreviated action}, the area under the momentum\textit{ vs. }position curve in the phase diagram, have the smallest possible value consistent with conservation of energy. Minimizing abbreviated action predicts particle trajectories in two (and three) dimensions and leads to the more powerful principle of least action of Hamilton, which not only generates conservation of energy but also predicts motion even when potential energy changes with time.   

Session J6: Extreme Computing

Scientific Discovery through Advanced Computing in Plasma Science

Advanced computing is generally recognized to be an increasingly vital tool for accelerating progress in scientific research during the 21st Century. For example, the Department of Energy's ``Scientific Discovery through Advanced Computing'' (SciDAC) Program was motivated in large measure by the fact that formidable scientific challenges in its research portfolio could best be addressed by utilizing the combination of the rapid advances in super-computing technology together with the emergence of effective new algorithms and computational methodologies. The imperative is to translate such progress into corresponding increases in the performance of the scientific codes used to model complex physical systems such as those encountered in high temperature plasma research. If properly validated against experimental measurements and analytic benchmarks, these codes can provide reliable predictive capability for the behavior of a broad range of complex natural and engineered systems. This talk reviews recent progress and future directions for advanced simulations with some illustrative examples taken from the plasma science applications area. Significant recent progress has been made in both particle and fluid simulations of fine-scale turbulence and large-scale dynamics, giving increasingly good agreement between experimental observations and computational modeling. This was made possible by the combination of access to powerful new computational resources together with innovative advances in analytic and computational methods for developing reduced descriptions of physics phenomena spanning a huge range in time and space scales. In particular, the plasma science community has made excellent progress in developing advanced codes for which computer run-time and problem size scale well with the number of processors on massively parallel machines (MPP's). A good example is the effective usage of the full power of multi-teraflop (multi-trillion floating point computations per second) MPP's to produce three-dimensional, general geometry, nonlinear particle simulations which have accelerated progress in understanding the nature of plasma turbulence in magnetically-confined high temperature plasmas. These calculations, which typically utilized billions of particles for thousands of time-steps, would not have been possible without access to powerful present generation MPP computers and the associated diagnostic and visualization capabilities. In general, results from advanced simulations provide great encouragement for being able to include increasingly realistic dynamics to enable deeper physics insights into plasmas in both natural and laboratory environments. The associated scientific excitement should serve to stimulate improved cross-cutting collaborations with other fields and also to help attract bright young talent to the computational science area.   

Exploring the Physics of Supernova Explosions

There is a growing body of evidence that core-collapse supernova explosions are inherently asymmetric. The origin of this asymmetry may arise in the first few hundred milliseconds after core collapse, when the nascent shock wave is susceptible to the spherical accretion shock instability. As part of the Terascale Supernova Initiative, we are using large-scale three-dimensional simulations to investigate the role of hydrodynamic instabilities in core-collapse supernovae. We show that the collapse of a stationary, spherical star can, through the development of the SASI, produce an asymmetric explosion, leave behind a rapidly spinning neutron star, and impart a significant neutron star kick.   

Does the 2D Hubbard model describe high-temperature superconductors?

With more than a thousand publications yearly over the past ten years, the 2D Hubbard model has been widely used as a theoretical tool to investigate the physics of the high-temperature superconducting cuprates. Here we present the first numerically exact solution of the conventional 2D Hubbard model. We systematically study the cluster size dependence of superconductivity in the doped model using the dynamical cluster approximation and quantum Monte Carlo as a cluster solver. Due to the non-locality of the d-wave superconducting order parameter, the results on small clusters show large size and geometry effects. These become weaker as the cluster size increases and finite transition temperatures are found in large enough clusters. The extrapolation to infinite cluster size will be discussed.   

Session J7: Biological Microsystem Technologies Using Microfluidics and Integrated Circuits

Soft Lithography and Microfluidic Devices

Complex three dimensional (3D) nanostructures can play important roles in microfluidic devices. High resolution, conformable phase masks provide a means to fabricate, in an experimentally simple manner, classes of 3D nanostructures that are useful for these systems. In this approach, light passing through a phase mask that has features of relief comparable in dimension to the wavelength generates a 3D distribution of intensity that exposes, through a one or two photon process, a photopolymer film throughout its thickness. Developing this polymer yields a structure in the geometry of the intensity distribution, with feature sizes as small as 50 nm. Rigorous coupled wave analysis reveals the fundamental aspects of the optics associated with this method. A broad range 3D nanostructures patterned with it demonstrates its patterning capabilities. Filter elements, passive mixers, separators and optical sensors built inside microfluidic channels represent examples of the many types of devices that can be constructed.   

Programmed Adsorption and Release of Proteins in a Microfluidic Device

Microfluidic devices are under development for the preconcentration, separation, sensing, and analysis of proteins from small solution volumes (ultimately the contents of single cells). As system dimensions continue to shrink, interfacial interactions become more and more important in dictating device performance. Research is in progress to develop self-assembled monolayers (SAMS) that can be programmed using ``on-chip'' stimuli including heat, light, and electric fields to manipulate interfacial interactions including electrical double layer forces, hydrations forces, and hydrophilic/hydrophobic interactions within confined microchannel environments. While several examples of such SAMS will be provided, the focus of this talk will be on thermally-activated thin films of the polymer poly(n-isopropylacrylamide)(PNIPAM) that can be used for the reversible trapping of proteins. At room temperature, measurements obtained using the interfacial force microscope (IFM) show that PNIPAM films swell to generate a repulsive hydration force that inhibits protein adsorption. Above a transition temperature of 35$^{\circ}$C, the ordered water within PNIPAM ``melts,'' allowing proteins to come into contact with the substrate and form an adsorbed protein monolayer. PNIPAM films have been integrated into a microhotplate device that allows the adsorption and desorption of proteins to be switched in a controlled fashion. Results obtained using ellipsometry and the quartz crystal microbalance show that the resulting reversible protein trap can be used for protein preconcentrations, crude protein separations, and (in conjunction with antibody trapping) highly selective systems for trapping and releasing specific antigens.   

Hybrid IC / Microfluidic Chips for the Manipulation of Biological Cells

A hybrid IC / Microfluidic chip that can manipulate individual biological cells in a fluid with microscopic resolution has been demonstrated. The chip starts with a custom-designed silicon integrated circuit (IC) produced in a foundry using standard processing techniques. A microfluidic chamber is then fabricated on top of the IC to provide a biocompatible environment. The motion of biological cells in the chamber is controlled using a two-dimensional array of micro-scale electromagnets in the IC that generate spatially patterned magnetic fields. A local peak in the magnetic field amplitude will trap a magnetic bead and an attached cell; by moving the peak's location, the bead-bound cell can be moved to any position on the chip surface above the array. By generating multiple peaks, many cells can be moved independently along separate paths, allowing many different manipulations of individual cells. The hybrid IC / Microfluidic chip can be used, for example, to sort cells or to assemble tissue on micrometer length scales. To prove the concept, an IC / Microfluidic chip was fabricated, based on a custom-designed IC that contained a two-dimensional microcoil array with integrated current sources and control circuits. The chip was tested by trapping and moving biological cells tagged with magnetic beads inside the microfluidic chamber over the array. By combining the power of silicon technology with the biocompatibility of microfluidics, IC / Microfluidic chips will make new types of investigations possible in biological and biomedical studies.   

Biotechnology Research at Intel

The Intel biotechnology research program is sponsored by Intel Research of Intel Corporation, aimed at developing novel detection technologies for ultra-sensitive analysis of biomolecules. Towards this goal, we have developed techniques that can be used to isolate and manipulate individual DNA molecules, as well as to place molecules in desired positions in a microfluidic system for effective detection. We have also developed a Raman spectroscopy system together with novel colloidal chemistries that allow single molecule detection of nucleotides and protein bioanalytes. We view this internal research effort as a long-term opportunity. We hope our gained expertise in the biotechnology area will allow us to develop tools that can be used to diagnose diseases and select the best treatments at early disease stages.   

Biomagnetics and Cell-Based Biochips

This presentation will review various micro- and nanotechnologies that we have developed over the past decade in our efforts to manipulate and probe living cells. In early studies, we used magnetic micro-particles to apply controlled mechanical forces to surface membrane receptors. We did this to probe cellular mechanical properties, and to investigate the molecular basis of mechanotransduction -- how mechanical forces are transduced into changes in intracellular biochemistry. The magnetic beads were coated with ligands for adhesion receptors, such as synthetic RGD (arginine-glycine-aspartate) peptides or antibodies that bind to membrane integrin receptors. Controlled twisting (torque) or pulling (tension) forces were exerted on the integrin-bound beads using magnetic twisting or pulling cytometry. To investigate the cellular response to dynamic forces, and to increase the level of stress applied, an electromagnetic needle was developed to apply a temporally varying magnetic field controlled by a user-defined solenoidal current; the end of the needle also was electropolished to produce a nanoscale pole tip. Magnetic forces applied to integrin receptors, but not other cell-surface receptors, induced force-dependent recruitment of cytoskeletal linker (focal adhesion) proteins to the site of bead binding, resulting in assembly and mechanical strengthening of the adhesions. Stress application to integrins also resulted in force-dependent increases in cAMP signaling and induction of gene transcription. These experiments revealed that integrins and the cytoskeleton play a central role in cellular mechanotransduction.studies in collaboration with George Whitesides (Harvard U.), we used microcontact printing techniques with self- assembled monolayers of alkanethiols to microfabricate extracellular matrix-coated adhesive islands of defined size, shape, and position on the micrometer scale. When cells were plated on these islands, the spread to take on the form of the island. These studies revealed that cells can be switched between growth, differentiation, and death (apoptosis) by varying the degree to which a cell physically can distend. When cells grown on islands with corners (e.g., squares, triangles) were stimulated with motility factors, the cells preferentially extended new motile processes from the corner regions, whereas cells on circular islands showed no bias. These findings demonstrated that much of cell behavior is controlled through physical interactions between cells and their adhesive substrate, and that microfabrication methods may be useful for tissue engineering, as well as creation of ``laboratories on a chip'' or biosensor devices that incorporate living mammalian cells. addition, in experiments with Bob Westervelt and Donhee Ham (Harvard U.), we have demonstrated the feasilibility of using microelectromagnetic circuits and CMOS technology to physically pull cells out from medium magnetically, and to move them in a directed manner. This approach may have great value for cell separation applications. Finally, with Whitesides group, we also demonstrated that microfluidics technologies may be used to deliver chemicals or probes to different regions of the same living cell under flow conditions. This provides a novel way to create chemical gradients at the subcellular scale and thereby probe the relation between cell structure and function. We also are currently exploring novel uses of microfluidics technologies, including their application for clinical cell separation applications. Taken together, this body of his work clearly demonstrates the great value of microsystem and microfluidic approaches for the analysis and manipulation of living cells. These approaches may have great value, both for fundamental scientific research and for clinical applications.   

Session J9: Focus Session: Spin Transport/Magnetism Theory

A Full-Potential Linearized Augmented Plane-Wave Method for Calculating Transport Properties: Application to Fe/MgO/Fe Tunnel Junctions

In order to calculate on the basis of the single particle picture as provided by the density-functional theory (DFT), the spin-dependent tunneling through barriers and interfaces of materials with increasing chemical and structural complexity, an extention of the full-potential linearized augmented plane- wave method (FLAPW) as realized in the {\tt FLEUR} code is introduced. The volume in which the electrons scatter is sandwiched between two semi-infinite leads. The leads and the scattering volume are described by an embedding Green function formalism. Different scenarios of electron transport such as sequential and coherent tunneling is formulated and will be compared. Several applications will be presented. The method is used to understand the spin-polarized scanning tunneling microscope. For a three- layer heterosystem SrRuO$_3$/SrTiO$_3$/SrRuO$_3$, the effect of different orbital characters of the states at the Fermi level on the tunneling conductance was investigated. The main focus is on the Fe/MgO/Fe system for which we show that very small changes at the interface can have drastic effects on the conductance.   

\emph{Ab-initio} scattering-state based method for calculating transport in nanostructures

We will present an \emph{ab-initio} computational method for calculating the transport properties of nanostructures. In contrast to the commonly employed scattering-state approaches it directly utilizes supercells with periodic boundary conditions, which makes it ideal for use with planewave density functional codes.   

Spin Torque in Magnetic Tunnel Junctions

The effect of current-induced switching in the orientation of magnetic moments has attracted much attention both experimentally[1] and theoretically in the past several years due to its potential application to spin electronics. The origin of the current-induced switching is the spin torque due to the local exchange interactions between the conduction electrons and the magnetic moments. Using a simple tight-binding Hamiltonian[2] for the magnetic metal-insulator-magnetic metal tunneling junction and the Keldysh formalism for the non-equilibrium Green functions, we have calculated the spin torque and the tunneling current in non-collinear magnetic junctions. We have studied the effect of the bias voltage, the thickness of the barrier and the angle between the magnetizations of the ferromagnetic electrodes on the spin torque and the tunneling current. \begin{enumerate} \item E. Myers, D. Ralph, J. Katine, R. Louie, R. Buhrman, Science \textbf{285,} 867 (1999). \item C. Caroli, R. Combescot, P. Nozieres, D. Saint-James, J. Phys. C: Solid St. Phys., \textbf{4}, 916 (1971). \end{enumerate} *Supported through the NSF grant No. DMR-0097187   

Electronic Structure and Schottky Barrier Formation in Fe/GaAs Magnetic Junctions

In the most successful experiments, spin injection efficiencies of 2\% at room temperature and 30\% at low temperature were achieved [1,2]. The role of intrinsic Schottky barriers in controlling spin-dependent tunneling through the interface is crucial since the barriers significantly reduce the conductivity mismatch, which would otherwise eliminate the possibility of spin injection almost entirely. In this work electronic and magnetic properties of Fe/GaAs magnetic junctions are investigated using the first-principles plane wave based pseudopotential method. It has been shown that the properties of such junctions can differ significantly depending on the interface structure [3]. Therefore, here we calculate wide range of properties of Fe/GaAs junctions, including Schottky barrier heights, magnetization profiles, charge distributions, potential profiles, and equilibrium structures of such junctions, and show how these properties depend on the interface structure. [1] H.J.Zhu et al., {Phys. Rev. Lett.} {\bf 87}, 016601 (2001). [2] A.T.Hanbicki et al.,{Appl. Phys. Lett.} {\bf 80}, 1240 (2002). [3] S.C.Erwin et al., {Phys. Rev. B} {\bf 65}, 205422 (2002).   

{\it Ab inito} study of molecular spin valves

The combination of spin and molecular electronics poses exciting perspectives both for basic science and technological applications. By manipulating the spin degree of freedom at the atomic level we enter a new and exciting era where entire spin devices can be substituted by single a molecule performing analogous tasks. In this talk, we will present a through theoretical study of electronic transport through molecular spin valves \footnote{Towards Molecular Spintronics, A.~Reily Rocha, V.~M.~Garcia~Su\'arez, S.~W.~Bailey, C.~J.~Lambert, J.~Ferrer and S.~Sanvito, submitted to Nature Materials.} obtained by sandwiching a molecule between two Ni electrodes. The calculations are performed with our novel code Smeagol (www.smeagol.tcd.ie), which combines density functional theory with non-equilibrium Green function transport method. We will show results for two types of molecules with distinct transport mechanisms, namely: tunneling and metallic conductance. In both cases we analyze the effects of the contacts on the molecule and the particular states contributing to the transport. We will demonstrate that it is not only possible to obtain large magnetoresistance effects in both types of molecules, but also to engineer the signal by an appropriate choice of end-groups.   

Coulomb Correlation Effects in Variable-Range Hopping of Magnetic Polarons

We study electrical conductivity due to spin polaron hopping in disordered solids with wide distributions of the localized electron energies and polaron shifts taking into account the Coulomb correlation effects. By means of the percolation theory we demonstrate that in such materials a hard polaron gap does not manifest itself while the soft Coulomb gap persists at low temperatures. As a result, the variable-range \textit{polaron }hopping conductivity, $\sigma $, as a function of temperature, $T$, obeys the stretched-exponent law: $\ln \left( \sigma /\sigma _{0}\right) =-\left( \widetilde{T}/T\right) ^{p}$ , where $p=$ 4/7 (3/5) for $3D\;(2D)$ case. It differs from the standard Shklovskii-Efros law for which $p=1/2$. In addition, parameter $\widetilde{T}$ is shown to depend on the dispersion of the polaron shift distribution. Therefore, it decreases with the application of an external magnetic field thus leading to giant negative magnetoresistance in the variable-range hopping regime where for paramagnetic materials $p=$ 5/7 (4/5).   

Tunable spin filter and molecular hybridization in a quantum dot molecule

Spin filtering using few electron semiconductor quantum dots formed in two-dimensional electron gas systems has attracted much recent attention in spintronics. Spin filtering has been achieved in a quantum dot via universal conductance fluctuations and electron magnetic focusing [1]. A bipolar spin filter (\textbf{SF}) has been realized recently using a semiconductor quantum dot which can operate practically as a perfect SF, provided there is a large enough Zeeman splitting [2]. In this work we present calculations showing that the tunable (molecular) hybridization between two quantum dots with few electrons and connected ``in parallel,'' produces a singlet-triplet transition in the ground state which can be used as a robust bipolar SF in both the linear and non-linear regimes of transport. The bipolar SF is found to be \textit{fully tunable by only electrical gating} at low temperatures. We show that a singlet-triplet transition in the energy spectrum gives rise to the natural spin selectivity in the odd-to-even electron number transition in Coulomb blockade experiments. The competition between the Zeeman, Coulomb, and tunneling energies is studied in detail to determine the optimal conditions to achieve the singlet-triplet transition, so that it becomes broadly useful as a bipolar SF. [1] J. A. Folk et al., Science \textbf{299}, 679 (2003). [2] R. Hanson et al., cond-mat/0311414 (2003). *Supported by DGAPA-UNAM project 1N114403, CONACYT, projects J40521F and 143673F, and NSF-IMC.   

Ferromagnetic Resonance in FeZrN

We report here the in-plane ferromagnetic resonance in FeZrN . The parameters measured were intensity I$_{0 }$, linewidth $\Delta $H, and resonance field H$_{r }$.The FMR studies were performed using a conventional X-band ($\mu $=9.1 GHz) ESR spectrometer equipped with a TE$_{011 }$cavity. The samples about 3 m.m. by 3 m.m were placed onto a quartz sample holder, put into the cavity and held in either parallel or perpendicular orientation to the sample plane of the applied magnetic field. The samples were prepared by R.F. sputtering with Zr chips bonded to the target. The films were approximately 0.4 micron thick on glass substrates.   

Entanglement and Quantum Phase Transition in Low Dimensional Spin Systems

Entanglement of the ground states in $XXZ$ and dimerized Heisenberg spin chains and in two-leg spin ladder is analyzed by using spin-spin concurrence and the entanglement entropy between a selected block of spins and the rest of the system. Quantum critical points as well as phase boundaries can be in some cases identified straightforwardly by analyzing the local extreme of the entanglement. We show that various subsystem partitions may provide complementary description of a quantum phase diagram.   

An LAPW Study of d - Plutonium and the (001) Surface

The electronic structure properties of bulk fcc $\delta $-Plutonium and the quantum size effects in the surface energies and the work functions of the (001) ultra thin films (UTF) up to 7 layers have been investigated with periodic density functional theory calculations within the full-potential linearized augmented-plane wave approach.$^{1}$ Several levels of theory, namely NSP-NSO, NSP-SO, SP-NSO, and SP-SO, have been examined and our calculated equilibrium atomic volume of 178.3 a.u.$^{3}$ and bulk modulus of 24.9 GPa at the fully relativistic level of theory are in good agreement with experimental results. The energy difference brought by spin-orbit coupling, about 7-8 eV, is dominant, but the energy difference brought by spin-polarization, from a few tenths to 2 eV, has a stronger dependence on the atomic volume. Density of states show that 5f electrons are more itinerant when the volume of $\delta $-plutonium is compressed and they are more localized when the volume is expanded. The surface energy converges rapidly and the semi-infinite surface energy is predicted to be 0.692eV. Quantum size effects for the work function is not pronounced for (001) surface. $^{\ast }$Work supported by the Department of Energy (Grant No. DE-FG02-03ER15409) and the Welch Foundation (Grant No. Y-1525). $^{1 }$P. Blaha, K. Schwarz, G. K. H. Madsen, D. Kvasnicka and J. Luitz, \textit{WIEN2k} (Technische Universitat Wien, Austria, 2001)   

A First Principles Electronic Structure Study of Quantum Size Effects in (111) Films of FCC Plutonium

First principles linear combinations of Gaussian type orbitals -- fitting function (LCGTO-FF) electronic structure calculations are used to study thickness dependencies in the surface energies and work functions of ultra-thin (111) films of fcc Pu, up to five layers thick. The calculations are carried out at both the scalar- and fully-relativistic (with and without spin-orbit coupling) levels of approximation. The surface energy is shown to be rapidly convergent, while the work function exhibits a strong quantum size effect for all thicknesses considered. The surface energy and work function of the semi-infinite solid are predicted to be 1.12 J/m$^{2}$ and 2.85 $\pm $ 0.20 eV, respectively, for the fully-relativistic case. These results are in substantial disagreement with results from previous electronic structure calculations. The present predictions are in fair agreement with the most recent experimental data for polycrystalline fcc Pu, namely 0.91 J/m$^{2 }$and 3.1-3.3 eV, for the surface energy and work function, respectively. \textbf{*}The work of AKR is supported by DOE (Grant No. DE-FG02-03ER15409) and the Welch Foundation (Grant No. Y-1525). The work of JCB is supported by DOE under contract W-7405-ENG-36 and the LDRD program at LANL.   

An Ab Initio Study of Molecular Oxygen Adsorption on Pu (111) Surface

We will present a study of oxygen molecule adsorption on a Pu (111) surface using the generalized gradient approximation to density functional theory using the DMol3 suite of programs [1]. Horizontal approaches on center site, with and without spin polarization, were found to be the highest chemisorbed sites among all the cases studied here with chemisorption energies of 8.365eV and 7.897eV, respectively. The second highest chemisorption energy occurs at the vertical approach on bridge site with chemisorption energy of 8.294eV (non-spin-polarized) and 7.859eV (spin-polarized). In general, with spin polarization, dissociative adsorption with a layer by layer alternate spin arrangement of the plutonium layer is found to be energetically more favorable compared to molecular adsorption. Non-spin-polarized chemisorption energies are usually higher than the spin-polarized energies. We also find that 5f electrons are more localized in spin polarized case, than the non-spin polarized counterparts. The ionic part of O-Pu bonding plays a significant role along with the Pu 5f-O 2p hybridization. [1] B. Delley, J. Chem. Phys. \textbf{92}, 508 (1990); J. Chem. Phys. \textbf{113}, 7756 (2000).   

First-principles study of alloy formation at Fe/GaAs interfaces

The combination of the high Curie temperature of Fe, high-quality epitaxial growth of Fe on GaAs, and demonstrated high-efficiency spin injection in GaAs, together make Fe/GaAs heterojunctions very attractive candidates for room-temperature spintronic applications. However, little is known about the structure of Fe/GaAs interfaces, and there is a range of conflicting experimental results describing them---including findings of magnetically dead layers [1], an intermediate FeGaAs phase [2], and bulk-like magnetic moments at the interface [3]. Motivated by these findings, we use density-functional theory to study the structural and magnetic properties of interfaces involving possible alloyed phases occurring between Fe and GaAs. From the calculation of interface formation energies we report results on the stability of various structural and magnetic configurations, and provide microscopic parameters which may be used in studies of spin transport to assess the device potential of these heterostructures. [1] J. J. Krebs, B. T. Jonker, and G. A. Prinz, J. Appl. Phys. 61, 2596 (1987). [2] J. Deputier, R. Guerin, B. Lepine, A. Guivarc'h, and G. Jezequel, J. Alloys Comp. 262, 416 (1997). [3] J. S. Claydon, Y. B. Hu, M. Tselepi, J. A. C. Bland, and G. van der Laan, Phys. Rev. Lett. 93, 037206 (2004).   

Session J10: Focus Session: Spin Dynamics in Semiconductors

Bias Dependent Spin Relaxation in a [110]-InAs/AlSb Two Dimensional Electron System

Manipulation of electron spin is a critical component of many proposed semiconductor spintronic devices. One promising approach utilizes the Rashba effect by which an applied electric field can be used to reduce the spin lifetime or rotate spin orientation through spin-orbit interaction. The large spin-orbit interaction needed for this technique to be effective typically leads to fast spin relaxation through precessional decay, which may severely limit device architectures and functionalities. An exception arises in [110]-oriented heterostructures where the crystal magnetic field associated with bulk inversion asymmetry lies along the growth direction and in which case spins oriented along the growth direction do not precess. These considerations have led to a recent proposal of a spin-FET that incorporates a [110]-oriented, gate-controlled InAs quantum well channel [1]. We report measurements of the electron spin lifetime as a function of applied electric field in a [110]-InAs 2DES. Measurements made using an ultrafast, mid-IR pump-probe technique indicate that the spin lifetime can be reduced from its maximum to minimum value over a range of less than 0.2V per quantum well at room temperature. This work is supported by DARPA, NSERC and the NSF grant ECS - 0322021. [1] K. C. Hall, W. H. Lau, K. Gundogdu, M. E. Flatte, and T. F. Boggess, Appl. Phys. Lett. 83, 2937 (2003).   

Nondegenerate time-resolved Faraday rotation in quantum wells: the role of exciton-exciton interactions

Time-resolved Faraday rotation (TRFR) has been widely used in studies of spin related phenomena in semiconductors. Understanding the physical origin of TRFR is thus of special importance to these studies. In this paper, we report experimental studies based on the use of nondegenerate TRFR, in which we measured the spectral response of TRFR by varying the detuning between the pump and probe. Nondegenerate TRFR studies in GaAs and InGaAs quantum wells revealed that simple atomic-like model for TRFR fails to describe the spectral TRFR response. Theoretical analysis further indicated that manybody exciton-exciton interactions fundamentally modify the TRFR response in these semiconductor systems.   

Inducing electron spin coherence in GaAs quantum well waveguides: Spin coherence without spin precession

We report the experimental demonstration of inducing and detecting electron spin coherence in a GaAs quantum well without the use of either an \textit{external} or \textit{internal} magnetic field. We have taken advantage of the spin-orbit coupling in the valence band and have used light-hole transitions in a waveguide to induce coherent superposition of the electron spin states. In the absence of spin precession, the induced spin coherence is detected through quantum interference in the spectral domain, instead of time domain, coherent nonlinear optical response. We interpret the experimental results qualitatively using a few-level model with only the optical transition selection rule as its basic ingredients.   

Time-resolved Spin Dynamics in Semiconductor Microdisk Lasers

Optical microcavities offer unique means of controlling the interaction of light and matter, which have led to the development of a wide range of applications in optical communications and have stimulated discussions of quantum computational schemes based on cavity QED. We present a study of the dynamics of optically injected spins in GaAs/AlGaAs multiple quantum well microdisk lasers, where emission intensity and line width measurements reveal very high quality factor modes, using a pump-probe, time-resolved Kerr rotation technique with picosecond resolution. We measure the spin decoherence as a function of the pump wavelength and input power and find that the spins in these structures couple selectively to the cavity modes at the resonant wavelengths. This is manifested by an enhancement of the spin decoherence time at the lasing threshold of the cavity modes followed by a sharp decrease at greater pump power, where the stimulated emission dominates the radiative decay.   

Time-resolved spectroscopy of electron spin dynamics in ZnO

The prediction of room temperature ferromagnetism in magnetically-doped wide band gap semiconductors has galvanized interest in using zinc oxide for spintronic applications. In addition, ZnO has small spin-orbit coupling and a naturally low abundance of nuclear spins, which is expected to contribute to long spin coherence times. Time-resolved Faraday rotation is used to monitor electron spin dynamics in commercially available bulk single crystal ZnO wafers and thin films grown by pulsed laser deposition on sapphire substrates\footnote{S. Ghosh, V. Sih, S. Y. Bae, S. Wang, G. Chapline, S. Vaidya and D. D. Awschalom, \textit{in preparation} (2004)}. Measurements are performed over a range of temperatures and magnetic fields, with spin coherence persisting to room temperature in both bulk and thin film samples. We investigate the role of intrinsic defects in ZnO on spin decoherence through a systematic comparison of thin films grown under varying oxygen partial pressures and bulk samples.   

Coupling of a Single Nitrogen-Vacancy Center to Nitrogen Spins in Diamond

Confocal microscopy with photon-correlation detection is employed to optically probe single Nitrogen-Vacancy color centers in diamond at room temperature. Photon anti-bunching is measured to distinguish one center from multiple centers within the laser focus. Polarization-dependent excitation in conjunction with magneto-photoluminescence enables the orientation of an N-V center's symmetry axis to be discerned. For a single center with symmetry axis parallel to the magnetic field, a dip in the photoluminescence intensity is observed when the ground-state spin sublevels anti-cross at 0.1 T. We find an additional dip at 0.05 T that is attributed to resonant dipolar coupling to nearby substitutional Nitrogen spins.   

Microsecond spin-flip times for localized donors in GaAs

One of the central tasks in developing spin-based quantum computing is the development of materials which have long spin lifetimes. Observations of long electron spin lifetimes (hundreds of ns) in n-type GaAs dating back to Kikkawa and Awschalom in 1998 [1] have stimulated much excitement in the field, and many groups have similarly made observations of the inhomogeneous T$_{2}^{\ast }$ lifetime of electrons in GaAs in the ns regime. The homogeneous dephasing time, T$_{2}$, has not yet been measured, although it is expected to be much longer. Here, a series of measurements of lifetimes [2] are described for donors in lightly n-type GaAs doped at 3E14, 1E15, and 3E15 cm$^{-3}$ that mimic spin memory in doped quantum dots. Hanle effect measurements yield T$_{2}^{\ast }$ at close to 0T, magnetic resonance measurements provide T$_{2}$* at 40 mT, and Kerr rotation measurements provide T$_{2}^{\ast }$at higher fields. The measured T$_{2}^{\ast }$ values for the 3E14 sample are consistent with full electron localization. A new pump-probe technique using electronic delays between pulses has been used to measure spin lifetimes into the $\mu $s range. This time-resolved technique provides measurements of the spin-flip time (often labeled T$_{S}$, which is essentially the same as T$_{1})$ for two of the samples at a range of fields and temperatures. T$_{S}$ is greater than 1 $\mu $s for B$>$0.6T at 1.5K and for B$>$2.5T at 6 K. Since T$_{2}$ is limited by the spin-flip time, these measurements show the range of temperature and magnetic field where very long T$_{2}$'s are possible. [1] Phys Rev Lett \textbf{80}, 4313 (1998). [2] J.S. Colton et al., Phys Stat Sol B \textbf{233}, 445 (2002); Phys Rev B \textbf{67}, 165315 (2003); Phys Rev B \textbf{69}, 121307(R) (2004); Solid State Comm \textbf{132}, 613 (2004).   

Optical Excitation of Spins in Lightly Doped GaAs layers and Quantum Wells

The transfer of a quantum state from light to localized spins in a solid is of great interest for potential applications in quantum information and optoelectronics. Here, ultrafast pump-probe experiments have been used to excite and detect spin states in n-doped GaAs layers and remotely doped wide quantum wells. Strong signals whose g-factors indicate electron spin precession were observed with 1.5 ps circularly polarized exciting pulses and linearly polarized probe pulses analyzed through Time-Resolved Kerr Rotation (TRKR). We use a simple theory for the trion in the quantum well to describe the results with resonant light. The trion recombination time and hole spin relaxation time are important parameters. However, the functional form of the data partially disagrees with the theory. Reasons for this, such as a contribution from four-wave mixing, are discussed along with explanations of the results for other wavelengths and samples.   

Interface-sensitive study of ultrafast spin dynamics in multilayer semiconductors

We report the first application of pump--probe second harmonic generation (SHG) measurements to characterize optically induced magnetization in non-magnetic multilayer semiconductors GaAs/GaSb/InAs. A circularly-polarized pump beam has been used to inject electrons into the conduction band of GaAs, where the photons impart their angular momentum to electron-hole pairs. Because of the interface-sensitive method, the spins accumulated at the GaSb/InAs interface have been monitored. Subsequent precession of these spins about the applied magnetic field has then been detected by a time-delayed probe pulse as an interfacial magnetic field induced SHG response. The electron and spin transport through the heterostructure takes place on the time frame of 15-20 ps, and it is followed by the relaxation of interfacial magnetic and electric fields on the time scale of 100 ps.   

Far-infrared photo-response of 2D electrons in a single InAs/AlGaSb quantum well in the region of EDSR

Motivated by predictions of strong electric dipole spin resonance (EDSR) and possible spin manipulation by electric fields$^{1}$ we have studied the far-infrared (FIR) photo-response of a two-dimensional electron gas (2DEG) in an asymmetric 15\,nm InAs quantum well (Al$_{0.35}$Ga$_{0.65}$Sb barriers). Changes in $R_{xx}$ induced by a FIR laser beam ($E_{FIR}$ = 3.15\,meV) in Hall-bar geometry were measured vs. magnetic field magnitude ($<$10\,T) and tilt angle $\theta$ of the sample normal with a double lock-in technique. We have observed a sharp minimum (20\,mT wide) in a non-resonant background photo-response for $\theta \approx$ 38$^{\circ}$. This line splits into two sharp minima with increasing angle $\theta$ and vanishes for $\theta < 38^{\circ}$. The dip occurs close to the field (odd filling factor $\nu$ = 7) at which we expect EDSR in this highly non-parabolic system. Possible explanations for the splittings will be discussed.\\ $^{1}$E. I. Rashba, Al. L Efros, Appl. Phys. Lett. {\bf{83}}, 5295 (2003).\\ Work supported by DARPA ONR\# N00014-00-1-0951.   

Observation of Spin-Spin Interaction in InSb Quantum Wells

For 2D electron systems in InSb, the energy of spin-conserving transitions between neighboring Landau levels (LLs) depends on spin orientation. The spin up transition will have a higher energy than the spin down transition due to the non-parabolicity of the InSb conduction band. These spin-resolved cyclotron resonance features allow us to probe the spin dependence of avoided-level crossings between LLs associated with different subbands. Previous studies in GaAs, where cyclotron resonance peaks were not spin resolved, demonstrated that the anti-crossing energy gaps depend linearly on the tilt of the magnetic field. In our spin-resolved studies, we do observe a linear dependence for anti-crossings between levels with the same spin. However, the strength of this dependence is different for the two spin orientations. More notably, we observe anti-crossings between levels with opposite spin, as would be expected for a spin-spin interaction. The gap for these opposite-spin crossings is essentially tilt-angle independent. None of the observed spin dependencies are expected from the conventional theory behind subband LL anti-crossings. Although spin-orbit effects are strong in InSb, the origin of the observed spin dependencies has not yet been definitively identified. This work is supported by the NSF under Grants No. DMR-0080054 and DMR-0209371.   

Hole spin relaxation in diluted mag\-netic semi\-conductors

Diluted Magnetic Semiconductors are postulated to serve as efficient spin injectors for various spintronic applications. Hence, a detailed theoretical investigation of carrier spin dynamics in these materials is of particular importance. The introduction of Mn ions influences the itinerant carrier spin dynamics in many ways. First, the orbital scattering of the itinerant carriers with the Mn impurities can affect spin relaxation through Elliot-Yaffet and/or Dyakonov-Perel' mechanisms. Secondly, the diagonal part of exchange interaction results in the large spin splitting of energy bands. Finally, the off-diagonal part of the carrier-magnetic ion exchange interaction forms an additional channel of spin relaxation. In this work we theoretically investigate the hole spin dynamics in III, Mn-V bulk semiconductors such as InMnAs or GaMnAs. We study the changes in spin-relaxation with the increase of Mn concentration. In our model we take into account the valence band complexity and treat the exchange interaction within the mean field approximation. We consider the Elliot-Yafet spin relaxation mechanism for impurity and phonon scattering. Results of our calculations are compared with experiment.   

Two-dimensional magnetoexcitons in the presence of spin-orbit interactions

We study excitonic energy spectrum and optical absorption in narrow-gap semiconductor quantum wells in strong magnetic field. We find that, in the presence of an in-plane field, the interplay between Zeeman, Coulomb, and spin-orbit terms leads to a drastic change in the magnetoexciton energy spectrum. When separation between adjacent Landau levels with opposite spins becomes of the order of the magnetoexciton binding energy, the bright and dark exciton dispersions exhibit anticrossing, resulting in a pronounced minimum at finite momentum for the higher-energy eigenstate. With varying in-plane field, the anticrossing moves to zero momentum leading to a spin-orbit-induced splitting of the excitonic absorption spectrum. In the presence of both Rashba and Dresselhaus spin-orbit terms, the spectrum is anisotropic and it depends explicitly on the in-plane orientation of the magnetic field. In particular, by varying the azimuthal angle, the splitting of excitonic absorption peak can be tuned in a wide interval. Experimental implications for InAs and InSb quantum wells are discussed.   

Session J11: High Pressure II

Magnetism in High Pressure Cobalt from X-ray Spectroscopy

We investigate the electronic and magnetic properties of high pressure cobalt using X-ray emission spectroscopy (XES) and magnetic circular dichroism (MCD) measurements in a diamond anvil cell. We ascribe the changes in the line-shape of the K$\beta$ emission as signature of an electronic (spin) transition, similar to those reported in iron and manganese compounds. We further interpret the observed pressure-induced decrease in the MCD signal as evidence of a gradual loss of magnetic order in high-density cobalt. Experiments were conducted at the Advanced Photon Source in Argonne, IL. The work at LLNL was performed under the auspices of the U.S. Department of Energy by University of California, under Contract W-7405-Eng-48.   

LDA+U Picture of the Moment Collapse under Pressure in MnO

The transition metal monoxide MnO crystallizes in the rock-salt structure and is a high-spin antiferromagnetic insulator at low temperatures. Under pressure, experimentally it is observed to undergo a metal-insulator transition along with a structural change to the nickel arsenide phase.[1,2,3] As the first step in a concerted effort to obtain a realistic theory of the pressure behavior of MnO, we have performed full potential local orbital (FPLO) LDA+U calculations in the rock-salt phase. The outcome is a first order moment collapse at reduced volume V/V$_0$ $\approx$ 0.73, whereas within LDA the collapse is smoother and is centered around V/V$_0$ $\approx$ 0.68. The moment collapse is from high spin 4.9 $\mu_B$ to a lower but nonzero spin value of 0.8 $\mu_B$. The strong influence of symmetry-lowering (cubic to rhombohedral) by antiferromagnetism will be discussed. [1]. Kondo et al., J. App. Phys, 87, No.9, 4153 (2000) [2]. C.S. Yoo et al. (preprint 2004) [3]. Patterson et al., PR B 69, 220101 (2004)   

Pressurre-induced magnetic phase transitions in the two-dimensional ferromagnet Cs2CuF4

Since both the interlayer and intralayer ferromagnetic exchange interactions in Cs$_{2}$CuF$_{4}$ are closely correlated with the orbital state of Cu$^{2+}$,we expect pressure-induced magnetic phase transitions in this compound because pressure can change the orbital state of Cu$^{2+}$. To confirm this expectation, we performed magnetic susceptibility measurements at several pressures up to around 25 GPa over the temperature range from 1.5 to 18 K, using a diamond anvil cell and a SQUID vibrating coil magnetometer. The results show successive magnetic transitions around 2 GPa and 23 GPa, which correspond to changes of the interlayer and the intralayer exchange interactions from ferromagnetic to antiferromagnetic, respectively.   

Band gap closure in yttrium hydride under high pressure

Trivalent rare-earth metals demonstrate spectacular change in electronic properties by hydrogenation. With increase in hydrogen concentration beyond $x \sim 2.7$, yttrium hydride, YH$_x$, shows metal-insulator phase transition with structural change from the fcc to hcp of yttrium metal lattice. Band gap opening due to orbital hybridization between $1s$ (H) and $4d$ (Y) has theoretically been proposed for the metal- insulator transition. Theoretical studies have also predicted that the volume reduction by applying hydrostatic pressure would lead to metallization in association with band gap closure. We have investigated structural properties of yttrium hydrides by means of x-ray diffraction and infrared absorption beyond a predicted metallization pressure of $\sim 18$~GPa. Hydride specimen was prepared by hydrogenation reaction of yttrium powder or foil with liquid hydrogen in a diamond anvil cell at room temperature. With increase in pressure beyond $\sim 10$~GPa, the hcp lattice of YH$_3$ transforms gradually to a fcc structure. Infrared spectra show peak position change in the hydrogen vibrational region of 450-1500 cm$^{-1}$ above $\sim 11$~GPa, corresponding to the x-ray diffraction results. The H-Y bonding state and expected metallization are discussed on the basis of the high pressure experimental results obtained x-ray diffraction and infrared absorption.   

Calculation of PETN molecular crystal vibrational frequencies under hydrostatic pressure

First-principles calculations of the effects of hydrostatic pressure on pentaerythritol tetranitrate (PETN) are performed using the all-electron CRYSTAL03 program. The procedure for applying hydrostatic pressure by performing a series of volumetric changes coincident with lattice constant and internal coordinate optimization using various scripts and support programs is described. Once the optimized internal coordinates and lattice constants have been obtained for a given hydrostatic pressure, a separate algorithm consisting of additional scripts and programs is employed for performing a complete normal-mode analysis, with analytic first derivatives and numeric second derivatives of the total energy. The eigenvalues obtained provide the vibrational frequencies and the eigenvectors are used for mode identification. The role of the Gaussian basis sets chosen and the exchange-correlation potential used is discussed. The vibrational frequencies obtained at ambient pressure are shown to compare well with experiment and gas-phase calculations. The shift of the vibrational frequencies under hydrostatic pressure is compared with experiment.   

Fluorescence XAFS Study of Heterogeneous Samples and Material Composition.

Fluorescence XAFS spectroscopy is an appropriate technique for in situ study of the dilute and heterogeneous samples. Application of the standard expressions for calculating the fluorescence radiation from homogeneous samples when applied to inhomogeneous samples can introduce significant errors, which limit the accuracy of this spectroscopic method for determination of material speciation and structure. Hence, development of generalized models are of value. In this study we show that the environmental samples must be treated as heterogeneous samples and  also investigate  heterogeneity effects, of which particle size is the most important, and their impact on determination of material speciation and structure by XAFS. Our work includes the calculation of fluorescence radiation from arbitrary shaped convex particles by Monte Carlo methods, and adapting and expanding early work on particle size effects from X-ray Fluorescence Spectroscopy to XAFS spectroscopy. We have developed several theoretical models to calculate the  fluorescence radiation from  heterogeneous samples with arbitrary particle size distributions, compared them  and discussed the complementary and importance of heterogeneity effects in each of them. The importance of accounting for these physical effects in interpreting experimental data is emphasized.   

Crystal Structure Characterization Using High Pressure - Temperature Optical Properties

Recent developments in tailored dynamic compression techniques have given us the ability to explore the dynamic phase space along prescribed thermodynamic paths. However, our ability to characterize the crystal structure under ultra-fast (sub-ns) and extreme pressure-temperature conditions is lacking. Here, we will report a novel idea of using optical properties to characterize phase transitions and crystal structures under such conditions. Preliminary measurements on three phase transitions will be reported: Fe ($\alpha $--$>\varepsilon )$, Sn and Bi (solid --$>$ liquid). Changes in complex optical constants across these phase transitions have been observed. We will discuss the implications of these observations in emissivities, temperature measurements and on phase diagrams such as iron. We will also discuss the possibility of using this technique to explore the differences between the dynamic and static phase diagrams.   

First principle study on high pressure phase of LiH: discovery of B2 phase

Recent progress in high-pressure experimental method expanded the accessible P-T condition significantly, and it has already been established as a powerful method for material design. However, characterization of the new phase is still challenging especially at high-temperature. For example, melting line of hydrogen under mega bar range is already accessible for laser heated Diamond Anvil Cell (DAC) technique, yet, it is not possible to determine the structural change upon the melt. On the other hand, ab-initio calculation method, in principle, does not have the limitation both in the physical condition and in structural characterization. Lithium hydride is only the alkali hydride, in which B2 phase has not yet been found experimentally. The B1-B2 phase boundary at 0K suggested by previous ab-iniito calculations are around 4 mega bar, which is still out of reach for DAC, however, the temperature axis has not yet been explored yet. We demonstrate, using ab-initio two-phase simulation method, that B1-B2 phase boundary near melting line is as low as 150Gpa, which is accessible with the laser heated DAC method. The detailed discussion will be given at the presentation.   

Melting mechanisms and shocked superheated alkali halide single crystals

Even after decades of experimental work, increasingly detailed MD simulations, and refined theoretical approaches, our understanding of the physics of melting remains incomplete at best. Consideration of superheating of solids has been important in studying the melting process through its frustration by means of surface constraint, lack of nucleation sites, and dynamic process. Mechanisms of Lindemann, of Born, of Mott, of Fecht, of Tallon, and of others have been investigated recently by MD simulations of Jin et al. [1] and by Luo et al. [2]. Reanalysis of experimental results on shocked single crystals of the alkali halides KBr and CsBr [3] (with our unpublished data on NaCl and KCl), in light of the recent MD superheating computations [1, 2], leads to a more complete picture of melting mechanisms on superheated dynamically compressed single crystals in the [100] orientation. These shock experiments combine temperature measurement of the region just behind the shock front with elastic wave propagation sampling of the entire shocked region of the crystal. $^1$Z. H. Jin et al., Phys. Rev. Lett. 87, 055703 (2001). $^2 $S.-N. Luo et al., Phys. Rev. B 68, 134206 (2003). $^3$D. A. Boness and J. M. Brown., Phys. Rev. Lett. 71, 2931 (1993).   

RIXS across the volume collapse transition in Gd metal at high pressures

Gadolinium (Gd) undergoes a volume-collapse phase transition at 59 GPa as a result of pressure-induced change in 4f-electron correlation. Similar correlation-driven electronic phase transitions occur in many other rare-earth metals, some (Ce, Pr, Dy, for example) with large volume collapses and others (Nd, Pm, Sm) without. The exact relationships between crystal structure, volume collapse, and electronic correlation in these materials, however, are not well understood. In this study, we have investigated the electronic structure change of Gd to 113 GPa in a diamond anvil cell, by using resonant inelastic x-ray scattering(RIXS) at the HPCAT/APS. Utilizing the resonance at the LIII-absorption edge of Gd, we were able to probe the dipole allowed 3d-5d transition as well as the quadrupole 3d-4f transition as a function of pressure where the degree of 4f electron correlation should manifest as a change in relative intensity of 4f and 5d transition peaks. This work has been supported by the LDRD-04-ERD-020 at the LLNL, University of California, under the auspices of the U.S. DOE under Contract No. W- 7405-ENG-48.   

First-principles phase diagram for Ce-Th system

{\it Ab initio} total energy calculations based on the exact muffin-tin orbitals (EMTO) theory are used to determine the high pressure and low temperature phase diagram of Ce and Th metals as well as the Ce$_{43}$Th$_{57}$ disordered alloy. The compositional disorder for the alloy is treated in the framework of the coherent potential approximation (CPA). Equation of state for Ce, Th and Ce$_{43}$Th$_{57}$ has been calculated up to 1 Mbar in good comparison with experimental data: upon compression the Ce-Th system undergoes crystallographic phase transformation from an fcc to a bct structure and the transition pressure increases with Th content in the alloy. This work was performed under the auspices of the U.S. Department of Energy by the University of California Lawrence Livermore National Laboratory under contract W-7405-Eng-48. A.R. and L.V. are grateful to the Swedish Research Council, the Swedish Foundation for Strategic Research, and the Royal Swedish Academy of Sciences.   

Optical Studies of f-electron Materials Under High Pressure

We are currently developing optical techniques for the study of f-electron materials under high static and dynamic pressures. Static pressures will be obtained using a diamond anvil cell and dynamic high pressures will be obtained using a pulsed laser system. Laser induced fluorescence using continuous wave ultraviolet and visible laser lines will be used for the purpose of characterizing the electronic structure of the materials and to obtain signatures of phase transitions. Preliminary data will be presented.   

Localization of 5f electrons and phase transitions in americium

Density-functional electronic calculations have been used to investigate the high-pressure behavior of americium. The phase transitions calculated agree with the recent sequence obtained experimentally under pressure; double hexagonal close packed $\rightarrow$ face centered cubic $\rightarrow$ face centered orthorhombic $\rightarrow$ primitive orthorhombic. In the first three phases the 5f electrons are found localized, only in the fourth phase (Am IV) the 5f electrons are found delocalized. The localization of the 5f electrons is modeled by an anti-ferromagnetic configuration which has a lower energy than the ferromagnetic ones. In this study the complex crystal structures have been fully relaxed.   

Effect of pressure on the superconducting transition temperature of doped and neutron-damaged MgB$_2$

Measurements of the superconducting transition temperature for Al-doped, C-doped and neutron-damaged-annealed MgB$_2$ samples under pressure up to $\sim 8$ kbar are presented. The $dT_c/dP$ values change systematically with the decrease of the ambient pressure $T_c$ in a regular fashion for each group of the samples. The evolution of the pressure derivatives can be understood assuming that the change in phonon spectrum is a dominant contribution to $dT_c/dP$.   

Session J12: Superconducting Properties of MgB2

Superconducting Mg(B$_{1-x}$C$_x$)$_2$ with Titanium Additions

We have studied the superconducting properties of Mg(B$_{1-x}$C$_x$)$_2$ up to x$~=~0.021$ with and without 0.5$\% $ titanium impurities to determine the feasibility of simultaneously enhancing both the upper critical field and critical current density via chemical additions. Carbon substitutes for boron, increasing H$_{c2}$ by increasing scattering within the $\pi$ band. Titanium forms inter and intragranular precipitates of either TiB or TiB$_2$, which enhance flux pinning and J$_c$. The two effects appear additive and result in an increase in J$_c$ and H$_{c2}$ values.   

Effects of Carbon Doping and Neutron Irradiation on MgB2

We have irradiated Mg(B$_{0.962}$C$_{0.038}$)$_2$ with thermal neutrons and performed post exposure annealing studies to probe the interplay between two different sources of defects. Carbon is believed to act as a point defect, enhancing H$_{c2}$ due to an increase in scattering in the $\pi$ band, without significantly enhancing flux pinning. Neutron damage studies on pure MgB$_2$ wire segments show a suppression of H$_{c2}$ that approximately scales with T$_c$, and an increase in the critical current density at low fields. Irradiation of carbon doped fibers results in a similar scaling of the enhanced H$_ {c2}$ with T$_c$, and the cumulative effects enhance J$_c$ at intermediate fields.   

Effect of Al doping on the upper critical field of MgB$_{2}$ single crystals

We use magnetization measurements to investigate the effect of Al substitution on the temperature dependence of the upper critical field, H$_{c2}$(T), of MgB$_{2}$ single crystals. We find that as the Al concentration is increased, the shape of H$_ {c2}$(T) changes from that for dirty $\sigma$ bands to that for dirty $\pi$ bands, which verifies that Al doping enhances intraband scattering mainly in the $\pi$ bands. Thus, one of the characteristics of the two-gap nature of MgB$_{2}$, i.e., the strong temperature dependence of the H$_{c2}$(T) anisotropy $\gamma_{H} = H_{c2}^{ab}/H_{c2}^{c}$ in pure MgB$_{2}$, is drastically affected by Al doping.   

The Specific Heat of Mg(B$_{1-x}$C$_{x}$)$_{2}$: Two-Gap Superconductors

Two polycrystalline samples of Mg(B$_{1-x}$C$_{x})_{2}$ were measured in magnetic fields (B) to 9 T. The samples show no evidence of magnetic impurities and have only small non-superconducting fractions. For x = 0.1, T$_{c}$ = 32 K and for x = 0.2, T$_{c}$ = 20 K. The specific heats of both samples can be fit with two superconducting energy gaps as was the case for MgB$_{2}$, although the magnitudes and fractions for each are different. For the two carbon-substituted samples the evolution of $\gamma $(B) with B, the normal state $\gamma $ values, and the Debye thetas will be compared to those of MgB$_{2}$.   

Magnetic Field and Carbon-doping Dependent Far-infrared Studies of Epitaxial MgB$_{2}$ Films

Magnetic field dependent (up to 10T) far-infrared transmission studies have been carried out for a series of pure and carbon- doped epitaxial MgB$_{2}$ films (between 30-40nm) on SiC substrates. While the carbon-doped film (30nm) exhibits the typical characteristics for a dirty BCS superconductor in the T$_{S}$ /T$_{N}$ ratio, the pure MgB$_{2}$ films show anomalous behavior: the peak in the T$_{S}$ /T$_{N}$ ratio is broad in frequency; and as the film thickness increases the peak height decreases. Both of these observations could be the direct results of multi-gap nature of the superconducting state in MgB$_{2}$. As a function of magnetic field, the pure MgB$_{2}$ films show saturation behavior beyond 8T which gives the value of H$_{c2}$ at 8T for these films. Interestingly, the carbon- doped film also exhibits saturation behavior at around 8T, suggesting that the carbon impurities are not effective pinning centers for the vortex flux. Reflectivity measurements are underway to deduce the absolute scattering rates for both pure and carbon-doped films in order to construct an effective BCS model with multiple gaps based on the infrared data.   

Probing the Superconducting Gaps of Carbon Substituted MgB2 Films using SIS Tunnel Junctions

We report scanning tunneling spectroscopy of superconducting-insulating-superconducting (SIS) tunnel junctions consisting of a polycrystalline MgB$_{2}$ micro-wire as the probe tip and a series of C-substituted thin films as samples. The MgB$_{2}$ micro-wire is affixed to a 250$\mu $m Pt-Ir wire and approached to each C-substituted film using an Oxford CryoSXM in STM mode. We are then able to scan over the film surface and probe different areas of the film locally using this SIS configuration. The micro-wire tip, for which we have previously characterized the gaps, is used as a reference for the C-substituted films and to enhance energy resolution by pushing the spectral features to higher energies. The films were grown by hybrid physical-chemical vapor deposition and extensive magnetic and structural characterization of identical films have been reported previously. We investigate the evolution of $\Delta _{\pi }$ and $\Delta _{\sigma }$ with carbon substitution and correlate these results with T$_{C}$ and H$_{C2}$ results.   

Observation of Non-Rigid-Band Alloying of Al- and C-doped MgB$_2$ by Electron Energy-Loss Spectroscopy

Angular-resolved electron energy loss spectroscopy in a scanning transmission electron microscope was used to study the B K-edge in pure and doped MgB$_{2}$. We have shown that the p$_{xy}$ states have a high density up to 0.8 eV above the Fermi level before dropping to near zero and then starts to rise again 5 eV above the Fermi level; the density of p$_{z}$-states changes only very little over the first 10 eV. The incompletely filled p$_{xy}$ states at the Fermi level are believed to play an important role in the superconductivity of MgB$_{2}$. Samples with carbon substituted for boron display a shift of the B K-edge towards lower energy but leave the pre-peak structure undisturbed, thus suggesting that carbon's extra electron enters mainly the $\sigma $ band and has little effect on the $\pi $-band states.~ Samples with aluminum substituted for magnesium display dramatic changes in the B K-edge pre-peak, which implies that the Mg site dopant contribute strongly to $\pi $-band states and only weakly to the $\sigma $-band states. These site-specific changes in local density of states are discussed in terms of intra- and interband scattering scenarios.   

Effect of carbon doping on pinning force in MgB2 films

MgB$_{2}$ thin films were doped with carbon by adding carbon containing bis(cyclopentadienyl) magnesium to the carrier gas during the hybrid physical-chemical vapor deposition process. The superconducting properties were studied by transport and magnetization measurements. It was observed that the carbon doping greatly increased upper critical field Hc2 and the irreversibility field Hirr, and significantly improved the magnetic field dependence of critical current Jc. For example, Jc in carbon doped films could reach 3$\times $10$^{5}$A/cm$^{2}$ at 5T and 4.2K when the field was applied perpendicular the film plane, while Jc in the undoped film was already diminished at 5T of field. This indicated that extra stronger pinning centers were introduced into the samples by carbon doping. Analysis of the pinning scaling law indicated that the extra pinning mechanism could be normal core pinning and large precipitates pinning depending on the doping level. The effect of carbon doping on the flux dynamics will also be discussed.   

Probing 2-band superconductivity of Al and C-substituted MgB2 with heat capacity measurements

We study the changes in the heat capacity Cp(T) in Mg$_{1-x} $Al$_{x}$B$_{2}$ (x $\le $ 0.19) and Mg(B$_{1{\-}y}$C$_{y})_{2}$ (y $\le $ 0.08). The two band model is used to fit Cp(T) and extract the two energy gaps and electron-phonon coupling matrix for the different dopant concentrations. These fitting routines are sensitive to background subtraction, and we discuss what constraints this places on sample quality and preparation. Like previous results, fits for Al doping do not indicate merging of the gaps, suggesting Al does not increase interband scattering. However, we also notice trends that are different from those seen in previous experiments, which we also discuss. For instance, the main peak due to the sigma band does not smear with increasing Al content, but remains fairly abrupt. Other characterization suggests the Al doped samples are of very high quality. Results for C doping are also discussed in terms of filling the sigma band hole states with electrons and increased interband scattering.   

Synthesis and Superconducting Properties of the Yb Intercalated MgB$_2$

The effect of the lattice parameters on the superconducting transition temperature (T$_{c})$ of MgB$_{2}$ was studied as a function of the Yb metal doping concentration. The intercalated MgB$_{2}$ samples, Mg$_{1-x}$Yb$_{x}$B$_{2}$, were prepared in a relatively small range of Yb concentration, 0.02$\le $x$\le $0.10. The phase analysis and characterization of the superconductivity were carried out by X-ray diffraction (XRD) and AC susceptibility measurements. The results show that a 0.15{\%} increase in the $c$-parameter for a starting composition of Mg$_{0.95}$Yb$_{0.05}$B$_{2}$ samples.   

Effect of ion damage on the electrical properties of MgB2

The effect of point defects introduced by 2MeV alpha particles on Tc, resistivity and Hc2 of MgB2 films produced was studied. The damage controllably alters the material so the physical mechanisms that determine the widely varying electrical properties of the MgB2 samples and, in particular, the role of the 2 superconducting gaps can be investigated. The residual resistivity increases linearly by a factor of 50 as Tc decreases with damage to below 10K, while the change in resistivity from 300K to 40K decreases by less than 50{\%}. Our results indicate damage increases the residual resistivity of the grains themselves, but has no significant effect on connectivity. These results, and the common resistivity at which Tc extrapolates to 0K, indicate a direct correlation between Tc and intra-grain resistivity. dHc/dT near Tc increases with damage, reaches a maximum for Tc of 30K and then decreases. Hc2 extrapolated to 0K are as high as 57T for fields perpendicular to c-axis.   

A Study of the Electronic Structure and the Effects of Oxygen on the Superconducting Properties of MgB$_2$ by Electron Energy Loss Spectroscopy

The transport properties of MgB$_2$ have a strong dependence on the incorporation of impurities such as oxygen. Different phases of oxygen precipitates have been characterized and it has been found that the presence of oxygen in MgB$_2$ systematically changes the electronic structure of the boron atoms. Among the different phases of oxygen precipitates found in MgB$_2$, those forming coherent superlattice structures of MgB$_2$- MgB$_x$O$_y$ were studied in detail by first principles calculations. This kind of precipitate has been reported to increase the upper critical fields and critical current density without decreasing the critical temperature of MgB$_2$. This effect is reflected by the low critical temperatures calculated for coherent oxygen precipitates with different concentrations of oxygen using density functional theory. These low critical temperatures explain the behavior of the oxygen precipitates as pinning centers and highlight the importance of oxygen on the superconducting properties of MgB$_2$. Additionally, due to the presence of two carrier species given by the B and Mg states in MgB$_2$, a low energy plasmon mode was theoretically proposed. This work presents the first experimental evidence of this plasmon mode, which has a quadratic dispersion, $\omega_p\propto q^2$.   

Study on the formation of MgB2 and other phases in Cu-sheathed /MgB2 wires.

We report the results from a systematic x-ray diffraction, SEM, and critical current density measurements for Cu-sheathed MgB$_{2 }$wires fabricated using the powder-in-tube method and ultra-fine Mg and B precursor prepared by high-energy ball milling. The samples were sintered at temperatures ranging from 450 \r{ }C to 900 \r{ }C for 5 minutes. It is found that MgB$_{2}$ phase can be formed in this whole temperature range. Below 550 \r{ }C, the weight of the Mg$_{2}$Cu phase increases with sintering temperature while the Mg-phase decreases. Between 550 \r{ }C and 725 \r{ }C, the Mg$_{2}$Cu phase disappears, only MgB$_{2}$ and MgCu$_{2}$ phase co-exist. At or above 725 C, MgB$_{2}$, MgB$_{4}$, and Mg$_{1+y}$Cu$_{32+\delta }$ phase coexist, and the fraction of the Mg$_{1+y}$Cu$_{32+\delta }$ phase increases with sintering temperature while the other two phases decrease. These results are supported by our SEM and $J_{c}$ measurement results.   

Processing, Microstructure, and Properties of Doped and Undoped MgB2 Fibers

A composite consisting of continuous MgB$_{2}$ fibers within an Mg matrix is synthesized by reacting CVD boron fibers with liquid magnesium. Over the reaction temperature range of 700 - 1100 \r{ }C, the reaction time varies by several orders of magnitude. Much slower kinetics are observed for reaction of B fibers doped with C or Ti by co-deposition during the CVD process, which has been shown to improve superconducting properties after reaction to MgB$_{2}$. The reaction times, resulting microstructures, and superconducting properties are compared for both doped and undoped fibers.   

Session J13: Defects and Other Structure in Superconductors

Impact of inhomogeneities on various properties of novel superconductors and magnetic oxides

Novel superconducting compounds such as cuprates and borocarbides are in the ``pseudogap state'' at T$>$Tc: they display anomalous diamagnetism, an energy gap, a ``giant'' proximity Josephson effect, etc. The compound is intrinsically inhomogeneous because of the effects of doping and pair-breaking. The phase diagram is characterized by three energy scales (T*$>$Tc*$>$Tc;res). When cooled below T*, the system begins to display phase separation, i.e., coexistence of metallic and insulating phases as well as other possible effects, e.g., a CDW gap.Below Tc* superconducting ``clusters'' embedded into the normal metallic matrix appear, leading to diamagnetism, pairing gap, a.c.losses, and the ``giant'' proximity effect. The Meissner and resistive transitions are split, and the ``intrinsic'' critical temperature (Tc*) greatly exceeds Tc;res. Further cooling towards Tc=Tc;res. increases the volume taken up by the superconducting ``islands'' and eventually results in a percolation transition at Tc, so that a macroscopic coherent dissipationless phase forms. The percolation transition is similar to that in manganites [L.Gor'kov, V.Kresin, Phys. Rep. 400, 149 (2004)].   

Electron Ising Nematic in High Tc Superconductors

There is theoretical and experimental evidence that electronic liquid crystal phases can arise in strongly correlated electronic systems. The electron nematic breaks the rotational, but not the translational, symmetry of the host crystal. In the cuprates, Cu-O bonds are often favorable directions for such charge-ordered states. Then the electron nematic has Ising symmetry, corresponding to orienting the easy and hard local directions of conduction along the Cu-O bond directions. Quenched disorder in the host material acts as a random field. Using this mapping to the random field Ising model, we construct a random resistor network model which provides a direct connection between local ordering and local transport properties. Interesting behavior reminiscent of this model has been reported in recent experiments on the cuprates: switching noise with slow time dynamics was observed in the resistivity of YBCO nanowires (Bonetti et al., PRL 2004), and magnetic hysteresis at intermediate field strengths has been reported in LSCO (Panagopoulos et al., PRB 2004).   

Superconducting Fluctuations in 2D

Using microwave cavities over a broad frequency range, we have performed a comprehensive temperature-dependent study of the AC fluctuation superconductivity in two-dimensional disordered InOx films. A number of distinct regions of superconducting fluctuations can be observed. Various scaling relations concerning these regions can be extracted.   

Exploring the influence of out-of-plane impurities on the electronic structure of BSCCO-2212

Nonstoichiometric atoms (NSA) are universally found in the High-Tc superconductors. BSCCO-2212 (the most studied of these compounds on the nanoscale) shows inhomogeneity (K. M. Lang et al Nature 2001) and the effects of weak scattering of low energy quasiparticles in much of its phase diagram (K. McElroy et al, Nature 2003; cond-mat/0406491). However, the interaction of these nanoscale low energy phenomena with the NSA is unknown. We report on scanning tunneling misroscope (STM) experiments that identify these NSA and characterize their effects on the low energy electronic structure.   

Atomic arrangement and charge distribution in YBCO tilt grain boundaries

It is well known that the critical current, J$_{c}$, in high-Tc superconductors is reduced at grain boundaries. Recent high resolution holography experiments show the [100] tilt grain boundaries in YBa$_{2}$Cu$_{3}$O$_{7-x}$ to have an excess negative charge localized at the boundary dislocation core. Upon doping with Ca, this charge is reduced and the critical current increased. To shed light on this behavior at an atomic scale, we carried out Tight Binding (TB) calculations of these boundaries. Our TB scheme is charge self consistent to allow charge transfer typical for ionic materials. We present the arrangement of atoms and charge in YBCO tilt grain boundaries as determined by a combination of TB calculations, recent high resolution Scanning Transmission Electron Microscopy and Electron Energy Loss Spectroscopy measurements.   

Transport Properties of Amorphous Tantalum Thin Films Near the Superconductor-Insulator Transition

In amorphous superconducting thin films, the superconducting transition temperature continuously decreases with decreasing film thickness, and eventually the film becomes an insulator. This is known as the superconductor-insulator transition (SIT). Because the SIT is a phase transition between two different zero temperature ground state, it is an example of quantum phase transition. We observe such a SIT in amorphous tantalum thin films occurring at a sheet resistance of $\sim h^2/2e$ when the film thickness is $\sim 1nm$. We report detailed study on the transport characteristics of amorphous tantalum films at the vicinity of the superconductor-insulator transition.   

Cooperative doping mechanism of YBCO grain boundaries

A major impediment to large-scale applications of high-Tc superconductors has been the low critical crurrent across grain boundaries. Grain-boundary doping, in particular by Ca, was shown to increase the critical current, but an atomic-scale understanding of the underlying mechanism is still lacking. Here we report first-principles, density-functional calculations of Ca-impurity and O-vacancy formation energies in YBCO. Using biaxial strain to mimic the effect of local strain at the grain boundaries, we find that Ca segregates at Cu or Ba sites, depending on the sign and magnitude of the local strain field. In bulk YBCO, Ca$^{2+}$ ions are known to substitute for Y$^{3+}$ ions, thereby providing additional holes. However, the doping mechanism proposed for bulk YBCO does not apply to grain-boundary doping, as Ca substitutes for Cu or Ba atoms. Instead, we propose that Ca doping provide strain relief at the grain boundary, thereby reducing the propensity of O vacancies to segregate, and effectively restoring high hole concentrations.   

Investigation of Current Characteristics in YBCO RABITS Tape

Using Variable Temperature Scanning Laser Microscopy (VTSLM), the current transport properties of high temperature superconducting YBCO RABITS tape are studied. Voltage response images are taken to map the critical temperature (Tc*), as well as the critical current (Jc*) of the YBCO RABITS sample. These maps are used to investigate the way the current flowed through the YBCO RABITS film. They showed that the current flows through the YBCO RABITS film by percolation. The size of percolation cluster is about 50- 150 micrometers. Some high voltage response areas in temperature scanning are related to the lower value of Tc* and Jc*. This means that some defects result in the crowded current in those areas.   

X-ray diffraction study of charge stripe order in La$_{1.875-x}$Ba$_{0.125-x}$Sr$_{x}$CuO$_{4}$

The charge stripe order in La$_{1.875-x}$Ba$_{0.125- x}$Sr$_{x}$CuO$_{4}$ with $0.05\leq x\leq 0.10$ and its relevance with high-$T_{\rm c}$ superconductivity have been investigated by synchrotron X-ray diffraction. For $x=0.05$, as temperature decreases, incommensurate superlattice peaks associated with the stripe order appear just below the structural phase transition temperature $T_{\rm d2}$, indicating the strong relevance between the formation of the charge stripe order and the structural phase transition. However, in $x=0.075$ and 0.09, the superlattice peaks emerge far above $T_{\rm d2}$ as a short range correlation, indicating a precursor of charge ordering. Furthermore, temperature dependences of the superlattice peak intensity, correlation length, and incommensurability for $x=0.05$ are different from those for $x=0.075$ and 0.09. These results suggest that the transition process into the charge stripe order strongly correlates with the order of the structural phase transitions. A quantitative comparison of the structure factor associated with the charge order have been also made for all the samples.   

Magnetic stripe formation of in-plane c-axis aligned YBCO thin films

The magnetic anisotropy of YBCO (110) thin films, with c-axis aligned in-plane, has been investigated by Magneto-Optical Imaging (MOI) method. The (110) YBCO thin films were fabricated by magnetron sputtering on SrTiO$_{3}$ substrates. The MOI measurement yields a stripe pattern of vortex-penetration deep into the film in the [110] direction. This pattern is superimposed on that predicted by the Bean model. The stripe is interpreted in the context of micro- or nano-irregularity on the sample edge. This stripe geometry is analyzed as the limiting case for a large assembly of parabolic contours typically found in an isotropic sample, such as c-axis oriented ones, in the presence of a region of non-superconducting irregularity.   

Session J14: Focus Session: Anisotropic Building Blocks: Synthesis and Assembly

Self-assembly on curved surfaces: a tool to generate novel Nano-Materials

It is known that thiolated molecules spontaneously form poly-crystalline self-assembled monolayers (SAMs) on flat gold surfaces. Scanning tunneling microscopy (STM) studies have shown that, in SAMs composed of more than one type of molecule (mixed-SAMs), domains of random shape and size phase-separate. Here we will show that, when mixed SAMs are formed on gold nano-crystals with a radius of curvature $<$ 20 nm, they spontaneously phase-separate into highly ordered domains of unprecedented size. In the case of binary mixture of thiolated ligands on gold particles, domains, only 0.5 nm wide, of alternating composition encircling or spiraling around the metallic core spontaneously assemble. This new family of nano-structured nano-materials[1] shows properties that are determined by this unique morphology, such as solubility. Also, due to the ordered alternation of hydrophobic and hydrophilic regions, surfaces coated with these particles show the ability of suppressing protein nonspecific adsorption. Recent results show that these particles can be forced to assemble into chains, rings and triangles. [1] Jackson A. M., Myerson J. W., Stellacci F. Nature Materials, 3, 330, 2004   

Biphasic nanoparticles made by electrified jetting

Nano-colloids have recently attracted intense attention due to unique properties that are distinctly different from bulk solid-state materials; including unique magnetic, electronic, optical, chemical, and biological characteristics. The vision that these nano-objects could essentially act as functional components in novel device generations, which ``magically'' assemble following a master blueprint void any human manipulation, has resulted in a new ``gold rush'' in materials science. These concepts have results in the synthesis of a multitude of nano-objects, such as nano-wires, nano-rods, nano-disks, or nano-prisms.$^{ }$ Recently, nano-particles with anisotropic materials distributions (biphasic nano-particles) moved in the focus of research. Our approach differs fundamentally from the above-mentioned methods in that it takes advantage of electrified polymer jets to create anisotropic materials distributions in nano-objects. jetting is a process to generate liquid jets by use of electrostatic forces. It is well-known that high electrical potentials (typically several thousand volts) applied between the jetting liquids that are fed through a capillary and a collecting substrate will induce jetting of a charged liquid. The differences in the final morphologies from similar processes are mainly determined by the properties of the jetting liquids and the process parameters. transmission electron microscopy, scanning electron microscopy, and scanning laser confocal microscopy, we demonstrate the applicability of the process to control size, shape, and materials distribution at the nanoscale. The resulting anisotropic nanoparticles may have potential applications for targeted drug delivery or as electro-rehological fluids.$\backslash $ a) F. M. Van der Kooij, K. Kassapidou and H. N. W. Lekkerkerker, \textbf{Liquid crystal phase transitions in suspensions of polydisperse plate-like particles,} \textit{Nature }\textbf{406}, 868 (2000); b) C. A. Mirkin, R. L. Letsinger, R. C. Mucic and J. J. Storhoff, \textbf{A DNA-based method for rationally assembling nano-particles into macroscopic materials}'' \textit{Nature }382, 607 (1996); c) N. B. Bowden, M. Weck, I. S. Choi and G. M. Whitesides, \textbf{Molecule-mimetic chemistry and mesoscale self-assembly}, \textit{Accounts of Chemical Research }\textbf{34}, 231 (2001).   

Guiding Rules for Self-Assembly of Patchy Particles

The functionalization and patterning of nanoparticle and colloidal building blocks with organic and biomolecular ligands provides new possibilities for directing their self-assembly into complex structures for novel materials and devices. We seek to develop an intuitive and general framework for predicting the assembly of building blocks functionalized at specific locations with patches of attractively-interacting molecules. We present the results of molecular simulations of the self-assembly of spherical and cone-shaped particles decorated with sticky patches. ~We relate the geometry of polyhedral terminal structures formed from small numbers of particles, to geometrical details of the building blocks and the anisotropy of the patch patterns. ~We compare the structures obtained in our simulations with colloidal polyhedra formed by droplet evaporation.   

Self-assembly of particles with anisotropic interactions

We investigate the self-assembly of hard core particles with additional dipolar and higher order (in particular octopolar) interactions using a model system of vertically vibrated magnetic spheres. Self-assembly in such a driven dissipative system is similar to transitions to self-assembly seen in equilibrium polymerization. We show the crucial role of the anisotropy of interaction on the pattern of self-assembly in our experimental model system. In particular, we observe clusters, chains, and branched networks. We show that energy minimization in a simple point charge model can be used to predict the preferred self-assembly pattern. We also show that such a model containing a few carefully placed representative charges can successfully recreate self-assembly patterns in several related physical systems, including biological macromolecular self-assembly of e.g. tubulin.   

Molecular Dynamics Simulation of Colloidal Nanoparticle Forces

An improved understanding of the forces between colloidal nanoparticles could lead to new strategies for achieving their selective assembly for a variety of different applications. We employ molecular dynamics simulations to study the interplay between solvation and van der Waals forces for model colloidal nanoparticles. We consider the influence of nanoparticle size, shape, and surface roughness, as well as solvent type (Lennard-Jones vs. n-decane) and solvent-solid interaction (“solvophobic” vs. “solvophilic”). We find that solvation forces can be comparable to van der Waals attraction and, thus, they can play an important role in determining the stability of colloidal suspensions. Surface roughness causes nanoparticles to rotate so they approach one another via paths of minimum free energy. This rotation causes crystalline (icosahedral) nanoparticles to approach one another via alternating face-face and vertex-vertex conformations, suggesting that solvation forces can control nanoparticle alignment during assembly. Finally, our simulations of solvophobic nanoparticles in n- decane yield insight into how the drying transition is influenced by the relative sizes of the solvent molecules and nanoparticles.   

Error-Free DNA Directed Self-Assembly of Nanoparticle Clusters

We study DNA directed self-assembly of colloids into structures with controllable geometries. By introducing a soft-core repulsive potential between colloids, we can overcome excluded volume effects. A toy model is constructed to demonstrate these ideas, and its phase behaviour is studied numerically. We explain the route by which our proposal can be implemented experimentally.   

Nano hybrid shish-kebab: towards periodically functionalize carbon nanotubes

Both chemical and non-covalent wrapping methods have been used to functionalize carbon nanotubes (CNT). Periodical functionalization of CNT remains a challenging task and few works have been dedicated to this research field. We report a novel method of functionalizing CNT surface using controlled polymer crystallization. CNTs were periodically decorated with polymer lamellar crystals, resulting in ``nano hybrid shish-kebabs'' structure. The periodicity of the polymer lamellae varies from 20 - 70 nm. Both polyethylene and Nylon 6,6 have been successfully decorated on multi-walled as well as single-walled CNTs. This method opens a gateway to functionalizing CNTs in an ordered and controlled manner, an attractive research field that is yet to be explored. It also directly leads to the synthesis of the ``ideal'' polymer/CNT nanocomposites with controllable tube-to-tube distance.   

Shape Separation of Gold Nanorods using Centrifugation

We describe the shape separation of colloidal gold nanorods using centrifugation. Nanoparticle synthesis is characterized by a polydispersity in the shape and size of particles. Since the shape and size determine the properties and applications of nanoparticles, the separation of nanorods from a mixture of different shapes is necessary. We describe the hydrodynamics of nanorods and nanospheres undergoing centrifugation, elucidating how this can be efficiently exploited for the shape and size separation. For nanoparticles in dilute concentration, the relative sedimentation velocity of rods and spheres is obtained by describing Brownian motion of the particles in presence of external forces, accounting for shape dependent drag, as well as hydrodynamic interaction. The hydrodynamic arguments illustrate the effect of shape and size on both relative sedimentation velocities and concentration profiles. The arguments advanced here, with described caveats, are quite general and applicable to shape and size separation in organic, inorganic and biological systems. In present study, we report the efficient separation of gold nanorods from mixture of shapes obtained from synthesis by the seed mediated method.   

DNA Templating of Au Nanowires

We have developed a process for fabricating nanoscale wires using DNA templates. The templates were subsequently decorated with gold nanoparticles to make metallic wires. We have successfully deposited linear, straight sections of random ($\lambda$-phage) and regular-repeat sequences of DNA, of various lengths, on oxidized silicon substrates. We have also successfully deposited thiolated DNA on gold electrodes, allowing the DNA to electrically bridge gaps between electrode pairs. Electrode gaps ranged from 50 nm to 300 nm, fabricated using electron beam lithography. We decorated the DNA with gold nanoparticles with diameters in the range of 1-13 nm, and have used the nanoparticles as nucleation sites for the growth of continuous gold wires. We have performed AFM characterization of all surfaces and structures. In addition, we have performed current-voltage measurements on the undecorated DNA, the nanoparticle-decorated DNA, and the gold nanowires.   

Manipulation and Assembly of Semiconductor Nanowires with Holographic Optical Traps

Semiconductor nanowires are versatile building blocks for the assembly of functional electronic and photonic devices. Yet to realize their potential will require assembly into increasingly complex architectures with placement at specific locations in a parallel process. Here we describe progress towards the use of the holographic optical tweezer (HOT) technique for manipulating nanowires in solution. The HOT technique can create hundreds of individually controlled optical traps with the ability to manipulate objects in three dimensions. Our results show that individual nanowires can be aligned along a line of optical traps. Single traps cannot manipulate individual nanowires suggesting that the mechanism of trapping may be different than previously observed for dielectric microspheres. Our results also show that individual nanowires can be rotated in circles using an optical vortex, and that it is possible to fuse nanowire junctions and deposit nanowires irreversibly on substrates. Efforts towards creating nanowire arrays and other complex structures will be discussed.   

Guiding 3-D Self Assembly of Nanostructures by DNA Hybridization

The directed three dimensional self-assembly of microstructures and nanostructures through the selective hybridization of DNA is the focus of great interest toward the fabrication of new materials. Single stranded DNA is covalently attached to polystyrene latex microspheres and functions as a ``smart Velcro'' by only bonding to another strand of DNA of complementary sequence. The attached DNA increases the charge stabilization of the microspheres and allows controllable aggregation of microspheres by hybridization of complementary DNA sequences. The process is perfectly selective and reversible by heating, with a characteristic ``aggregate dissociation temperature'' that is dependent on salt concentration, and the evolution of aggregate dissociation with temperature is observed with optical microscopy.   

Session J15: Focus Session: Strained Si and Other Semiconductors for Device Applications

Straining Si on Insulator

Transistor scaling has been the primary factor driving mainstream Si CMOS performance improvement. Approaching the fundamental limits of conventional bulk transistor scaling makes it increasingly difficult to remain on the historic scaling trend. To solve the two major scaling issues, namely increases in transistor leakage and decrease in performance improvement, new materials and device architectures are demanded. Two parallel developments in Si CMOS technology have created new opportunities in the control of channel electrostatics and the improvement of channel transport, for leakage reduction and performance enhancement. The two developments are: silicon-on-insulator (SOI) and strained channel. Due to excellent channel electrostatics, SOI transistors are considered very scalable, with their architecture scaling from partially depleted SOI for the current generation to fully depleted variety for future generations. Appropriately applied strain to the device channel can~significantly increase channel mobility, and consequently increase drive current. Both technologies can be incorporated into the CMOS device structure to significantly improve its scalability and boost its performance. In this paper, we will first describe how these two advances allow further scaling of CMOS, which include scalability improvement in SOI devices and performance enhancement by channel strain engineering. Of particular interest is the strain engineering for SOI platforms, including strained substrates and process-induced strain, leading to SiGe-on-insulator (SGOI), strained-Si-on-insulator (SSOI), and other process-induced strain techniques based on SOI substrates. We will describe the formation of such engineered substrates, implementation of the strain-engineered processing, as well as their impact on MOSFET performance.   

Surface Roughness and Dislocation Distribution in Compositionally Graded Relaxed SiGe Buffer Layer with Inserted Strained Si Layers

We report the experimental investigation of surface roughness and dislocation distribution of 1 $\mu $m-thick, compositionally graded, relaxed SiGe buffer layer with a final Ge surface content of 30{\%}. Tensile-strained Si layers are inserted at various locations in the graded buffer during SiGe epitaxial growths. Slight reduction in surface roughness from about 10.3 nm to about 7.8 nm by inserting two 20 nm thick tensile-strained Si layers followed by SiGe growths. It turns out that majority of the residual surface roughness is developed during the SiGe growths on top of the topmost strain Si layer. The surface immediately after the growth of tensile strained Si is very flat with about 1.1 nm RMS roughness and without crosshatch morphology. Cross-sectional TEM shows clear signs of increased interaction between dislocation half-loops at the top surface of the strained Si layers. Our observation shows that although thin Si layers under tensile-strain are effective in reducing cross-hatch, they could in the meantime impede dislocation propagation leading to higher threading dislocation density. Considerations for an optimized scheme exploiting the flattening function of tensile-strained layers will be discussed.   

Ultra-thin ambipolar germanium on insulator field effect transistors

As semiconductor technology shifts towards semiconductor-on-insulator, material combinations other than Si/SiO$_{2}$ are becoming more attractive. We will report on the transistor characteristics of ultra-thin germanium layers (less than 100 {\AA}) that have been epitaxially grown on a lattice matched epitaxial high-$\kappa $ crystalline oxide (La$_{0.27}$Y$_{0.73})_{2}$O$_{3}$, in turn grown on (111) silicon substrate. This enables the use of Ge, which has higher electron and hole mobilities than Si. Our back-gated germanium on insulator field effect transistors show good transistor characteristics, especially for the very thin layers (30 {\AA}). The devices exhibit a high I$_{on}$/I$_{off}$ ratio and they can be fully depleted and inverted, enabling both P and N channel operation in the same device. Current-voltage measurements at room and low temperature will be presented and compared with device simulations. Hall effect measurements will be used to characterize the quality of the ultra-thin Ge channels.~   

Defect-Free Strained Si-on-Insulator Structures

Unlike graded SiGe buffer layers that are used for strained Si devices, free-standing Si/SiGe/Si structures that undergo elastic strain relaxation are essentially defect free and can be used to fabricate strained Si-on-insulator (SSOI) slabs suitable for SSOI MOSFETs [1-3]. We present an alternative method to form defect-free strained Si-on-Si (SSOS) or SSOI slabs by in-place bonding. An SOI wafer having a pseudomorphic SiGe layer and a Si cap layer is etched to form slabs. As the buried SiO$_2$ layer is completely etched away, the Si/SiGe/Si slabs are bonded in place to the Si substrate in the etch solution. The slabs remain bonded to the substrate by van der Waals forces when the wafer is removed from the etch bath. X-ray diffraction and AFM measurements show that the SiGe layer has relaxed elastically, i.e. no misfit dislocations are formed, and that the Si layers are under tensile strain. Subsequent annealing at high temperature forms a covalent bond. This method allows direct bonding of strained Si to Si. By suitable choice of the layer structure of the starting wafer, bonded SSOI structures can also be fabricated. The different forces involved at each stage of this in-place bonding process will be discussed. 1. G.M. Cohen, et al., Mat. Res. Soc. Symp. Proc. 768, 9 (2003). 2. P.M. Mooney, et al., Appl. Phys. Lett. 84, 1093 (2004). 3. P.M. Mooney, et al., Mat. Res. Soc. Symp. Proc. 809, 27 (2004).   

Mechanical Stability of Ultra Thin Ge/Si Film on SiO$_2$:the Effect of Si/SiO$_2$ Interface

We perform two-dimensional linear elastic finite element analysis to investigate mechanical stability of ultra-thin Ge/Si film grown on or bonded to SiO$_2$, using imperfect interface elements between Si and SiO$_2$ to model Si/SiO$_2$ interfacial slippage. We show that the overall composite film is stable when only the tangential slippage is allowed. But it becomes unstable when normal slippage is allowed: the coherently strained Ge island induces a large local bending of Si layer, and debonds the Si layer from the underlying SiO$_2$ forming a void at the Si/SiO$_2$ interface. Thus, the quality of Si/SiO$_2$ interface is expected to play an important role in controlling the stability of those device structures employing the strained Si/SiO$_2$ film. *This work is supported by DOE.   

PECVD growth of SiGe layers for high speed devices and MEMS.

We will report on SiGe layers deposited by Plasma Enhanced Chemical Vapor Deposition (PECVD; MV Systems, Colorado) for use in high speed devices, MEMS and Bolometry. Increasing the germane concentration allows the deposition temperature to be decreased, which decreases the thermal conductivity of the samples and improves their properties for bolometry. The samples were deposited up to 580$^{o}$C and doped with either diborane or phosphine. Films as deposited had predominantly $<$111$>$ texture and some $<$110$>$ texture as determined by X-ray diffraction. Annealing produced crystalline material as determined by resistivity and confirmed by X-ray diffraction with no evidence of cracking. Annealing tends to produces a variation of crystallite orientation. The crystallite sizes and orientations in the films will be discussed. Spectroscopic ellipsometry provided thickness and alloy composition. Research supported in part by NSF under grant {\#} 0073004.   

The Technology of Strained Si on Insulator

Scaling of MOSFET dimensions is no longer sufficient to continue performance enhancements that are expected in each subsequent silicon device generation. Improvements in charge carrier mobility are also required to stay on the Moore's curve. To achieve uniform high mobility in Si wafers, it is necessary to introduce a biaxial strain into silicon lattice. This is typically done by epitaxial growth of silicon on a virtual substrate of relaxed SiGe. Thin pseudomorphic layers of Si grow under tensile strain in order to preserve the epitaxy with a larger lattice spacing of the SiGe template. In order to take full advantage of strained Si, the film is transferred to a new substrate in such a way that there is a layer of SiO$_{2}$ between strained Si and the silicon handle wafer. Smart Cut{\texttrademark} technology, which involves wafer bonding and controlled exfoliation from the donor wafer, is the most practical way to accomplish layer transfer while preserving lattice strain. Formation of the virtual substrates is not trivial -- control and minimization of misfit dislocations are critically important. Nevertheless, strained Si on insulator (sSOI) wafers with 200 and 300mm diameter are rapidly becoming an industrial reality. Strain of $\sim $0.8{\%} is induced by growth on relaxed SiGe with 20{\%} Ge content. This can double the electron mobility in n-type transistors. Enhancing p-type devices requires higher strain values or non-standard crystal orientations. The range of strained Si thicknesses from 10-60nm allows fabrication of both fully depleted and partially depleted MOSFETs. Formation of sSOI, its properties and applications will be reviewed.   

Defect reduction in HgCdTe layers by MBE growth on CdTe mesas

The performance of infrared detectors is limited by the high defect density in the HgCdTe epilayers especially in the long wavelength region. This necessitates the growth of low defect density material for device fabrication. Patterned CdTe mesas have been proposed to grow low defect density HgCdTe epilayers by MBE. The reduction of defect density by growth on patterned substrates has been reported for different heteroepitaxial systems\footnote{E.A.Fitgerald \textit{et.al }`` Nucleation mechanisms and the elimination of misfit dislocations at mismatched interfaces by reduction in growth area'', J. Appl Phys.65 (6), 1989.}\footnote{S.Guha \textit{et.al} ``Defect reduction in strained InGaAs via growth on GaAs (100) substrates patterned to sub-micron dimensions'' Appl.Phys.Lett.55 (23),1990.} We report the growth of HgCdTe epilayers on CdTe mesas, fabricated from CdTe epilayers grown on silicon. A bright field mask with circular features of different sizes (ranging from 80$\mu $m-310$\mu $m) was used to fabricate mesas by contact lithography and wet isotropic etching. HgCdTe epilayers were grown in a Riber 32P MBE system. Etch pit density measurements were made on the epilayers and a reduction of EPD was observed on the mesas compared to the planar regions of the sample. This reduction of EPD could provide a breakthrough in the infrared technology.   

Low Temperature Epitaxial Growth of Antimony Doped Silicon for Broadband Astronomical Charge-Coupled Devices

Future NASA missions will require exceptionally large focal plane arrays to explore the large-scale structure of the universe. High-purity, p-channel silicon CCDs provide a unique combination of high resolution, extended response in the near infrared, and improved radiation tolerance necessary for these missions. We have demonstrated low temperature growth of antimony-doped silicon on the back surface of high purity silicon charge-coupled devices (CCDs), enabling imaging at full depletion with high resolution, high quantum efficiency, and broadband response. Using molecular beam epitaxy, we were able to grow silicon layers less than 5 nm thick with an integrated dopant concentration greater than 10$^{14}$ cm$^{-2}$. Our low-temperature process kept the device temperature below 450 C at all times, enabling growth on fully-processed CCDs. We will discuss the effects of surface preparation, temperature, Sb dose, and thickness on the leakage current and quantum efficiency of these detectors.   

Dislocations in Ge/Si$_{1-x}$Ge$_x$ films: atomistic simulations and elastic-theory calculations

Molecular dynamics simulations based on Tersoff potentials are used to investigate 60$^\circ$ dislocation stability and mobility in compressed Ge films on Si$_{1-x}$Ge$_{x}$ [1]. For low misfit values glide dislocations appear as partials (90$^\circ$ and 30$^\circ$), separated by a stacking fault. By increasing the misfit, dissociation is no longer observed, and the core geometry becomes the perfect, Hornstra one. Elastic-theory calculations provide an explanation of the observed behavior, caused by the stress- dependent effective force acting on the two cores. Shuffle dislocations, on the other hand, behave very differently. Under high compressive strain conditions, indeed, the perfect Hornstra core is conserved during the fast gliding motion observed in the simulations. The above described misfit-dependent behavior is consistent with recent experimental results [2]. [1] A. Marzegalli, F. Montalenti, and Leo Miglio, Appl. Phys. Lett. (in press). [2] D. Chrastina et al., Thin Solid Films 459, 37 (2004).   

Session J16: Focus Session: Nano-spectroscopy of Quantum Dots

Learning the Rules of Extreme Quantum Confinement: Ultrafast Photophysics of Semiconductor Nanocrystals

Zero-dimensional (0D) semiconductor nanocrystals (NCs) allow the realization of new types of strongly interacting multiexciton states that do not occur in bulk semiconductors. Using sub-10 nm colloidal nanoparticles one can generate states, in which several excitons occupy a volume comparable to or smaller than the volume of a bulk exciton. Such ``squeezed'' exciton states are characterized by greatly enhanced mutiparticle interactions resulting from a forced overlap of electronic wavefunctions and a reduced dielectric screening. In this paper, we utilize various ultrafast optical technique in order to study spectroscopic and dynamical signatures of multiexciton states in size- and shape-controlled CdSe and PbSe NCs. Specifically, by using a series of elongated CdSe nanoparticles (quantum rods), we investigate the effect of the 0D-to-1D transition on the efficiency of multiparticle Auger recombination. Further, by applying a femtosecond photoluminescence up-conversion technique, we detect the emission from neutral and charged biexcitons and directly measure their interaction energy. Finally, we use PbSe NCs to study the effect of carrier muliplication, in which relaxation of a single, high-energy exciton produces multiple excitons. In addition to providing new insights into the physics of exciton-exciton interactions in the regime of extreme quantum confinement, our studies have a direct relevance to the number of emerging applications of NCs in such areas as optical amplification and lasing, nonlinear-optical switching, and photovoltaics.   

Ultrafast carrier capture dynamics in laterally ordered InGaAs/GaAs quantum wires

Carrier capture into semiconductor nanostructures has been a popular research area in recent years, in part due to efforts to improve the efficiency of nanoscale devices which require carrier capture into an active region after optical or electrical injection. Terahertz (THz) pulse spectroscopy is a powerful tool for investigating this capture process due to its sensitivity to free carriers, sub-picosecond time resolution and non-contact nature. In this talk, we present results of recent time-resolved THz pulse spectroscopy experiments investigating carrier capture into a single layer of strain-induced laterally-ordered InGaAs quantum wires on a [311] GaAs substrate after photoexcitation by 400 nm, 100 fs pump pulses. The temperature and fluence dependence of the carrier capture process is examined both perpendicular and parallel to the wires by using the polarization of the THz probe pulse. The authors acknowledge financial support from NSERC, CIPI, iCORE, NSF, and DMR-0080054 (C-SPIN).   

Polarization and Pumping Intensity Effects on the Energy Transfer Rate in Quantum Dots

We study the dependence of the excitation transfer rate between coupled quantum dots (QDs) on the polarization and the intensity of the exciting light. We use the density matrix to study the dynamics of the luminescence polarization of the QDs [1]. We consider a detailed description of the band edge fine structure of the exciton in the QDs based on an effective mass description with eight exciton levels [2]. The QDs are coupled via the dipole-dipole Forster-like interaction with realistic, experiment-relevant parameters. We investigate the dependence of the luminescence polarization on the polarization of the exciting light and how such measurement give us information about the angular orientation of the excited dipole in the donor dot and the transferred dipole in the acceptor dot. We also study the dynamics of the QD system in the case of multiple-excitons, obtained by increasing the intensity of the exciting light. In this case, the coupling between the QDs includes all the possible interactions between the excitons. The exchange interactions are found to strongly influence the energy transfer rate and affect the resulting polarization. Supported by The Indiana 21$^{st}$ Century Research and Technology Fund. [1] A. N. Al-Ahmadi and S. E. Ulloa, Phys. Rev. B \textbf{70}, 201302(R) (2004). [2] Al. L. Efros and M. Rosen, Annu. Rev. Mater. Sci. \textbf {30}, 475 (2000). Al. L. Efros, phys.rev. B 46, 7448 (1991).   

Spin memory effect in single magnetic quantum dots

We image zero-field spin polarization of single CdMnTe quantum dots (QDs) using a slit-confocal microscopy and a solid immersion lens. For non-resonant excitation, where excitons randomize their spins before being captured by QDs, both $\sigma \quad ^{+}$ and $\sigma \quad ^{-}$ - polarized PL emission lines are observed. This strongly ($>$25{\%}) polarized emission implies a preferred direction of spin alignment of magnetic impurities in single QDs, and thus a preferred orientation of exciton magnetic polarons (EMPs). Since the single dot emission lines include accumulation of $\sim $ 10$^{4}$ photons this magnetization must persist through many recombination events. Therefore, we conclude that the EMP formed in a QD by a subsequent exciton often follows the spin alignment of the EMP formed through the previous exciton occupation. We attribute the zero-field spin memory effect observed for single CdMnTe QDs to the decay time of the Mn magnetization being significantly longer than the time intervals between consecutive exciton occupations. The work was supported by NSF grants nr 9975655 and 0071797 and PBZ-KBN-044/P03/2001.   

Time-Resolved Spectroscopy of Single Excitons Bound to Te Isoelectronic Pairs in ZnSe

Single Te impurity centers in ZnSe were probed with time-resolved photoluminescence spectroscopy. Resolution-limited peaks with an ultra-low spatial density originate in the recombination of excitons deeply bound to single nearest-neighbor isoelectronic Te pairs. This interpretation is confirmed by ab-initio calculations. The peaks reveal anti-bunched photon emission and a doublet structure polarized along $[110]$ and $[\bar {1}10]$. We analyze the time-resolved PL decay to clarify the role of the dark states in the spin relaxation and radiative recombination of single fine-structure split excitons.   

Population oscillations of two orthogonal states in a single quantum dot

We investigated the exciton dynamics in a single self-assembled quantum dot with a V-type three-level structure. Using tailored pulse pairs we generated population oscillations between two orthogonal excitonic states without a direct transition. We found good agreement between measured data and theoretical calculations.   

Tunneling control and phase dependent phenomena in a two-level system

The search for an ideal qubit has resulted in the study of structures composed of effective two level systems, including double quantum dots, small molecules, and superconducting junctions. The dynamical manipulation of such systems is then of great broad interest. One common approach is to drive the system with an oscillating field, which is known to produce the interesting phenomenon of coherent destruction of tunneling (CDT) [1]. We have reported previously on the degree of localization (or the ability to block the tunneling) in this system, and how it decreases by lowering the frequency of the driving field [2]. In this work we show that this localization is in fact highly dependent on the {\em phase} of the drive, which makes this phenomenon even richer and more interesting, with possible application to quantum information. We report here how to use this effect to control the rotation of the equivalent qubit on the Bloch sphere and how the phase can produce a well- controlled `revival’ of the localization. Supported by the Indiana 21st Century Research and Technology Fund, and FAPESP-Brazil. [1] M. Grifoni and P. H\"{a}nggi, Phys. Rep. \textbf{304}, 229 (1998). [2] J. M. Villas­-B\^{o}as, W. Zhang, S. E. Ulloa, P. H. Rivera, and N. Studart, Phys. Rev. B \textbf{66}, 085325 (2002).   

Interdot coupling through common excited state in II-VI self-assembled quantum dots

We study the excitation coupling in CdTe/ZnTe self-assembled quantum dots (QDs) by means of photoluminescence excitation (PLE) imaging. We use a solid immersion lens in combination with slit-confocal microscope and a multi-channel CCD detector to simultaneously resolve the single dot emission energy, excitation energy and position. The PLE spectra feature sets of several different single dot emission lines coupled to identical excitation resonances with energies about 100 meV above the QD emissions. This result is a signature of an excitation coupling through a common excited state between QDs. A very narrow linewidth of these high-energy resonances of 0.5 meV suggests a very efficient relaxation from these excited states. Using spatially resolved PLE imaging spectroscopy we show that these coupled QDs occur within a 500 nm spatial cluster. This suggests that there is a quasi zero-dimensional electronic state which extends across 500 nm and is coupled directly to the cluster of strongly confined CdTe dots. The work was supported by NSF grants nr 9975655 and 0071797 and PBZ-KBN-044/P03/2001.   

Optical properties of quantum dot systems

Interlevel electromagnetic response of different quantum dot (QD) systems is theoretically investigated within the self- consistent field approach. It is shown that the Coulomb coupling must be taken into account for correct description of optical spectra of the systems. Fundamental importance of the problem of the electron self-interaction in QD systems is established. It is shown that the shape of QD can dramatically affect the spectra, in particular, depending on the polarization of incident radiation and number of electrons in the dot. It is found that the effects of the intradot and interdot Coulomb interactions on the response can be analyzed separately. It is established that the approximation of the point dipole-dipole interaction can be used for adequate representation of the dynamic interdot electron-electron interaction in the lattice. Also it is shown that the approach of the modified oscillator strength very well reproduces the absorption spectra of the considered systems with interacting modes of the collective excitation.   

Optical spectroscopy of single impurity centers in semiconductors

Using optical spectroscopy with diffraction limited spatial resolution, the possibility of measuring the luminescence from single impurity centers in a semiconductor is demonstrated. Selectively studying individual centers that are formed by two neighboring nitrogen atoms in GaAs makes it possible to unveil their otherwise concealed polarization anisotropy, analyze their selection rules, identify their particular configuration, map their spatial distribution, and demonstrate the presence of a diversity of local environments. Circumventing the limitation imposed by ensemble averaging and the ability to discriminate the individual electronic responses from discrete emitters provides an unprecedented perspective on the nanoscience of impurities.   

A theoretical study of the structural and electronic properties of CdSe/CdS and CdS/CdSe core/shell nanoparticles

We present a theoretical study of the structural and electronic properties of CdSe/CdS and CdS/CdSe core/shell nanoparticles. The results are relevant not only for understanding the properties of these nanoparticles but also for understanding those of quantum dots. We have considered nanoparticles whose structures were obtained as relaxed structures of essentially spherical parts of the zincblende crystal structure and with one semiconductor compound outside the other one. The electronic properties and the total energy for a given structure were calculated using a parameterized density-functional tight-binding method. The results give information on structure, electronic properties, the HOMO and LUMO orbitals, the charge distribution, and the stability of these core/shell nanoparticles. The results depend critically on the size of both core and shell, and only in one single case we find a charge separation upon excitation. We have also investigated the energetics related to the interchange of a S and a Se atoms between the core and the shell. Although the total energy may be lowered upon interchange, the energy barrier for this process is so large that the systems should be stable against degradation.   

Exciton radiative lifetime and the ``dead-layer'' effect in ZnO quantum dots

ZnO quantum dots (QDs) have recently attracted significant attention due to the proposed optoelectronic and nanoelectronic applications. We theoretically investigated the exciton radiative lifetime and the thickness of the ``dead-layer'' (layer where exciton does not penetrate) for ZnO QDs with radii from 1 to 3 nm [1]. Our calculations show that the dead layer formed near the QD surface is rather thick for ZnO QDs, what is attributed to the large hole-electron effective mass ratio in ZnO. The excited exciton states are also investigated as a function of the ZnO QD size. The small radiative lifetime and thick dead layer found in ZnO QDs can be beneficial for device applications owing to better luminescence and isolation of the exciton from surface defects [2]. The obtained results can be used for the optimization of ZnO QD arrays for optoelectronic applications. The authors acknowledge the support of MARCO and its Functional Engineered Nano Architectonics (FENA) Focus Center. [1] V.A. Fonoberov and A.A. Balandin, Phys. Rev. B 70, 195410 (2004). [2] V.A. Fonoberov and A.A. Balandin, Appl. Phys. Lett. 85, in press (Dec. 20, 2004).   

Session J17: Focus Session: Materials and Device Physics for Quantum Computing

Single electron transistor (SET) devices for probing single donors in Si and for microscopic CV characterization

We will describe SET devices fabricated in our group for measurement of single donors. The Al/Al$_{2}$O$_{3}$/Al SET is fabricated with standard electron-beam lithography and double-angle thermal evaporation. A SiO$_{2}$ barrier layer about 20 nm thick isolates the SET from the lightly n-doped silicon, and the substrate is heavily boron doped using high energy ion implantation, and hence conducting at low temperature, beginning a few hundred nm below the SiO$_{2}$/Si interface. We will describe our fabrication process and characterization of our devices made with capacitance voltage (CV) measurement. CV measurement is a traditional tool for probing doping, carrier densities and interface trap densities. It can be extended to microscopic regime using SET electrometers. In the SET measurements, after the voltage on the substrate was swept, the recorded coulomb blockade peaks were counted and the capacitance could be extracted. We will compare the macroscopic and microscopic CV measurements in our devices. Finally, we will present the results of electrostatic modeling of our device design and discuss improvements to the design that will enhance the sensitivity to motion of single electrons at donors..   

Field ionization of individual donors in Si measured with a single electron transistor

Many proposals for spin qubits in semiconductors rely on spin-charge conversion combined with charge measurement for determination of the final state. In pursuit of such a measurement we have engineered devices consisting of a single electron transistor (SET) on a lightly n-doped, oxidized (thickness 25 nm) Si wafer. A p++-doped region 250 nm below the oxide serves as a back gate for donor ionization, creating an electric field towards the SET island and perpendicular to the surface. It also pins the Fermi level in the substrate, so that we can empty or fill the donors above by applying an appropriate bias. To empty the donors we apply a strong positive voltage to the back gate, pulling any donor electrons into the substrate. To fill the donors we apply a negative voltage and momentarily shine an LED at the sample, flooding it with electron-hole pairs. We will present recent results that give strong evidence for the filling and emptying of donors and for their ionization and recapture in an electric field as measured by an SET. We will conclude with a discussion of our efforts to extend these techniques to a measurement of spin.   

Detection of Quantum State of Electrons Bound to Shallow Donors via Resonance Fluorescence: Ensemble Measurements

Electrons bound to shallow donors in GaAs have a hydrogenic spectrum with principal transitions in the THz range and below the optical phonon energy. The 1S and 2P levels serve as a model solid-state qubit. Bound excitons can be resonantly excited from donors in their 1S state. Since the probability of decay to the 1S state is high, donors in the 1S state scatter light; donors in excited states do not. This is similar to the cycling transition used for readout in ion trap quantum computers. When the bound exciton recombines, some energy may be transferred to the donor, leaving it in an excited state. This Auger process is the limiting factor in making nondemolition measurement of the qubit. We present measurements of the change in resonance fluorescence of ensembles of donor-bound electrons due to THz excitation, corresponding to a decrease in the fraction of electrons in the 1S state. Resonance fluorescence is less destructive to the qubit than photoconductivity-style measurements, and may be used to measure lifetimes of states of bound electrons. Research supported by CNID, DARPA-QUIST, and Sun Microsystems.   

Quantum computing on long-lived donor states of Li in Si

We predict a gigantically long lifetime of several excited states in the ground-state ({\em 1s}) manifold of an interstitial lithium donor in silicon. The nature of this effect roots in the anomalous level structure of the {\em 1s} Li manifold under external stress. In particular, the coupling via the deformation potential between the lowest two states of the opposite parity is very weak and occurs via intervalley phonon transitions only. We propose to use these states under the controlled ac and dc mechanical stress to process quantum information. We find an unusual form of the elastic-dipole interaction between different donors. This interaction scales with the inter-donor distance $R$ as $R^{-3}$ or $R^{-5}$ for the transitions between the states of the same or opposite parity, respectively. The long-range $R^{-3}$ interaction provides an extremely high fidelity mechanism for 2-qubit operations.   

Gate voltage control of exchange interaction for phosphorous donors in silicon.

We perform realistic calculations for coupled phospherous donors in silicon delta-doping sheet, which is relevant for silicon-based quantum computation. With the help of generalized unrestricted Hartree-Fock method, we study the influence of valley-orbit interaction on the exchange coupling. We also solve the tunable gate potential by Poisson's equation and study the gate votage dependence of the exchange splitting. The implications are examined for silicon-based quantum computer architecture, where phospherous donor electron spin encodes logic qubit and exchange interaction is employed to generate entanglement among qubits.   

Pulsed Electrically Detected Magnetic Resonance of 2D Electrons in a Si/SiGe Quantum Well

We have developed a new method of pulsed EDMR (Electrically Detected Magnetic Resonance) and applied it to measure spin relaxation times of 2D electrons in a Si/SiGe quantum well. The method is based on spin-dependent transport in the 2D channel: Conduction electrons scatter off each other, and their scattering cross section depends on the relative orientation of their spins [1]. The initial, thermal polarization of 2D electron spins (at H=350 mT and T=4 K) is altered by applying the resonant 10 GHz microwave pulses. A change in the spin polarization results in a variation of the device conductivity ($\sim $10$^{-4})$, and its recovery back to the thermal equilibrium is measured after the microwave pulse. The recovery time measures the spin relaxation, and we find T$_{1}$ = 1.4 $\mu $s for 2D electrons in a modulation-doped Si quantum well, the same time as we measure with conventional pulsed spin resonance. This new pulsed EDMR method will allow the measurement of T$_{1}$ and T$_{2}$ on small semiconductor structures with sensitivity down to a few spins, possibly a single spin. [1] Ghosh and Silsbee, Phys. Rev. B 42, 12508(1992).   

Microwave Spectroscopy of the Valley Splitting in a Silicon/Silicon-Germanium Two Dimensional Electron Gas

The strain in silicon/silicon-germanium quantum wells reduces the usual six-fold degeneracy of the silicon conduction band, leaving a pair of degenerate bands in the growth direction. Quantum confinement in the silicon well further splits this degeneracy, leading to a small, but extremely important energy gap (the valley splitting) between these lowest two levels. We perform microwave spectroscopy, electron valley resonance (EVR), between these two states. Transport measurements at 0.25 K in a silicon/silicon-germanium two dimensional electron gas (2DEG) are used to detect microwave absorption at the valley splitting energy. The lineshapes are similar to those observed in electrically detected electron spin resonance signals. The valley splitting is found to increase linearly with an applied perpendicular magnetic field. The valley splitting peak shows a dramatic (seven-fold) increase in width as the temperature is increased from 0.23~K to 0.35 K. These results indicate that in moderate magnetic fields the silicon valley degeneracy can be completely removed in low temperature quantum devices.   

Valley splitting in low-density quantum-confined heterostructures: Superposition, not Spin!

Although valley splitting in Si quantum-confined heterostructures has been studied for many years, it is far less well understood than one might expect. Because valley degeneracy is problematic in spin quantum computing as a potential source of decoherence and other difficulties it is essential that its origins be thoroughly explained. We explain the valley splitting in Si quantum wells using a simple tight-binding model which eliminates the artificial valley coupling constants found in multiband/multi-valley effective mass treatments. The results of the simple tight-binding model agree well with multiband tight-binding results, and explain the changing parity of the ground state, and the behavior of the splitting as a function of well width. The results show that the valley splitting has nothing to do with spin, but is instead purely due to the superposition of states in the quantum well.   

Long-lived spin and valley states in silicon for quantum information processing

Quantum electronics in silicon offers both challenges and opportunities for information technology. Here we take a challenge, the doubly degenerate conduction band minima of silicon quantum wells, and make it into an opportunity, stable valley states for quantum storage and control. We calculate the coupling between the two valley states using both tight-binding and approximate-analytic techniques for a lateral quantum dot. This determines the valley relaxation and optical excitation rates. Not only are the relaxation times uncharacteristically long for excited orbital states, but we find that for finite quantum wells there are 'magic' electric fields where the coupling goes to zero, suppressing valley relaxation and excitation. In the process we derive new expressions for single-valley orbital relaxation (which is very fast) and qubit spin-flip times (due to spin-orbit coupling) and compare them to valley state relaxation. We discuss important implications for 'valley qubits' in silicon quantum information processing and technology.   

Fabrication of Encapsulated H-Passivated Si(111) Surfaces for 2D Electron Systems

H-passivated silicon surfaces may provide an excellent high-mobility substrate for 2-D electron systems (2-DES) and, potentially, atomic-scale quantum devices. We have prepared H-Si (111) surfaces \textit{ex situ} using NH$_{4}$F and incorporated these atomically flat surfaces into novel field effect devices. Using Si-SiO$_{2}$ contact bonding, we encapsulate the H-Si (111) surface in a vacuum cavity, which both isolates the surface from the environment and provides a dielectric through which we can gate electrons. However, successful bonding requires both surfaces being bonded to be atomically flat (rms roughness $<$ 0.5 nm). We have observed that making ohmic contact to the 2-DES via P implantation into the Si (111) affects the surface flatness where significant height variations are created at the contact boundaries due to differential oxidation and etch rates between doped and undoped Si regions. Such topographical irregularities can inhibit contact bonding. Using AFM, we have studied these topographic features on our device surfaces, and report methods for obtaining an overall rms surface roughness $<$ 0.2 nm and for reducing the doping-induced height difference to $\le $ 0.5nm by controlling implantation, annealing, and etching parameters. Finally, we discuss ongoing work and the possible implications for quantum computing architectures.   

Electron Transport of a 2-D Electron System Gated on a Hydrogen-Passivated Si (111) Surface via a Vacuum-Silicon Interface

Creating a 2-D electron system to couple with atoms on a surface or to nuclear spins buried in semiconductors are non-trivial due to the inherent presence of disorder at the semiconductor-dielectric interface. However, it has been shown that the H-passivated Si surface in vacuum is an ideal candidate to build such devices because it can be atomically flat and entirely free of dangling bonds. We will discuss the characterization of a new field effect transistor which creates a 2D electron system on a H-passivated Si (111) surface gated through a vacuum-silicon interface. The H-Si (111) surface is preserved and encapsulated in a vacuum cavity via contact bonding of two silicon substrates (*). We report for the first time low temperature electron transport on H-Si (111) surfaces. Hall mobilities at 4.2K were measured to be $\sim $ 4000 cm$^{2}$/Vs which are higher than previous results in Si (111) MOSFETs. We also investigated the possible six-fold degeneracy of the H-Si (111) surface through low temperature (T$<$4K) magnetoconductance measurements up to B=12T. Results of the longevity of these new devices along with potential applications in quantum computing will also be discussed. * Discussed in talk given by R. N. McFarland, ``Fabrication of Encapsulated H-Passivated Si (111) Surfaces for 2-D Electron Systems''.   

Top-gated Quantum Dots in Silicon / Silicon-Germanium Two-Dimensional Electron Gases

Electrons in silicon/silicon-germanium two-dimensional electron gas quantum dots are a promising architecture for spin based quantum computation. Top gated quantum dots allow precise tuning of electron shape and interdot coupling. We report the observation of Coulomb blockade in Si/SiGe quantum dots defined by a combination of etching and metal top gating. A narrow channel or mesa is fabricated by electron beam lithography and subsequent reactive ion etching. Metal gates are deposited across the channel to define the leads of the dot. The sides of dot are defined by surface depletion from the etched sidewalls. Low temperature measurements (250mK) show a single electron charging energy of about 0.8meV. We use an etch- defined side gate to vary the potential in the dot, observing several conductance oscillations as the blockade is lifted, with a period of 280mV.   

Quantum Dot Spin Qubits: Decoherence from Nearby Impurities

We study the undesired exchange coupling between quantum dot spin qubits and other nearby electronic spins trapped on dopant impurities. Such coupling is a source of decoherence. The problem is treated in the context of a 2DEG heterostructure, with strong, local electric fields. For silicon-based systems, we develop the theory of the Stark effect for P:Si in a degenerate conduction band. We investigate the resulting Stark energy spectrum and the field dependence of the valley composition parameters.   

Single spin detection in endohedral fullerenes

Reading out single spins is challenging. We study the endohedral system of a nitrogen atom trapped inside C$_{60}$. The qubit here is the total electronic spin of the three valence nitrogen 2p electrons (a spin quartet). We propose a method of measuring this spin by placing the endohedral fullerene in a circuit and passing a current through it. If an electron hops onto the fullerene it becomes an anion, and we use the energy splitting between the triplet and quintuplet state of the N@C$_{60}$ anion to make the spin-polarisation of the current passed by the fullerene depend on the state of the qubit inside it. We estimate the size of this energy splitting and of the hopping matrix element between the fullerene and a nearby source or drain electrode. From these data we estimate temperature ranges and experimental geometries necessary for our read-out scheme.   

Electron Spin Resonance on a Single Carbon Nanotube

Little is known about the spin properties of carbon nanotubes (CNT) such as their spin-coherence time. We are in process of directly determining the electron spin coherence time of a single walled carbon nanotube by measuring the microwave reflection (S11) off a single CNT in a magnetic field at 0.3K. We expect to observe resonant microwave absorption at the Zeeman frequency, which is 27GHz/Tesla. The linewidth of these absorption peaks will provide a direct measurement of the spin-coherence time of the CNT electrons which is currently lacking in the research literature. Absorption peaks associated with the Coulomb energy, the quantum energy level separation, the energy mismatch between bands are also expected to be measured. A homodyne reflectometer has been constructed in our lab that can resolve S11 changes of 1 part in 10\^{}5. We expect that our technique of measuring the microwave reflection off of a single nanostructure will be a power spectroscopic tool to investigate a wide variety of quantum excitations in nanostructures, an important prerequisite for powerful quantum information processing based on integrated nanosystems.   

Session J18: Focus Session: Wide Band Gap Semiconductors I

Ferromagnetism in Transition Metal Doped GaN and Related Materials

There is high current interest in the development of dilute magnetic semiconductor (DMS) materials exhibiting ferromagnetic behavior for spin-based light-emitting diodes, sensors, and transistors. Such materials are formed through the introduction of transition metal (TM) ions, such as Mn and Cr, into semiconductor hosts. Unfortunately many DMS materials, such as GaMnAs, have a relatively low magnetic ordering temperature ( 170 K for GaMnAs), which severely limits their usefulness. In the past few years, several groups have reported achieving ferromagnetism at room temperature in wide bandgap materials, such as GaMnN. This property makes these materials attractive for use as ultra-low-power switching elements, where the bit state of the device is determined through control of electron spin. Furthermore, these materials may also allow for the integration of photonic (laser and light-emitting diodes), electronic (field-effect and bipolar transistors) and magnetic (information storage) devices on a single substrate, leading to a new class of electronic devices that offer multi-purpose functionality. However, to realize such devices, several challenges remain. One concern to date has been the relatively low thermal stability of the III-Mn-N compounds. Doping with Cr in place of Mn, however, appears to greatly enhance the ability of the material to retain its magnetic properties even after annealing at temperatures up to 700C, easing the road to practical device fabrication. In addition, the ability to achieve magnetic behavior in a semi-insulating barrier material such as AlCrN opens new device possibilities. The most evident application of ferromagnetic AlN is as a ferromagnetic tunnel barrier, similar to EuS, but unlike EuS should allow for operation at room temperature. Growth of tunnel devices using Al-TM-N as a barrier and Ga-TM-N as a spin injector will be discussed. This work is supported by the Army Research Office under ARO-DAAD19-01-0-0701 and NSF under ECS-0224203.   

Demonstration of carrier mediated ferromagnetism in GaMnN by co-doping and heterostructures

We demonstrate carrier mediated ferromagnetism via GaMnN films co-doped with Si and Mg, heterostructures, and p-i-n junction devices that were grown by metal-organic chemical vapor deposition. The magnetic properties of GaMnN are affected by intentional introduction of donor or acceptor states into the film. Si or Mg co-doping of GaMnN films led to either ferromagnetic or paramagnetic behavior depending on the concentration. The magnetic properties of the GaMnN material system correlates with the Fermi level. Ferromagnetism was observed only when the Fermi level was near the Mn energy band resulting in a partially occupied Mn energy level; a prerequisite for conduction within this band. This allows carriers to be present in this band to mediate ferromagnetic behavior. In addition to co-doping, the dependence of ferromagnetic properties of GaMnN films on carrier transfer across heterojunctions was also studied. The magnetic properties of GaMnN, as a part of GaMnN/GaN:Mg heterojunctions depend on the thickness of both the GaMnN film and the adjacent GaN:Mg layer. These results are explained based on the occupancy of Mn energy band and how this occupancy is altered by carrier transfer at the GaMnN/GaN:Mg interface. Thus, the ferromagnetic properties result from a solid solution of Mn in GaN.   

Electronic structure and ferromagnetism of Mn $\delta$-doped GaN

Recently, Mn-doped GaN has attracted much attention because of the ferromagnetism observed in this material. However, experimental data so far are quite controversial, reporting the Curie temperatures ranging from 10 to 940 K. Very recent experiments showed that Mn $\delta$-doped GaN films have high hole carrier concentrations, which lead to the high Curie temperature and enhanced magnetization. In this work, we study the electronic and magnetic properties of Mn doped GaN and the origin of p-type conductance especially for Mn $\delta$-doped films through first-principles spin-density-functional calculations. The nature of magnetic interactions between two Mn ions is investigated by varying the Mn-Mn distance and their orientation. The ferromagnetic coupling has a short-range nature, effective for Mn-Mn distances up to about 7 $\rm \AA$. We also investigate the doping effect on ferromagnetism, and the energetics and ferromagnetism of Mn nanoclusters. Finally, we find that Ga vacancies near the Mn $\delta$-doped layer are more stable than in the bulk region of GaN due to the charge transfer from the Mn to Ga vacancy. We suggest that Ga vacancies near the Mn $\delta$-doped layer are likely to be the origin of p-type conductance.   

Transition from Ferromagnetism to Antiferromagnetism in Ga$_{1-x}$Mn$_x$N

Ga$_{1-x}$Mn$_x$N has attracted much attention recently because previous theoretical predictions suggested that it has a ferromagnetic (FM) ground state with T$_c$ above the room temperature. However, available experimental data often contradict each other. Some reports show that high T$_c$ FM phase is achievable in GaMnN; others show that the magnetic coupling of the Mn ions in GaMnN is actually antiferromagnetic (AFM). The exact nature of the magnetism observed in this system is also under debate. To help unravel the magnetic properties of this system, we developed a new band-coupling model to show that FM order in this system is facilitated by coupling between occupied majority d states inside the gap, whereas AFM ordering in this system is caused by coupling between majority spin d band and minority spin $d$ band. At low Mn concentration Ga$_{1-x}$Mn$_x$N is FM, whereas at high Mn concentration, under pressure, or if the hole at the Mn $d$ band is compensated, Ga$_{1-x}$Mn$_x$N will change to AFM ground state. Our {\it ab initio} total energy calculation based on spin density functional theory support this model.   

Antiferromagnetic coupling driven by bond length contraction near Ga$_{1-x}$Mn$_x$N film surface

Following the discovery of ferromagnetism in (Ga,Mn)As and the subsequent theoretical prediction that Mn doped GaN could be ferromagnetic at or above room temperature, numerous attempts have been made to synthesize this promising DMS material. However, the results have been rather confusing. Not only the reported Curie temperatures vary over a wide range (10K-945K), but also it is uncertain whether the ground state of (Ga,Mn)N is ferromagnetic (FM) or antiferromagnetic (AF). An understanding of the controversy between FM and AF is both important and challenging. Using first principles calculations based on gradient corrected density functional theory we show that Mn atoms, which couple ferromagnetically in bulk Ga$_{1-x}$Mn$_{x}$N, couple antiferromagnetically on its surface. This change in magnetic behavior is brought about a contraction of the Mn-Mn and Mn-N bond lengths which is significantly smaller on the surface than in the bulk. The present study provides new insight for explaining the numerous conflicting experimental observations in Mn doped GaN systems.   

Magnetism and Energetics of Mn Doped ZnO (10 0) Thin Film

First principles calculations based on gradient corrected density functional theory are performed on Mn doped ZnO thin film. Magnetism and energetics are studied for two Mn concentrations and varying Mn configurations. It is found that in the dilute limit when Mn atoms are far apart, the ferro- and anti-ferromagnetic states are energetically nearly degenerate. The resulting fluctuation would, therefore, make the system paramagnetic as found in the experiment. But, as the concentration of Mn atoms increases, there is a tendency for Mn atoms to form nearest neighbors and cluster around oxygen. For such a configuration, the anti-ferromagnetic coupling between Mn atoms is energetically more favorable. The results are compared with a diverse range of experiments on Mn doped ZnO thin film.   

Carrier-mediated Ferromagnetism in N Co-doped (Zn, Mn)O (10 0) Thin Film

Considerable experimental work is available on the (Zn, Mn)O system. However, the results have been rather controversial. While some groups have reported ferromagnetism in (Zn, Mn)O systems, others report observations of anti-ferromagnetic or spin-glass behavior. Using first principles calculations based on the density functional theory and generalized gradient approximation we show that the ground state of Mn doped ZnO (10$\overline 1 $0) thin film changes from antiferromagnetic to ferromagnetic when co-doped with N. The ferromagnetic coupling between Mn spins arises due to the overlap between N 2p and Mn 3d electrons in the spin up band, rendering the system half-metallic character.   

The role of Mn doping in ZnO based DMS studied by x-ray absorption and emission spectroscopy

Independent control of spin and charge of doped carriers has attracted much interest in diluted magnetic semiconductors (DMSs) because the combination of the two degrees of freedom is expected to open up new functionalities in optoelectronic and magnetoelectric devices. Recently dilute Mn doped ZnO was shown to have such a property when processed at temperatures below 500$^{\circ}$C. In general, electronic structure ultimately determines the properties of matter, and therefore a detail description of the electronic structure of DMS will lead to a better understanding its magnetic properties. We studied the electronic structure of Mn-doped ZnO using X-ray absorption (XAS) and emission spectroscopy (XES). Upon Mn doping, the top of O $2p$ valence band extends into bandgap and a distinct absorption feature appears at the bottom of the conduction band, which suggests the strong hybridization of Mn $3d$ and O $2p$. The evidence of the ligand-hole states of Zn \textit{3d -- O 2p} is shown. Furthermore, Mn $2p$-absorption and $L-$emission spectra indicate Mn$^{2+}$ replacing Zn site in tetragonal symmetry.   

Investigation of Magnetic Doping of High-Density GaN Nanorods

We investigate GaN nanorods as host structures for transition metal doping and alloying. The epitaxial growth of bulk-like films of GaMnN, which has been predicted to exhibit ferromagnetism at or above 300K, has been the focus of many recent investigations. Epitaxial growth of GaMnN involves lowering the substrate temperature to allow the mobile Mn to incorporate while sacrificing the underlying GaN crystal quality. This delicate balance is difficult to achieve, and with increasing Mn flux the GaMnN often contains magnetic precipitates or serious structural inhomogeneities. We investigate the layers of vertical GaN nanorods having a width of 60$\pm$5 nm which were grown by MBE on Al$_2$O$_3$(0001) and Si(111) substrates. These rods appear to be fully-relaxed, low- defect structures which have PL with a narrow (2.05 {\AA} FWHM) peak centered at 3572.6 {\AA}. We report on nanorods with continuous Mn doping as well as Mn doped GaMnN/InGaN quantum wells incorporated into the nanorods.   

Dielectric functions of as-grown and annealed Ga$_{1-x}$Mn$_{x}$As thin films

We have investigated the dielectric functions of a series of as-grown as well as annealed Ga$_{1-x}$Mn$_{x}$As thin films using spectroscopic ellipsometry. After determining the alloy compositions using x-ray diffraction experiments, a rotating analyzer spectroscopic ellipsometer was used to measure the complex reflection ratio for each of the films in the energy range between 0.9-6.5~eV. By modelling the ellipsometric data, the dielectric functions for each of the Ga$_{1-x}$Mn$_{x}$As samples were determined. All of the dielectric functions displayed the critical point structures related to the higher order electronic transitions, and showed differences between the as-grown and the annealed sample spectra.   

High Temperature Ferromagnetic and UV-Optic Properties of Co-Doped ZnO Nanoclusters Prepared under Different O2 Atmospheres

Co-doped ZnO nanocluster films are prepared at room temperature under different oxygen concentrations by, our novel nanocluster system, based on a technique that is a combination of high pressure sputtering and aggregation. Magnetic properties of the cluster films are measured by SQUID magnetometer. We measured hysteresis loops of these samples at various temperatures and with the increase of temperature the coercivity, remanence and saturation magnetization decreased. The UV-PL intensity of the samples prepared in high O$_{2}$ atmosphere is stronger, with low FWHM compared to the samples prepared in low O$_{2}$ atmosphere. The field cooling (FC) and zero-field cooling (ZFC) data are taken and analyzed. XRD pattern of these samples are quite similar to the bulk ZnO where as XPS data showed the presence of Co in the samples.   

Session J20: Surfaces: Novel Instrumentation and Techniques

Coherent Atom Beams to Probe Surface Dynamics

We are developing a probe of surface dynamics for time scales and size scales on the order of 1ms and 1 nm respectively. This is done by quasielastic scattering of a coherent thermal atom beam off a surface. Helium atoms and hydrogen molecules have de Broglie wavelengths of about 1 angstrom making them natural choices for nanoscale probes. Scattered coherent atoms interfere with themselves to create a speckle pattern. As in dynamic light scattering, monitoring the change in the reflected speckle pattern will probe the time scales of surface dynamical processes. We have created a continuous coherent beam of atoms. We have measured diffraction from single slits, pinholes, and sets of randomly oriented pinholes (to simulate a speckle pattern). We are at the stage where we are trying to reflect the beam off of a sample. The set up includes using a high voltage ionization tip and channel electron multiplier as detector, a micron sized glass capillary nozzle with 1000 PSI stagnation pressure, and micron sized skimmers and pinholes to create the conditions for coherence.   

Atom-by-atom extraction by controlling a scanning tunneling microscope tip- cluster interaction

We present a novel atom-by-atom extraction scheme using scanning tunneling microscope (STM) manipulations on a Ag(111) surface at 6K under an ultra-high-vacuum condition. At the initial step, a silver nanocluster is deposited on the surface by gently touching the silver coated tip onto the surface. Individual silver atoms from the cluster are then extracted by precisely controlling the tip-cluster interactions. The recorded STM tip height signals reveal atomistic details of the atom extraction dynamics. The experimental findings are corroborated by total energy calculations based on interaction potentials using the embedded atom method.   

Laser Speckle Interferometry for Measuring Three-Dimensional Mesoscopic Deformations in Polycrystalline Surfaces

A speckle interferometric microscope has been constructed to simultaneously measure mesoscopic deformations in all three directions at a surface. The purpose is to understand mesoscale mechanics in polycrystalline materials using direct observation. The microscope uses three different wavelength lasers to separate dimensional data (out-of-plane and two in-plane directions) while capturing the intensity of speckle reflected from a surface as a function of position and time. Images are captured before and after deformation, with an intermediate image taken before deformation but with a mirror in one arm of each interferometer tilted. Post-processing exploits the mirror tilts to produce carrier fringes that isolate deformation data in Fourier space and increase sensitivity by orders of magnitude over traditional interferometry. Data are taken with 1 $\mu $m spatial resolution, and 10 nm deformations have been resolved. Experiments underway are using the system to study polycrystalline creep.   

Ultrafast Electron Diffraction for Interfaces and Nanometer Scale Materials

Surfaces are essential in many fundamental processes in materials and biology. Atomic interfaces can be modified with layered structures to engineer properties for nanometer scale sensing and electronics, or be used as templates for monolayer-assemblies with control from surface chemistry. Ultrafast electron diffraction (UED) can be applied to resolve, for these materials, both structures and dynamics to elucidate the underlying mechanisms and functions. I will outline the recent developments of surface UED in which crystalline substrates were used as templates for making chemically modified layers or supramolecular assemblies; their local structures and periodic orders in the long range reflect their affinities to the substrates. With controls of laser fluences, energies, and surface characters, strongly driven (either from charges or from thermal strains) restructuring of the surfaces and adspecies were observed with sub-angstrom displacement of atoms following the ultrashort laser impulse in the far-from-equilibrium regime at short time and at near-equilibrium at long times.   

Towards a direct method for low energy electron diffraction

A direct method for crystallography is an algorithm that leads directly from a set of measured diffraction intensities to an atomic-scale structure without the need for guesswork or iterative model building. Recently there has been considerable progress towards the development of such a method in the area of surface x-ray diffraction, where knowledge of the bulk structure allows the problem to be treated as one of structure completion. The algorithm involves iteratively satisfying constraints in real and reciprocal space. Extension of such a methodology to LEED is much more difficult as there is no invertible Fourier transform relationship between relevant quantities in the two spaces, due to multiple scattering. Nevertheless, we have made significant progress towards the solution of this inverse problem. We will describe the algorithm and give examples of practical applications to measured experimental data.   

Single electron manipulation and imaging by Electrostatic Force Microscopy

A new scanning probe method capable of detecting single electron tunneling events to/from individual electronic states at the surface of an insulator has recently been demonstrated [1,2]. The approach has now been used to demonstrate the manipulation and imaging of single electrons at an oxide surface. With a positive bias voltage on the sample, single electron tunneling events are observed when the probe is brought within tunneling range. Subsequent imaging clearly shows a localized change in the surface potential each time an electron tunnels. When the polarity of the bias voltage is reversed, the electron at the surface tunnels back into the probe, with a corresponding change observed in the image of the surface. This method provides a means to characterize individual electron traps in insulator films. The details of the experiment and corresponding theory will be presented and the manipulation results will be discussed. 1. L. J. Klein and C.C. Williams, Appl. Phys. Lett. \textbf{81}, 4589 (2002). 2. E. Bussmann, D.J. Kim, and C. C. Williams, Appl. Phys. Lett. \textbf{85}, 2538 (2004).   

Localized Single Electron Tunneling Spectroscopy Measurements on SiO2

A new scanning probe method capable of detecting single electron tunneling events to/from individual electronic states at the surface of an insulator has recently been demonstrated [1,2]. The approach has now been developed for performing local electronic spectroscopy measurements. The surface is imaged in dynamic force microscopy mode. After imaging, single electron tunneling force spectroscopy at specific locations on the surface is performed either by scanning the probe at fixed bias voltage toward the surface, or moving the tip within tunneling range and scanning the bias voltage on the probe. When this is done, single electron tunneling events are observed to occur only a specific sites on the surface, at particular gaps and voltages. A theory has been developed to extract the energy of the electronic state to/from which the single electron tunneling occurs. 1. L. J. Klein and C.C. Williams, Appl. Phys. Lett. \textbf{81}, 4589 (2002). 2. E. Bussmann, D.J. Kim, and C. C. Williams, Appl. Phys. Lett. \textbf{85}, 2538 (2004).   

Observation of all dangling bond states and potential variation among them by noncontact atomic force microscopy

High-resolution non-contact atomic force microscope (AFM) images were successfully taken on the Ge(105)-(1x2) structure formed on the Si(105) substrate and revealed all dangling bonds of the surface regardless to their electronic situation, surpassing the scanning tunneling microscopy, whose images were strongly deviated from the atomic structure by the electronic states involved. An atomically resolved electrostatic potential profile by a Kelvin probe method with AFM shows potential variations among the dangling bond states, directly observing a charge transfer between them. These results clearly demonstrate that high-resolution non-contact AFM with a Kelvin probe method is an ideal tool for analyses of atomic structures and electronic properties of surfaces.   

Attonewton force detection near a surface

Magnetic resonance force microscopy (MRFM) is a promising new technique for acquiring magnetic resonance images of a single molecule; to date we have demonstrated an unprecedented sensitivity of $\sim $10$^{5}$proton spins. Moving forward requires that force microscopy enter a new regime, where attonewton (10$^{-18}$ N) forces are measured near a surface. To facilitate this we operate custom fabricated, low spring constant, high quality factor cantilevers with their motion parallel to the sample surface. We observe that cantilever force sensitivity degrades with decreasing tip-sample separation due to energy losses. Our measurements indicate that this effect is dependent on tip size, composition, and tip-sample voltage. Theoretical models suggest that this effect might be due to dielectric fluctuations within the sample or inhomogeneous charge distributions on the surface. We have designed experiments to test these hypotheses and to elucidate the detailed mechanism of energy losses between a cantilever and a surface.   

Contact-Less Electrical Characterization of Fully Depleted Silicon-on-Insulator

Ultra-thin (10 nm) silicon-on-insulator (SOI) has recently emerged as an important substrate, and at nominal doping levels of 10\^{}15 per cubic centimeter will be fully depleted of free carriers by interface states at the silicon-silicon dioxide interface[1]. Therefore, when imaged with contact-less characterization techniques such as electric force microscopy, one might expect the ultra-thin silicon layer to behave as if it has no free carriers. However, our electric force microscopy measurements show that the layer possesses sufficient free charge to have a 2D resistivity of at least 800 TOhm/square, and that the silicon layer behaves as a metal when charge is deposited on it [2, 3]. We believe that thermally activated charge hopping at silicon-silicon dioxide interface provides free carriers even when there are no free carriers from the silicon bulk. We discuss the implications of this conduction path to imaging of 10 nm SOI by electric force microscopy. Research supported by DOE and AFOSR. [1] S. Henaux, et al. J. Electrochem. Soc. 146, 2737 (1999), [2] Tevaarwerk, et al, Appl. Phys. Lett. 80, 4626 (2002) , [3] P. Zhang, et al, in preparation.   

High-throughput resistivity apparatus for characterization of combinatorial libraries

A combinatorial apparatus, capable of measuring the resistance versus temperature of 49 samples prepared by thin film deposition techniques has been designed and tested. Magnetron sputtering is used to deposit films through an aluminum mask consisting of 8 mm diameter holes cut on a 7 x 7 grid. Electrical contact to the thin film samples are made in a standard van der Paaw geometry using 196 spring-loaded, gold-coated pins - four pins for each of the 49 samples. The system is able to characterize the resistivity of any conductor, semiconductor or superconductor library from 32 K to 350 K. The resistivity of a highly conductive metal (silver) and semi-conductor (multi-layer film of Si-Ge) are presented to highlight the capabilities of the apparatus.   

Session J21: Lipid and Insulating Bilayers

New Dendritic Lipids for Improved Gene Delivery by Cationic Liposome-DNA Complexes

Cationic Liposome-DNA (CL-DNA) complexes are widely used in non-viral gene delivery, including clinical trials, but their efficiency still requires optimization. Membrane charge density is a universal parameter for transfection with lamellar CL-DNA complexes (Lin AJ et al., \textit{Biophys. J.} 2003; \textbf{84}: 3307; Ahmad A et al., \textit{J. Gene Med.}, accepted). Newly synthesized lipids with dendritic headgroups, based on an ornithine scaffold, have headgroup charges of +4e to +16e. These lipids form lamellar complexes if the headgroup charge is small or the fraction of dendritic lipid in the membrane (in mixtures with DOPC) is low. Higher contents of highly charged lipids exhibit a novel phase of CL-DNA complexes, whose structure was determined by synchrotron x-ray diffraction. Cylindrical micelles of lipid are arranged on a hexagonal lattice, with DNA rods placed around them in the interstices. Complexes with this structure are highly transfecting, preventing the previously observed drop in transfection efficiency at very high membrane charge densities. Funded by NIH GM-59288.   

Interaction Between Two Nonuniform and Flexible Bio-Interfaces

One of fundamental interactions between charged bio-interfaces is the Coulomb interaction. The Coulomb interaction at nano- scale is different from that in a conventional scale due to proximity between charged particles. The attraction between two uniformly charged surfaces with same sign has been well known in a strongly coupled system. However surface charges in a real biological system are not uniformly distributed but rather discretely. We have found that the non-uniformity makes a stronger correlation of bulk counter ions to surface charges and induces stronger attractive pressure between two interfaces. Furthermore we have investigated the effect of the bio-interface flexibility on the pressure between two interfaces as a function of coupling parameter and distance for a large coupling parameter. Surface flexibility allows the surface ions to respond to counter ions. As a consequence, surface reforms to minimize total energy. Numerical simulations and theoretical analysis will be presented.   

Fluctuations spectrum of passive and active giant vesicles measured by contour analysis.

We have developed a new method of contour analysis using phase contrast microscopy on giant vesicles [1]. Our set-up allows an accurate detection at video rate, and a direct comparison with theory in a planar geometry. We have been able to measure directly fluctuations spectra. For pure lipid vesicles, we measure bending rigidities corresponding to those of the literature. Our technique has also been extended to non-equilibrium membranes. We have set up a protocol to prepare giant vesicles containing bacteriorhodopsine[2], a light- activated protons pump. When the protein is pumping this system is a simple model of active membrane. We have measured the fluctuation spectra of these active liposomes. As a first analysis, our results cannot be explained by actual active membranes theory [4] and are not in agreement with micropipette experiments [3-4].\\[4pt] [1] J.P\'{e}cr\'{e}aux et al. (2004) Eur. Phys. J. E 13(3): 277-290.\\[0pt] [2] P. Girard, J.P\'{e}cr\'{e}aux et al (2004), Biophys. J. 87: 419-429.\\[0pt] [3] J.-B. Manneville et al. (1999), Phys. Rev. Lett. 82: 4356-4359.\\[0pt] [4] J.-B. Manneville et al. (2001), Phys. Rev. E 64(2): 021908.   

Small-angle neutron scattering study of lipid bilayers of magneto-vesicles

We present results from a small-angle neutron scattering (SANS) study of two types of magneto-vesicles (MVs): dioleoylphosphatidylcholine (DOPC) vesicles containing citrate-coated magnetic nanoparticles and those containing oleic-acid-coated magnetic particles. By using a polydisperse core shell model, these MVs were found to have similar sizes as their original vesicles without magnetic particles and their bilayer thicknesses to be consistent with unilamellar structure.   

Shape Transformation of Fluctuating Vesicles Filled with a Ferrofluid Emulsion

By transferring inverse double emulsion (O/W/O) droplets from an oil phase into a water phase we have assembled asymmetric vesicles containing monodisperse submicron-sized emulsion droplets, which are made of an oil-based ferrofluid. Under a magnetic field the submicron-sized ferrofluid droplets trapped inside flexible vesicles form chain structures, which depend on the size and the shape of vesicles. The formation of chains of trapped ferrofluid droplets can also induce shape changes in fluctuating vesicles. We examine the metastable shapes of lipid vesicles manipulated by an external magnetic field. The responsive vesicles provide a model system to study the topological and rheological properties of biological membranes. The equilibrium shapes and stability of the vesicles under various ionic strengths are also studied.   

Electrostatically driven spatial patterns at supported lipid membrane junctions

We have recently shown that mobile, membrane-bound proteins sandwiched at simple, cell-free junctions between lipid bilayers can organize themselves into micron-scale spatial patterns. This pattern formation is mechanical in origin, a consequence of the coupling of the lateral mobility of the proteins and inter-membrane adhesion forces. We find that these mechanically driven protein patterns can electrostatically generate patterns of charged membrane lipids. Measuring the magnitude of the electrostatic interaction as a function of lipid composition and ionic strength, and quantitatively analyzing the interplay between thermodynamics and electrostatics via a Poisson-Boltzmann approach, we are able to determine the charge densities and surface potentials of the components of our junctions -- properties that are difficult or impossible to measure by other means. Surprisingly, the electrostatic potential of the proteins is a minor factor in the lipid reorganization; the protein size and its modulation of the junction topography play the dominant role in driving the electrostatic patterns.   

Thermotropic liquid crystalline properties of amphotropic branched glycolipids

Glycolipids are amphotropic liquid crystals forming lyotropic liquid crystals in aqueous solutions and thermotropic liquid crystals in their dry form. We report studies of six different branched glycolipids: four maltoside and two glucoside lipids in their dry form. Optical birefringence, electrical conductivity, DSC, and dielectric spectroscopy measurements were employed to characterize the phase structures of the materials. In general they exhibit a wide ($>$100$^{o}$C) mesophase (smectic, columnar) range with low (0.01-0.04) birefringence. They have large (60-120) dielectric susceptibility mainly proportional to the number of polar sugar heads. Depending on the temperature the relaxation frequency of the susceptibility varies from $<$100Hz to $>$1MHz, mainly determined by the hydrogen bonding between the polar sugar heads. Interestingly in heating the materials with long hydrophobic chains show optically isotropic phase between the smectic and columnar phases. Such a situation when more phases appear in heating than cooling has never been observed in other thermotropic liquid crystals.   

Membrane Simulations Over Long Length and Time Scales

We present a simulation algorithm for the dynamical evolution of lipid bilayers over long length and time scales. The membrane is treated as an elastic sheet with bending energy and surface tension and includes hydrodynamic coupling to the surrounding solvent and thermal fluctuations. The method we have developed allows for arbitrary external forces acting on the membrane and is particularly useful for studying many biological processes inaccessible to detailed atomistic simulations. Using this method, we have studied the repulsive interaction between the cytoskeleton and the membrane and its effect on thermal height fluctuations. We present results on the above application, but focus on recent work involving nonthermal fluctuations in membranes due to the activity of protein pumps embedded in the surface. We quantify the influence of these pumps on the height fluctuations in the membrane and discuss the results.   

Kinetics of rupture of a lipid bilayer under tension, with and without peptides in solution

The rupture of fluid membrane vesicles with a steady ramp of micropipette suction has been shown to produce a distribution of breakage tensions. The width and mean of the distribution increases significantly with tension rate (E. Evans et al. Biophys. J. vol. 85, p. 2342 (2003)). Starting from a lattice model which incorporates the essential features of the lipid bilayers held together with hydrophobic forces (Phys. Rev. E vol. 67, no. 051908 (2003)), and developing it to handle varying tension rates, we reproduce the essential features of the experimental results. In essence we show that the rupture kinetics are driven for all tension rates by the nucleation and growth of pores. The role of peptides in solution that can adsorb and insert themselves into the bilayer is also considered. Parameters relevant to various peptides and bilayers are used to explore the different possible scenarios that can occur, including recent experiments.   

Microtubule – Cationic Liposomes Assemblies: Pathways to the Formation of Lipid-Protein Tubular Complexes

The self-assembly of microtubules and charged membranes has been studied, using X-ray diffraction and electron microscopy. Polyelectrolyte lipid complexes (PLC) usually form structures where the lipid phase acts as the template, when the polyelectrolyte curvature (Cp) is much larger than the membrane spontaneous curvature (Co). When Cp approaches Co, as in microtubules, two new structures emerge. Depending on conditions, vesicles either adsorb onto the microtubule, forming a ‘beads on a rod’ structure, or undergo a wetting transition, coating the microtubule, which now forms the template. Tubulin rings next coat the microtubule-lipid assembly, forming a lipid protein tubular complex (LPTC). The ‘beads on a rod’ structure is a non-templated, kinetically trapped assembly state. The kinetic energy barrier between the two states depends on the membrane bending rigidity and charge density. The LPTC is the ground state of the system. Finally we make a connection to earlier studies and describe the assembly pathways of PLC as a function of polyelectrolyte curvature, membrane bending rigidity and charge density. This project is supported by NIH GM-59288, NSF DMR- 0203755, CTS-0404444. U.R. is supported by the HFSPO fellowship.   

Temperature Effect on Transport Dynamics of a Cationic Molecule across an anionic Liposome

By using second harmonic generation (SHG), we have studied the transport dynamics of cationic triphenylmethane dyes across anionic liposome bilayers. Because the square root of SH signal is proportional to the difference between the surface density of dye molecules on both sides of the bilayers, the time dependence of the SH signal provides nondestructive and in-situ information on the transport of these dyes across liposome bilayers. In this experiment, we measured the time dependence of SH signal as a function of temperature and dye concentration. The temperature dependence of the transport rate follows the Arrhenius equation. We found that activation energy is about 1 eV and independent of dye concentration. This indicates that the charge interaction plays an important role for the transport. Furthermore, we observed some interesting phenomena related with dye concentration dependence of the transport dynamics.   

Oxygen Transport Across Space-Filling Biological Membranes

Space-filling fractal surfaces play a fundamental role in how organisms function and in how structure determines function at various levels. In this project we developed an efficient and powerful algorithm, rope-walk algorithm, for solving diffusion equations of transport of species across the space-filling fractal surface. We performed analytic computations of the oxygen current across the alveolar membranes in the lung, as a function of diffusion coefficient and membrane permeability, using the rope-walk algorithm, without adjustable parameters. The analytic calculation identifies the four cases as sharply delineated screening regimes and finds that the lung operates in the partial-screening regime, close to the transition to no screening, and in the no-screening regime, for respiration at rest and in exercise respectively. The gas exchange satisfies six criteria of optimal design: maximum current; minimum waste of surface area; minimum permeability; maximum fault tolerance; minimum waiting time and maximum current increase when going from rest to exercise. This extraordinary, multiply optimized performance is a direct consequence of the space-filling membrane architecture.   

Dielectric behaviors of polydisperse cell suspensions

It is of technological importance to separate various cells which differ from one another in their dielectric properties. For instance, living cells have different conductivity from that of dead cells resulting in polydisperse cell suspensions. As a theoretical model, we consider a bidisperse suspension in which two different types of spherical biological cells are dispersed in a host medium, subject to an external ac electric field. The Clausius-Mossotti factors of the cells with isotropic, lossless dielectric membrane and with an intrinsic dispersion due to the presence of mobile hydrophobic ions within the plasma membrane have been given by Roth and Dignam [1]. A dielectric dispersion spectral representation (DDSR) is employed to express the Clausius-Mossotti factors of the two types of spherical particles as a series of sub-dispersions [2]. With DDSR, the characteristic frequencies and the corresponding dispersion magnitudes of the various sub-dispersions are determined for the individual particles in suspension [3,4]. We will report the effects of medium conductivity and volume fraction on the dielectric dispersion spectra. [1]. J. Roth and M. J. Dignam, J. Opt. Soc. Am. 63, 308 (1973). [2]. J. Lei, J. T. K. Wan, K. W. Yu, H. Sun, Phys. Rev. E 64, 012903 (2001). [3]. J. P. Huang, K. W. Yu, G. Q. Gu, Phys. Rev. E 65, 021401 (2002). [4]. L. Gao, J. P. Huang, K. W. Yu, Phys. Rev. E 67, 021910 (2003).   

Session J22: Biochemical and Genetic Networks

Understanding and Improving Massively Parallel DNA Detectors for Biomedical Assays

Microarrays are highly parallel sequence specific DNA detectors used to quantitatively study genotypes and gene-expression levels. Commercial agitation systems aim to remove the coverslip diffusion bottleneck, but the efficiency increase provided by these devices is variable and occasionally even negative. The underlying causes of these variations are not well understood. We have investigated hybridization efficiency using liquid-on-liquid mixing, in which an impeller stirs a viscous oil phase covering a thin film of fluorescently labeled target solution, which lies on a glass microarray substrate. Absolute efficiency studies indicate the diffusion limit is generally well obeyed by static hybridizations, but stirring produces no significant improvement in efficiency. To check whether shear deformation of DNA is a limiting factor a pause cycle is added to the mixing procedure, but no further improvement is observed. Antibody delivery experiments with a comparable diffusion constant show a clear increase in efficiency due to mixing.   

Label-less Fluorescence-based Detection for DNA Microarray

Microarray technology is the key to rapid, inexpensive gene sequencing that is the corner stone of modern medicine with the potential to diagnose disease before clinical signs and personalize medicine. By coupling light scattering and fluorescence, we describe a quantitative, label-free assay for microarray analysis with a dynamic range of 1 in 10$^{4}$ at signal-to-noise ratio of 3:1. Since light scattering is intrinsically proportional to number of molecules, the change in fluorescence is highly linear with respect to percent binding of single stranded DNA (ssDNA) target with the immobilized ssDNA probes. Since the scattering is proportional to fourth power of refractive index, the detection of binding is an order of magnitude more sensitive compared to other optical methods based on change in thickness and refractive index, such as, reflectivity, ellipsometry and surface-plasmon resonance. Remarkably, polystyrene film of optimum thickness of 30 nm is the best fluorescent agent since its excitation wavelength matches (within 5 nm) with wavelength for the maximum refractive index difference between ssDNA and dsDNA.   

Label-free optical detection of biochemical reactions in microarray format

We have developed an oblique-incidence optical reflectivity difference (OI-RD) scanning microscope for detecting biochemical reactions involving unlabeled macromolecules such as DNA, protein, or lipid membranes in microarray format. This optical microscope detects changes in density, thickness, and conformation of macromolecules as a result of the reactions of probe molecules with target molecules immobilized on a solid surface such as a chemically functionalized glass microscope slide. Of particular interest to our current investigation are microarrays of small ligands and macromolecules that are targeted for protein binding. Our OI-RD microscope is particularly desirable for such microarray-based proteomic investigations as it offers the capability to detect activities of protein molecules without the influence of extrinsic "tag" molecules attached to the protein (such as organic fluorophore molecules) and other undesirable effects such as photobleaching. We have used our OI-RD scanning microscope in a series of proof-of-principle studies of oligonucleotide hybridization and antibody-antigen capture reactions \textit{without labeling}.   

Identifying pattern in microarray expression series using algorithmic information theory

We introduce a method of detecting pattern in data series independent of the nature of the pattern. This is achieved by calculating a lower bound on the Algorithmic Information Content (AIC) of the data series, the exact value of the AIC being fundamentally uncomputable. This bound also provides us with a measure of the algorithmic compressibility. Data series which are highly compressible are more likely to result from simple underlying mechanisms than series which are incompressible. We show that the compression in bits is a universal currency by which we can order data series according to their significance, even if they are from different experiments or exhibit different kinds of pattern or noise. We test our method on microarray time series of yeast cell cycle and show that is very successful at blindly selecting genes identified by independent experimental studies, without making any assumptions about what kind of pattern these data series contain.   

Microarray Studies of Arabidopsis Gene Response to High Magnetic Fields.

Microarray analyses indicate that a homogeneous magnetic field of 21~Tesla has a far reaching effect on the genome of Arabidopsis plants. Survey of an Affymetrix microarray populated with 8,000 genes from the arabidopsis genome reveals that although most of the genes in the array show less than a 2-fold difference in expression between the 21~Tesla treatment and control, many show striking differential expression (5-50 fold). These results were corroborated by quantitative real-time reverse transcriptase - polymerase chain reaction (qRT-PCR), a method often used in conjunction with microarrays to support the scatter plot data rendered from the two-way comparison (21~Tesla vs. control) of the arrays. Scatter plots of treatment vs.~control data are saturated where differential expression is less than 2-fold. In an attempt to extract additional information from this area, topographical plots were generated to reveal the numbers of genes represented by any given point on the plot, providing information that may prove insightful in future analyses of microarray data.   

Using mutual information to infer gene-gene interactions from microarray expression series

Identifying network structure from microarray data rests crucially on what is meant by `similarity' between two gene expression patterns. We introduce a method of inferring gene-gene interactions without making assumptions about what kind of expression correlations to look for. Our approach is to bound the mutual algorithmic information, measured in bits, between sets of measurements for two genes; a higher level of mutual information corresponds to a greater confidence of interaction. We have applied our method to yeast cell cycle and bladder cancer.   

Quantifying optimal accuracy of local primary sequence bioinformatics methods

Traditional bioinformatics methods scan primary sequences for local patterns. It is important to assess how accurate local primary sequence methods can be. We study the problem of donor pre-mRNA splice site recognition, where the sequence overlaps between real and decoy data sets can be quantified, exposing the intrinsic limitations of the performance of local primary sequence methods. We assess the accuracy of local primary sequence methods generally by studying how they scale with dataset size and demonstrate that our new Primary Sequence Ranking methods have superior performance. Our Primary Sequence Ranking analysis tools are available at {tt http://rna.williams.edu/}   

Evolution at the Nucleotide Level

We carry out a quantitative analysis that supports the viewpoint that DNA mutations do not occur with equal probabilities. We find evidence that the identity of the neighboring nucleotide within a codon influences the probability of a point substitution and we use a mutation model to quantify the strength of these interactions. We find a set of neighbor dependent mutation parameter strengths that does the best job of explaining the current frequency spectrum of appearance of amino acids. We also show that this optimal solution does not fully explain the current frequency of appearance of amino acids, and therefore other effects, such as externally imposed survival advantage of amino acids sequences, must also play a role in the evolution of nucleotide sequences. We also explain how the relative importance for genetic evolution of internal nucleotide mutation versus external selection can be determined if the frequency spectrum of amino acids could be determined at various times in the past.   

An entropic tool for genome analysis

Shannon information (SI) defines the difference of Shannon entropy and its global maximum value. It was found SI in a genome tends to be much larger than that in its random match for all extant prokaryotic and eukaryotic complete genomes in Chang et al's work. Thus a better sense of the magnitude of the SI in a sequence is obtained by measuring it relative to the SI in the random match, the reduced SI. They observed a linear relation between reduced SI and sequence length L, which implies a k-dependent but genome-independent constant. This forms a universality class that indicates that reduced SI is a signature of complete genomes undiminished by the enormous diversity in growth and evolution experienced by individual genomes. Although their studies revealed intriguing results, the mechanism was not clear. Our main goal here is to investigate it through the method of maximum entropy (ME). The rationale hinges on the use of relative entropy. ME indicates preferred probability distribution of frequency- occurrence of k-string in real genome sequences updated from random sequences is the one that maximizes relative entropy of genomes and random sequences subject to certain constraints. Our result shows the existence of universality classes to be simply a trivial consequence if frequencies-occurrence of k-string is chosen as the relevant variable. However, the use of this result is far from being exhausted, which may provide a track to develop a genomic growth model.   

Nonlinear degradation and the function of genetic circuits

The functions of most genetic circuits require a sufficient degree of cooperativity in the circuit components. We examine a simple source of cooperativity that stems from the nonlinear degradation of multimeric proteins. Ample experimental evidence suggests that protein subunits can degrade less rapidly when associated in multimeric complexes, an effect we refer to as ``cooperative stabilization,'' For homodimers, this effect leads to a concentration dependence in the protein degradation rate because monomers which are predominant at low protein concentrations will be more rapidly degraded. Theoretical analysis of two model gene circuits in bacteria, i.e. genetic switch and oscillator, demonstrates that a few-fold difference between the degradation rate of monomers and dimers can substantially enhance the function of these circuits. Our results suggest that cooperative stabilization needs to be considered explicitly and characterized quantitatively in any systematic experimental or theoretical study of gene circuits.   

Topological units of environmental signal processing in the transcriptional-regulatory network of Escherichia coli

Recent evidence indicates that potential interactions within biochemical networks are differentially utilized according to the environmental conditions in which a cell exists. However, the topological units of this differential utilization have not been investigated. Here, we use the transcriptional regulatory network of Escherichia coli to identify such units, called origons, representing regulatory subnetworks which originate at a distinct class of sensor transcription factors. Using microarray data, we find that specific environmental signals affect mRNA expression levels significantly only within the origons responsible for their detection and processing. We also show that small regulatory interaction patterns, called subgraphs and motifs, occupy distinct positions in- and between origons, offering insights into their role in environmental information processing.   

Power law rank-abundance relationships in marine phage populations

Phage are the most abundant biological entities in the biosphere, with an estimated 10$^{31}$ particles on the planet. About 25{\%} of oceanic organic carbon is cycled through phage every day. Metagenomic analyses show that the rank-abundance curve for marine phage communities follows a power law distribution. This distribution is consistent with a proposed, modified version of Lotka-Volterra predator-prey dynamics, where blooms of a specific microbial species leads to blooms of their corresponding phage and a subsequent decrease in abundance. The model predicts that the majority of phage types in a population will be rare and it is unlikely that the most abundant phage will be the same at different time points. The model is based on spatial-temporal heterogeneity and a power law phage decay, which are both supported by empirical data.   

Biological Networks: Does Function Follow Form?

Recently, studies of biological networks have focused on various topological measures (primarily degree distributions and subgraphs). Relating such graph-theoretic statistics to function is difficult, since a given topology does not uniquely determine function. In fact, a topology's ability to support multiple functions may itself provide a selective advantage to an organism, since a topology with multiple functions can be adaptable (on the time scale of the individual) or evolvable (on the time scale of the species). Here we present a quantitative measure of circuit function and use this measure to test if circuits with well-defined function or functions are common, and if evolvable topologies exist among them.   

Analysis of a yeast cell cycle model

We have analyzed a model network of yeast cell-cycle regulation, which consists of a set of ordinary differential equations with about 90 parameters. We show that this dynamical system has very stable and robust global fixed points which correspond to the biological checkpoints. The biological pathway corresponds to a globally attracting trajectory of the system.   

Self-Consistent Proteomic Field Theory of Stochastic Gene Switches

The need for a computationally efficient treatment of genetic networks and cascades, which, while acknowledging their stochastic character, at the same time allows us to gain a better and deeper understanding of the global attractor structure is widely recognized. Even treating the building blocks of these systems, genetic switches, generally requires some approximations. We propose a powerful generically applicable method, a self-consistent proteomic field approximation in which the mean influence of the proteomic cloud created by one gene on the action of another is computed self-consistently [1]. The stochastic nature of protein synthesis and degradation, and DNA binding events are treated stochastically and on equal footing. For a large class of problems, in which the output proteins of one gene influence other genes, the probability distributions may be determined exactly without any further assumptions within the self-consistent proteomic field approximation. We compare the results for various versions of a toggle switch composed of two mutually repressing genes to solutions of deterministic rate equations and find that when proteins are produced in bursts, the deterministic approach can fail dramatically.\\ 1. Walczak, A.M., Sasai, M., Wolynes P.G., Self Consistent Proteomic Field Theory of Stochastic Gene Switches, to be published in Biophysical Journal   

Session J23: Stochastic Processes and Nonlinear Dynamics

Stochastic Resonance in Bistable Nanomechanical Oscillators

The bistable doubly-clamped nanomechanical beam resonator has recently been shown [1] to be a promising step in the development of mechanical memory cells. One of the major obstacles to the full implementation of this scheme lies in the ability to effectively control the two states [2] in a noisy or high-temperature environment. Here, we present the observation of stochastic resonance in these nanomechanical systems. This is a counter-intuitive effect in which the addition of noise to a noisy system results in coherent response. Over the past two decades, it has been seen in a wealth of physical systems. Aside from adding to knowledge in this area, the observation of stochastic resonance here lays the foundation for its effective use in the areas of signal processing, quantum information, and quantum control. This work is supported by NSF Nanoscale Exploratory Research (NER) Program and NSF (DMR, CCF, ECS), DOD (ARL), ACS (PRF), and the Sloan Foundation. [1] R. Badzey et al., Appl. Phys. Lett. 85, 3587 (2004). [2] R. Badzey et al., (To be Published, Appl. Phys. Lett. 1/20/05).   

Precession aided magnetic stochastic resonance in ferromagnetic nanoparticles with cubic anisotropy

It is shown that the signal-to-noise ratio (SNR) in the magnetic stochastic resonance of single-domain ferromagnetic nanoparticles having cubic anisotropy exhibits a strong intrinsic dependence on the decay rate \textit{$\alpha $} of the Larmor precession. This dependence (precession aided relaxation) is due to coupling between longitudinal relaxation and transverse (precessional) modes arising from the lack of axial symmetry. It is most pronounced in the intermediate to low damping (Kramers turnover) region 0.1 $<$ \textit{$\alpha $} $<$ 1. The effect which does not exist for axially symmetric potentials may be used to determine \textit{$\alpha $}.   

Delayed stochastic control

Time-delayed feedback control becomes problematic in situations in which the time constant of the system is fast compared to the feedback reaction time. In particular, when perturbations are unpredictable, traditional feedback or feed-forward control schemes can be insufficient. Nonethless a human can balance a stick at their fingertip in the presence of fluctuations that occur on time scales shorter than their neural reaction times. Here we study a simple model of a repulsive delayed random walk and demonstrate that the interplay between noise and delay can transiently stabilize an unstable fixed-point. This observation leads to the concept of ``delayed stochastic control,'' i.e. stabilization of tasks, such as stick balancing at the fingertip, by optimally tuning the noise level with respect to the feedback delay time. \\ \\ References:(1)J.L.Cabrera and J.G.Milton, PRL 89 158702 (2002);(2) T. Ohira and J.G.Milton, PRE 52 3277 (1995);(3)T.Hosaka, T.Ohira, C.Lucian, J.L.Cabrera, and J.G.Milton, Prog. Theor. Phys. (to appear).   

Non-exponential time-correlation function for random physical processes

Dielectric relaxation in gases is reconsidered phenoemenologically and it is shown that the dipole moment correlation function must have an inflection point at the mean collision time. The exponential function, used in the Debye and Van Vleck-Weisskopf models, does not possess an inflection point at finite times and must be rejected. New models that correctly represent the effects of collisions are necessary. A new time-correlation function is proposed that differs little numerically from the exponential function, exhibits an inflection point, is analytic at t = 0 and its power spectrum has finite moments to all orders. Problems related to divergence vanish. A new lineshape function is obtained that is indistinguishable from the Lorentzian lineshape. The new correlation function implies that the process is non-Markovian, which is theoretically consistent for all processes where the derivatives have physical meaning, including those described in terms of linear response theory. In addition, its mathematical superiority implies that it is advantageous to use this function over the exponential function for all such processes.   

Mean-field behavior of the Burridge-Knopoff model with long-range interactions

The one-dimensional Burridge-Knopoff model with a variable-range Kac-like interaction is simulated using molecular dynamics, and the number of earthquake-like events is obtained. In agreement with Carlson and Langer (Phys. Rev. A 40, 6470 (1989)) the event size distribution is found to exhibit power law scaling for nearest-neighbor interactions over a limited range of event sizes. We find that long-range interactions yield mean-field exponents only if the parameter characterizing the ratio of the largest characteristic slipping speed to the speed at which the dynamical friction is appreciably reduced is sufficiently small. In this limit the dynamical behavior of the long-range Burridge-Knopoff model becomes similar to the cellular automaton model of Rundle, Jackson, and Brown and Olami, Feder, and Christensen.   

Repulsive Synchronization in an Array of Phase Oscillators

We study the dynamics of an array of phase oscillators with repulsive coupling. Globally-coupled network of identical oscillators settles on one of a family of synchronized regimes characterized by zero mean field. However, variations of oscillator natural frequencies destroy synchronization for sufficiently large number of coupled oscillators independently of the coupling strength. In locally coupled networks (with a finite range of coupling less than the system size), the synchronization occurs even for non-identical oscillators when coupling is sufficiently strong. In the synchronized regime, a ring of repulsively coupled oscillators approaches linear phase distribution.   

Periodic orbit theory of two coupled Tchebyscheff maps

Coupled map lattices have been widely used as models in several fields of physics, such as chaotic strings, turbulence, and phase transitions, as well as in other disciplines, such as biology (ecology, evolution) and information processing. This work investigates properties of periodic orbits in two coupled Tchebyscheff maps. Then zeta function cycle expansions are used to compute dynamical averages appearing in Beck's theory of chaotic strings. The results show close agreement with direct simulation for most values of the coupling parameter, and yield information about the system complementary to that of direct simulation.   

Flow Equation Approach to the Statistics of Nonlinear Dynamical Systems

The probability distribution function of non-linear dynamical systems is governed by a linear framework that resembles quantum many-body theory, in which stochastic forcing and/or averaging over initial conditions play the role of non-zero $\hbar$. Besides the well-known Fokker-Planck approach, there is a related Hopf functional method\footnote{Uriel Frisch, {\it Turbulence: The Legacy of A. N. Kolmogorov} (Cambridge University Press, 1995) chapter 9.5.}; in both formalisms, zero modes of linear operators describe the stationary non-equilibrium statistics. To access the statistics, we investigate the method of continuous unitary transformations\footnote{S. D. Glazek and K. G. Wilson, Phys. Rev. D {\bf 48}, 5863 (1993); Phys. Rev. D {\bf 49}, 4214 (1994).} (also known as the flow equation approach\footnote{F. Wegner, Ann. Phys. {\bf 3}, 77 (1994).}), suitably generalized to the diagonalization of non-Hermitian matrices. Comparison to the more traditional cumulant expansion method is illustrated with low-dimensional attractors. The treatment of high-dimensional dynamical systems is also discussed.   

Quantized Interest Rate At The-money for American options

In this work, we expand the idea of Shepp for stock optimization using the Bachelier model as our model for the stock price at the money ($X=K)$ for the American call and put options. At the money ($X=K)$ for American options, the expected payoff of both the call and put options is zero. Shepp investigated several stochastic optimization problems using martingale and stopping time theories, one of the problems he investigated was how to optimize the stock price using both the Black-Scholes and the Bachelier (additive) models for the American option above the strike price $K$ (exercise price) to a stopping point. In order to explorer the non-relativistic quantum effect on the expected payoff for both the call and put options at the money ($X=K)$, we assumed the stock price to undergo a stochastic process governed by the Bachelier model given above. Further, using Ito calculus and martingale theory, we obtained a differential equation for the expected payoff for both the call and put options in terms delta and gamma. We investigated the solution of the differential equation in the limit when delta is zero, this sometimes is called hedging or delta neutral in finance. By comparison, the delta-hedged differential equation corresponded to the non-relativistic time-independent Shroedinger equation in quantum mechanics with a constant diffusion constant. We solved exactly the non-relativistic Schroedinger equation at the money and obtained a quantized interest rate in terms of volatility and stock price.   

A Unified Approach to Attractor Reconstruction

Reconstruction of attractors for dynamical systems has typically focused on solving seemingly separate problems of finding a proper time delay and then finding a proper embedding dimension. Techniques for solving these problems are somewhat heuristic. We show that the two problems of time delay and embedding dimension are actually the same problem. Using Taken's theorem we derive a mathemetical criterion for adding new components to reconstruction vectors. We also show how several statistics that gauge functional dependence between multivariate data sets can fullfill a practical application of the theory and solve at once the problems of determining time delays, getting embedding dimension, and optimally choosing time series to use from a multivariate data set. This unified approach is compared to ``standard'' approaches and is shown to be superior in requiring fewer embedding dimensions.   

Perturbations of L\'evy Processes Using the Feynman Functional

The Wiener process is an example of a stable L\'{e}vy process, and the stable L\'{e}vy processes have been used as a model for many Physical systems as well as Financial Mathematics. Using the heat semigroup, the Feynman-Kac formula provides a connection between the Wiener process and Feynman's path integral. DeFacio, Johnson, and Lapidus were able to expand upon this method to formulate a rigorous Feynman Functional for certain semigroup operators. It is possible to add a potential term to these operators in order to investigate the effect perturbations have on stable L\'{e}vy processes. With restrictions on the potential, these perturbations can be calculated using the methods developed for the Feynman path integral, and formal manipulation of the perturbations yields previous results in the literature. An overview of the Feynman Functional formulation for L\'{e}vy processes will be given, a few examples of perturbed L\'{e}vy processes will be shown, and some Mathematical and computational difficulties will be discussed.   

Non-linear Logit Models for Binarized High Frequency Financial Data

We propose a non-linear logit model for binarized high frequency data of yen-dollar exchange rate indicating up or down price movement. We show a non-trivial probability structure from the binarized data, which is invisible from the price change itself. The model successfully captures the structure, which is not possible by the conventional analysis such as an AR model and a logit model. In addition, a similar and a stronger bias can be observed from other binarized high frequency active stock data on NYSE, for example GE, INTL, MSFT, WMT and so on. Our model could be useful for a wide range of binary time series with non-trivial dynamical structures. \\ References [1]N. Sazuka and T. Ohira, in {\it Computational Finance and its application}, pp.275-305, WIT press 2004. [2]N. Sazuka, et. al., Physica A 324 pp.366-371, 2003. [3]T. Ohira, et. al., Physica A 308 N1-4, pp.368-374, 2002.   

The Opinion Dynamics of Majority Rule

We investigate the long-time behavior of a majority rule opinion dynamics model in finite spatial dimensions. Each site of the system is endowed with a finite-state spin variable that evolves by majority rule. In a single update event, a group of spins with a fixed (odd) size is specified and all members of the group adopt the local majority state. For the case of two states, repeated application of this update step leads to a coarsening mosaic of spin domains and ultimate consensus in a finite system. The approach to consensus is governed by two disparate time scales, with the longer time scale arising from realizations in which spins organize into coherent single- opinion bands. The extension to more than two states leads to a surprising faster evolution as soon as one state establishes itself as a local majority somewhere in the system.   

Theoretical evidence for the link between geomagnetic reversal and glacial events: stochastic resonance in the geodynamo model

Not yet a theoretical analysis can explain the coincident temporal correlation between the geomagnetic reversal and glacial events, which both have a quasi-period of about 100 kyr, although there exists dozens of observational evidences for such correlation. The geodynamo has widely been thought to be an intuitive and self-sustained model of the Earth's magnetic field$^{10}$. In this letter we report how possible a signal with 100 kyr quasi-period can be embedded in the geomagnetic filed \textit{via} the mechanism of stochastic resonance in a forced Rikitake dynamo. We thus suggest one common triggering for the geomagnetic reversal and glacial events, neither the glaciation controls the geomagnetic reversal nor vice versa. Instead, both kinds of catastrophes may result from the cyclic variation of the Earth's orbital eccentricity.   

Multi-scale continuum mechanics: From global bifurcations to noise induced high dimensional chaos

Many mechanical systems consist of continuum mechanical structures, having either linear or nonlinear elasticity or geometry, coupled to nonlinear oscillators. In this paper, we consider the class of linear continua coupled to mechanical pendula. In such mechanical systems, there often exist several natural time scales determined by the physics of the problem. Using a time scale splitting, we analyze a prototypical structural/mechanical system consisting of a planar nonlinear pendulum coupled to a flexible rod made of linear viscoelastic material. In this system both low-dimensional and high-dimensional chaos is observed. The low-dimensional chaos appears in the limit of small coupling between the continua and oscillator, where the natural frequency of the primary mode of the rod is much greater that the natural frequency of the pendulum. In this case, the motion resides on a slow manifold. As the coupling is increased, global motion moves off of the slow manifold and high-dimensional chaos is observed. We present a numerical bifurcation analysis of the resulting system illustrating the mechanism for the onset of high dimensional chaos. Constrained invariant sets are computed to reveal a process from low dimensional to high dimensional transitions. Applications will be to both deterministic and stochastic bifurcations.   

Session J24: GSNP Student Presentation Session and Far From Equilibrium Systems

Dynamical heterogeneities and long-lived stress: signatures of jamming in a dense granular flow

Recent interest in understanding the dynamical arrest leading to a fluid $\rightarrow$ solid transition in both thermal and athermal systems has led to questions about the nature of these jamming transitions (PRL {\bf 86}, 111 (2001), Nature {\bf 411}, 772 (2001)). It is believed that these jamming transitions are dependent on the influence of extended structures on the dynamics of the system (Science {\bf 287}, 627 (2000)). Is it possible to construct a simple model of a driven, dissipative system in which these structures are seen to form? Simulations of steady-state gravity-driven flows of inelastically colliding hard disks show the formation of large-scale linear chains of particles with a high collision frequency even at flow velocities well above the jamming transition (EPL {\bf 66}, 277 (2004)). These chains can be shown to carry much of the collisional stress in the system due to a dynamical correlation that develops between the momentum transfer and time between collisions in these ``frequently-colliding'' particles. The lifetime of these chains is seen to grow as the flow velocity decreases towards jamming, leading to slowly decaying stress correlations reminiscent of the slow dynamics observed in supercooled liquids. Long-lived stress chains seem to be precursors to force chains in static granular piles and understanding the dynamical principles behind their formation and decay can lead to increased insight into the mechanism of dynamical arrest.   

Ordered Phases of Diblock Copolymers in Selective Solvent: Micelle Interactions and Lattice Geometry

Diblock copolymers in selective solvents are known to aggregate into micelles for copolymer concentration above the critical micelle concentration (CMC). Well above the CMC (volume fractions greater than about 10\%) micelles begin to overlap appreciably and interbrush repulsion becomes relevant. To minimize the effect of these interactions, micelles assemble into ordered phases. Recently, thermoreverisible transitions between such cubic phases of copolymer micelles have been observed [1]. Here, the thermodynamics of micelle aggregation couples to the strength of the intermicelle repulsions, tuning the relative importance of the constrained micelle entropy and average micelle interactions. We propose a model that captures both these thermodynamics as well as the dependence of micelle repulsion and translational entropy on lattice geometry. This model relies on effective theories for the brush regions of the micelles as well as an Einstein crystal description of the lattice. The core region can be treated as a molten polymer brush while the outer brush must be treated as a semi-dilute polymer brush. This ``multi-scale" approach allows us to predict the phase behavior of ordered phases in copolymer solutions under a variety of experimentally realizable conditions. [1] T. P. Lodge, J. Bang, M. J. Park, K. Char, Phys. Rev. Lett. 92, 145501 (2004).   

When does continuum theory describe mechanical contacts?

Continuum theories of contact assume that discrete atomic displacements can be described by continuously varying strain fields and also that surfaces are perfectly smooth at small scales. While the first assumption is know to fail as dimensions decrease to atomic scales, continuum results are routinely applied to atomic force microscope tips and other nanoscale contacts. We have used molecular simulations of contact between a rigid sphere or cylinder and a flat elastic half space to test the limits of continuum theory. The flat surface was a (100) or (111) surface of an fcc crystal. Tips were made by bending perfect crystals to a radius of curvature $R$, or cutting surfaces of mean radius $R$ from crystals or amorphous solids. The normal displacement vs. load curves for all tips are close to continuum predictions, as are extracted elastic moduli. Local quantities such as the width of the contact and local pressures can vary by more than a factor of two from continuum predictions. Friction forces and lateral contact stiffnesses vary by at least an order of magnitude with atomic scale geometry. Analysis of these results shows that the assumption of smooth surfaces is a greater source of error than use of continuous strain fields.   

Understanding ecosystems using statistical physics

I will show, based on analytic theory and computer simulations, that ecosystems are organized in the vicinity of a new type of phase transition quite akin to Bose-Einstein condensation but occurring in a living system without quantum features. A special case of our model is akin to neutral theory, which postulates that an ecosystem can be characterized by random birth and death processes influenced by immigration and speciation with the further simplifying assumption that all species behave similarly in terms of their birth and death rates. I will present a theoretical framework for the neutral theory of biodiversity and an analytical solution for the distribution of the species composition both for a large metacommunity and for a semi-isolated local community. I will demonstrate that the analytical solution provides an excellent fit to field data. I. Volkov, J. R. Banavar, S. P. Hubbell and A. Maritan, Neutral theory and relative species abundance in ecology, Nature 424, 1035, (2003). I. Volkov, J. R. Banavar and A. Maritan, Organization of ecosystems in the vicinity of a novel phase transition, Phys. Rev. Lett. 92, 218703, (2004).   

The effects of gas pressure on liquid splashing

The corona splash due to the impact of a liquid drop onto a dry smooth glass substrate is investigated with high speed photography. We find a striking phenomenon that the splashing vanishes when the surrounding gas pressure is lowered. The relationship of threshold gas pressure (where the splashing ceases to occur) to the impact velocity is measured. Four different gases (Air, He, Kr, SF6) as well as three different liquids (Methanol, Ethanol and 2-Propanol) are used in the experiment. We find a scaling relationship of the threshold pressure in terms of the gas molecular weight and liquid viscosity. A model considering compressibility of the gas is proposed to explain these observations. These experiments shed new light on the phenomenon of how a splash is generated when a liquid hits a smooth substrate.   

Critical Behavior of the Banded-Unbanded Spherulite Transition in a Mixture of Ethylene Carbonate with Polyacrylonitrile

Banded spherulites appear generically when materials with viscous melts are frozen at high undercoolings. The characteristic striped pattern observed in thin samples is believed to reflect a rotation of crystalline axes that occurs as the front propagates radially away from a nucleation site. Common features include an onset of banding at finite undercooling and a divergence of the wavelength near this critical undercooling. Here, by carefully considering systematic errors, we show that the band spacing diverges with a power-law form showing scaling over nearly two decades. We also observe that the bands disorder as the transition point is approached. The critical exponent is non-classical. One possible explanation is that the transition is actually weakly first order. An analogous situation exists for cholesteric liquid crystals in the vicinity of a cholesteric--smectic-A transition.   

Persistence in Conserved Order Parameter Coarsening

Persistence in conserved order parameter coarsening is studied via computer simulation of the Cahn-Hilliard equation. Persistence $P(t_1, t_2)$ is defined as the fraction of the system that has not been traversed by a domain wall between times $t_1$ and $t_2$. We measure persistence as a function of volume fraction and establish that it decays according to a power law $P \sim t_2^{-\theta}$ for all volume fractions studied. We find that the persistence exponent $\theta$ depends on the volume fraction. Our results are then compared with an exact calculation applicable in the dilute limit.   

Anisotropic Lifshitz-Slyozov Theory

We study Lifshitz-Slyozov theory for the dilute limit of conserved order parameter coarsening with the addition of an anisotropic surface tension. We calculate the drop shapes and drop size distribution perturbatively in anisotropy strength. We find the $L \sim t^{1/3}$ growth law unchanged with drop shapes that depend only on the scaled drop size. The drop shapes are nonspherical and do not have the equilibrium Wulff shape. The drop size distribution is modified from the isotropic Lifshitz-Slyozov result.   

Cahn-Hilliard Simulation of Anisotropic Coarsening

The influence of surface tension anisotropy on the dynamics of coarsening is studied via computer simulations. The Cahn-Hilliard equation in dimension $d=2$ is modified to include an arbitrary surface tension anisotropy. For all cases studied, we find asymptotic late-stage scaling with the growth law $L \sim t^{1/3}$ unchanged. The structure factor $S({\bf k},t)$ is binned into angular wedges, and is found to exhibit scaling collapse distinct for each wedge, indicating that the asymptotic domain structure is indeed anisotropic. The Porod tail is found to be a sensitive diagnostic, allowing for quantitative measurement of the degree of anisotropy in the domain structure.   

Test of the steady-state fluctuation theorem in turbulent Rayleigh-B{\'e}nard convection

Local convective heat flux in turbulent thermal convection is obtained from simultaneous velocity and temperature measurements in an aspect-ratio-one cell filled with water. It is found that large positive fluctuations of the vertical heat flux occurs more often in the plume-dominated sidewall region and their histograms are highly asymmetric. The statistical properties of the time-averaged local flux fluctuations are analyzed and the results are compared with the predictions of the steady state fluctuation theorem of Gallavotti and Cohen. Work supported by the Research Grants Council of Hong Kong SAR under Grant Nos. HKUST603003 (P.T.) and CUHK403003 (K.Q.X.).   

Experimental measurement of power input fluctuation in a turbulent flow

We study the power input fluctuations in a turbulent flow driven by body forces. The local velocity is measured in a system driven by a known pattern of Lorentz forces. The local power input is computed $P = \vec{F} \cdot \vec{v}$ and studied in the context of the Fluctuation-Dissipation theorem. This liquid sodium flow has a Reynolds number $R \sim 10^4$ leading to turbulent fluctuations in the local power inpug. The probability distribution of the power input is consistent with predictions from the Fluctuation-Dissipation theory, even though that should only apply to the spatially averaged input power. These results can be interpreted as suggesting a generalization to the theoretical ideas for far from equilibrium systems.   

The noise thermal impedance of a diffusive wire

The current noise density $S_2$ of a conductor at equilibrium is determined by its temperature $T$: $S_2=4k_BTG$ with $G$ the conductance (Johnson noise). The noise temperature $T_N=S_2/ (4k_BG)$ generalizes $T$ for a system even out of equilibrium. We introduce the noise thermal impedance of a sample as the amplitude of the oscillation of $T_N$ when heated by an ac power. It is the usual thermal impedance for a macroscopic sample. We show for a diffusive wire, how this (complex) frequency-dependent quantity gives access to the electron-phonon interaction time in a long wire and to the diffusion time in a shorter one, and how its real part may also give access to the electron-electron interaction time. We will also present experimental results in various limits.   

Nonequilibrium Statistical Mechanics Using Maximum Entropy Methods

Phenomena like Fick's Law of diffusion, and chemical decay processes belong to the domain of nonequilibrium thermodynamics. We believe that the principle of maximum caliber, formulated by E.T. Jaynes, can provide the necessary framework to explain such processes. In this work we formulate simple models for dynamical processes like particle diffusion, heat diffusion, and chemical kinetics. Following the maximum caliber principle, we identify the phase trajectories in each case and write down the corresponding entropy. We then maximize this entropy, subject to the physical/chemical constraints involved in the process, to obtain the probability distribution for its trajectories. From this probability distribution we can get the mean value and fluctuations for the variables of interest.   

Mathematical origin of time arrow

Laws describing the main types of physical interactions are symmetrical with respect to the direction of time flow. At the same time, many virtually irreversible processes are observed. This ``time arrow'' paradox usually is associated with the law of entropy increase. The fact that physical systems obey this law regardless of their physical nature suggests that it may be based on a certain, yet unknown, mathematical principle. Here it is demonstrated that, if, on a time \textit{micro} scale, the intensity of fluctuations of a certain parameter depends on the parameter's value, it would appear to an external observer on a time \textit{macro} scale that the parameter tends to be modified in the direction of fluctuation intensity decrease. It is shown that the law of entropy increase is a consequence of this principle, if it is applied to entropy as a state variable of a thermodynamic system. The fundamental nature of this principle suggests that it must operate on virtually every level of physical reality. The principle is of great potential value for understanding mechanisms of self-organization, learning, adaptation, and evolution.   

Session J25: Focus Session: Novel and Complex Oxides: Ruthenates and Osmiumates

Superconductivity in the Osmium-based Beta-Pyrochlore Oxides

Superconductivity is reported in recently discovered \textit{$\beta $}${\rm t}$pyrochlore oxides AOs$_{2}$O$_{6}$. The $T_{c}$ is 3.3 K, 6.3 K, and 9.6 K for A = Cs, Rb, and K, respectively. The highest $T_{c}$ of KOs$_{2}$O$_{6}$ is almost one order higher than the $T_{c}$ = 1.0 K of previously reported \textit{$\alpha $}-pyrochlore oxide superconductor Cd$_{2}$Re$_{2}$O$_{7}$ which is believed to be a conventional $s$-wave superconductor. Moreover, the upper critical field $H_{c2}$ of KOs$_{2}$O$_{6}$ is estimated to be 38 T, which seems to exceed Pauli's limit expected for conventional superconductivity. This is again in contrast to the case of Cd$_{2}$Re$_{2}$O$_{7}$, in which the $H_{c2}$ is 0.29 T, much smaller than the corresponding Pauli's limit. These distinct contrasts strongly suggest that the mechanism of superconductivity is essentially different between the two pyrochlore oxides. It is to be noted that the $T_{c}$ of these \textit{$\beta $}${\rm t}$pyrochlore oxides decreases with increasing the ionic radius of the alkaline metal ions, imposing negative chemical pressure upon the Os pyrochlore lattice. I believe that interesting physics is involved on the basis of strong electron correlations on the highly frustrated pyrochlore lattice.   

Magnetic excitations of Sr$_{3}$Ru$_{2}$O$_{7}$

Although transport measurements of the layered perovskite Sr$_{3}$Ru$_{2}$O$_{7}$ abound, a clear understanding of the underlying magnetic excitation spectrum is far from complete. Knowledge of the details of the magnetic fluctuations in this material has implications for both cuprate superconductors as well as other doped and undoped ruthenate compounds. We present a series of inelastic neutron scattering measurements as a function of temperature and wave-vector transfer in the (H 0 L) scattering plane. The magnetic response is clearly visible in constant E scans at the wavevector (0.75,0,L) measured carefully for T = 3.8 K up to T = 100 K up to $\hbar \omega $= 14 meV. ORNL is managed by UT-Battelle for the US DOE under contract DE-AC05-00OR22725.   

Magnetic Excitation Spectrum of Ca$_{2-x}$Sr$_{x}$RuO$_{4}$

We have studied the concentration dependence of the magnetic excitation spectrum in single crystal samples of the layered perovskite ruthenates, Ca$_{2-x}$Sr$_{x}$RuO$_{4}$ for 2$>$x$>$0.4. For large x, the spectrum is similar to that observed in pure Sr$_{2}$RuO$_{4}$ with incommensurate excitations strongly peaked in Q at ($\pm $0.3, $\pm $0.3, q$_{z})$, consistent with Fermi-surface nesting wavevectors. As the concentration approaches the x=0.5 quantum critical point, the spectrum becomes broadly distributed in \textbf{Q} with a sharp upturn at $\pm $0.3 in both $h$ and $k$ and a flat distribution of scattering across the 2d ferromagnetic zone center. Possible interpretations of this scattering and qualitative similarity to the excitation spectrum of Sr$_{3}$Ru$_{2}$O$_{7}$, also in close proximity to a quantum critical point, will be discussed. ORNL is managed by UT-Battelle for the US DOE under contract DE-AC05-00OR22725.   

Low Temperature Structural Phase Transitions in Novel Oxides

Analysis of the Raman active phonon modes offers a symmetry dependent determination of structural phase transitions. We have performed polarized Raman scattering measurements on oriented single crystals of the superconducting pyrochlore Cd$_{2}$Re$_{2}$O$_{7}$ and the layered ruthenate La$_{4}$Ru$_{2}$O$_{10}$ as a function of temperature. In Cd$_{2}$Re$_{2}$O$_{7}$ we resolve and assign each of the six Raman-active (A$_{1g}$ + E$_{g}$ + 4F$_{2g}$) modes of the room temperature cubic phase. Below the structural phase transition at 200K (and 120K) we observe new symmetry dependent Raman-active vibrations associated with a cubic-tetragonal (tetragonal-tetragonal) phase transition. We identify two ``soft" modes and discuss a structural order parameter with E$_{u}$ symmetry. We measure La$_{4}$Ru$_{2}$O$_{10}$ through the monoclinic-triclinic phase transition @ 150K and compare the symmetry dependent results with expectations based on x-ray structural analysis.   

Quasi-Two-Dimensional Metallic Ground State of Ca$_3$Ru$_2$O$_7$

Ca$_3$Ru$_2$O$_7$ is a three-dimensional antiferromagnetic metal between a first-order metal to nonmetal transition at 48~K and the antiferromagnetic ordering temperature, $T_{\rm N}$=56~K[1]. The crystal structure is the double layered Ruddlesden-Popper type with the $Bb2_1m$ space group, which has both the rotation and tiling of RuO$_6$ octahedra. We have succeeded in growing single crystals of Ca$_3$Ru$_2$O$_7$ using a floating-zone method for the first time. The temperature dependence of the electrical resistivity establishes that Ca$_3$Ru$_2$O$_7$ develops a quasi-two-dimensional metallic ground state below 30 K, from which the observed quantum oscillation derives. The specific heat measurement reveals the electronic specific-heat coefficient $\gamma$ to be as small as 1.7 mJ/Ru mol K$^2$[2]. From the results of powder neutron diffractions, we proposed the most possible magnetic structure with an antiferromagnetic ordering. The field dependence of the resistivity at the metamagnetic transition around 6 T can be explained by the tunneling magnetoresistance. This work was done in collaboration with S. I. Ikeda, N. Shirakawa, C. H. Lee, M. Kosaka, and S. Katano. [1] G. Cao et al., Phys. Rev. Lett. 78 (1997) 1751. [2] Y. Yoshida et al., Phys. Rev. B 69 (2004) R220411.   

Phonon Instabilities in Ca(1.4)Sr(0.6)RuO4

Phonon instabilities in Ca$_{1.4}$Sr$_{0.6}$ RuO$_{4}$ are investigated by inelastic neutron scattering techniques. Sr$_{2}$RuO$_{4}$ is an unconventional p-wave superconductor with the same structure as La$_{2}$CuO$_{4}$, the parent compound of the high T$_{c}$ superconductor La$_{2-x}$Sr$_{x}$CuO$_{4}$. By partial substitution of Ca$^{+2}$ for Sr$^{+2}$, induced structural stresses create a complex phase diagram with exotic phases. La$_{2}$CuO$_{4}$ has a temperature dependent $\Sigma _{4}$ phonon instability, correlated with the tetragonal to orthorhombic structural transition. It is anticipated that Ca$_{1.4}$Sr$_{0.6}$RuO$_{4}$ will exhibit similar behavior as a precursor to its tetragonal to orthorhombic phase transition. Indeed the instability exists, but two anomalies appear in the spectra. A new phonon mode appears mimicking the dispersion of the $\Sigma _{4}$ phonon and a new Bragg peak appears incommensurate with the tetragonal unit cell. The origins and implications of the anomalies will be discussed. Work supported by NSF- DMR 0105232, NSF-DMR0346826, and DOE DE-FG02-04ER46125. ORNL, managed by UT-Battelle, LLC, for the U.S. Dept. of Energy under contract DE-AC05-00OR22725   

Low temperature magneto-transport measurements on Ca$_{1.5}$Sr$_{0.5}$RuO$_4$

In-plane electrical transport measurements were performed on Ca$_{1.5}$Sr$_{0.5}$RuO$_{4 }$in the presence of magnetic fields up to 8 T applied in the direction perpendicular to the plane. Upon substituting Sr with isovalent Ca, Ca$_{2-x}$Sr$_{x}$RuO$_{4}$ shows an intriguing phase diagram ranging from p-wave superconductor at x = 2 to Mott insulator at x $\le $ 0.2. The x = 0.5 system investigated in this work is reported to be at the boundary between the magnetic metal (x $<$ 0.5) and the paramagnetic metallic phase. A small but distinct increase in resistance was observed at T* $\approx $ 450 mK on warming. In addition, T* decreases with the applied magnetic field, and the feature in resistance disappears around 500 G. Our detailed magneto-resistance measurements reveal unusual behavior in the low temperature and low magnetic field region that, we believe, is directly related to the resistance anomaly observed near 450 mK in zero magnetic field.   

Three-Dimensional Band Structure of Sr$_4$Ru$_3$O$_{10}$

The electronic structure and Fermi Surface (FS) of the triple-layer ruthenate Sr$_4$Ru$_3$O$_{10}$ is probed with angle resolved photoemission. Angle-dependent FS maps show distinctly different sized FS orbits as compared to the measured single- and double-layer ruthenate FS topologies. In addition, photon-dependent FS maps reveal a distinct k$_{\perp}$ variation with a periodicity corresponding to the inter-layer spacing of the triple-layer stack, indicating a three-dimensionality of the band structure. Comparison of the FS topology is made to available band structure calculations.   

Temperature and frequency dependence of the mid-infrared Hall effect in Ca$_x$Sr$_{1-x}$RuO$_3$

Ca$_x$Sr$_{1-x}$RuO$_3$ compounds exhibit unusual properties, such as metamagnetism, quantum criticality, non-Fermi liquid behavior and an anomalous Hall effect that continue to challenge the condensed matter community. The mid-infrared (115-238 meV) complex Faraday, Kerr, and Hall angles are studied in Ca$_x$Sr$_{1-x}$RuO$_3$ films. The magneto-optical signals in transmission are up to an order of magnitude larger than those obtained in reflection for the same sample. The frequency dependence of the low-temperature magneto-optical signals in SrRuO$_3$ is in good qualitative and quantitative agreement with first-principles band calculations [Z. Fang et al., Science 2003]. Striking qualitative similarities and differences are observed in the temperature dependence of the Hall angle at three probe energies (dc, 120~meV and 224~meV) in CaRuO$_3$ and SrRuO$_3$.   

Magnetoresistance Caused by Spin-polarized Variable Range Hopping--Perovskite Ruthenates

We report a detailed study of A-site and B-site substitution on magnetoreistance of perovskite ruthenates, which have broad implications. By progressively disrupting the conducting pathway in the ferromagnetic SrRuO$_{3}$, we found it first undergoes Anderson localization, then exhibits very large negative magnetoresistance (exceeding -60{\%} compared to the zero-field resistance). The implication is that any ferromagnetic metal should acquire a large magnetoresistance when it is rendered insulating by way of disorder, regardless of whether the disorder is caused by a magnetic or nonmagnetic impurity. This pathway is especially feasible in strongly correlated metals such as SrRuO$_{3}$. A model based on variable range hopping of spin-polarized electrons can quantitatively explain the data.   

Charges play musical chairs on the pyrochlore lattice

We study a highly idealized model for quantum ``charge order'' phase transitions on a pyrochlore lattice, loosely motivated by observations of a ``valence skipping'' structure in vanadium spinels (K. Matsuno et al, PRL 90, 096404, 2003). The model maps onto a particular 3+1-dimensional compact quantum electrodynamics. We describe the transition in terms of a dual theory of monopole defect proliferation. The model gives rise to a number of degenerate low energy excitations which can condense in various patterns. Latest results will be presented.   

Session J26: Nanotubes and Nanowires: Applications

Carbon Nanotube FETs as Chemical Sensors

Exposure to chemical molecules can greatly change the conductance of carbon nanotube FETs (CNFETs). The underlying sensing mechanisms may involve changes in the properties of the interface between nanotube and electrode [1], as well as the nanotube bulk response to chemical molecules [2]. We fabricate CNFETs by standard photolithographic techniques, both for catalyst and contact patterning [3], and characterize their response, i.e. changes in the threshold voltage and saturated conductance, upon exposure of the whole device to chemical molecules, such as nitrogen dioxide and ammonia. We find that nitrogen dioxide only changes the threshold voltage, whereas ammonia changes both the threshold voltage and the saturated conductance. We plan to protect the carbon-nanotube/electrode interfaces and expose only the carbon nanotube to the same concentration of chemical molecules, to measure the contribution to the response due only to the bulk of the nanotube and distinguish between different sensing mechanisms. This work is supported by the ACS (PRF-39152-G5M) and the NSF (DMR-0239721). [1] V.Derycke, R. Martel, J. Appenzeller and Ph. Avouris, Appl.Phys.Lett., 80, 2773 (2002). [2] J. Kong, etc., Science 287, 622 (2000). [3] A. Tselev, K. Hatton, M. S. Fuhrer, M. Paranjape and P. Barbara, Nanotechnology 15, 1475 (2004).   

Individual Single Wall Nanotubes for High Sensitivity Gas Detection

We report on the influence of O$_{2}$ adsorption on individual SWNTs, and show that the observed sensitivity is dramatically improved as compared with SWNT thin films or ropes. SWNTs were grown on a SiO$_{2}$ / Si substrate using chemical vapor deposition, and individual nanotubes were located and contacted with Au/Ti electrodes. The resistance of the SWNT devices was then monitored in a sealed chamber under a number of different gas environments. We observe a two order of magnitude decrease in the resistance of a SWNT device following exposure to oxygen. This is much larger than the 10-15{\%} decrease in the resistance that has been observed for SWNT ropes or mats. Theoretical analysis suggests that the O$_{2}$ molecules provide acceptor impurity states to the nanotubes, which shift the Fermi energy towards the valence band. The resulting increase in positive charge carriers leads to the observed conductance increase. We also observe that the resistance of the SWNT decreases substantially when exposed to inorganic vapors, leading to the possibility of using the nanotube device for gas sensing applications. \textit{Supported by the NSF (DMR-0112824 and ECS-0224114), the DoE (DE-FG02-00ER45832), NASA (NCC5-571) and the US Army SMDC (W9113M-04-C-0024)}.   

Effects of molecular adsorption on electron transport properties of carbon nanotubes

Highly sensitive response of semiconducting single-walled carbon naotubes (SWNTs) to molecular adsorption provides a simple yet efficient direction in exploiting their unique electrical properties. For example, simultaneous doping and nearly ideal gate efficiencies are achieved with polymer electrolytes. However, highly sensitive responses can also lead to difficulties in interpretation of many observations such as the controversy surrounding whether oxygen adsorption causes doping or changes in the nature of SWNT-metal contacts. Effects of molecular adsorption from oxygen in the ambient surrounding to polymers with varying chemical groups on the electrical properties of SWNTs will be discussed.   

Detection of supported lipid bilayers with carbon nanotube transistors

Supported lipid bilayers are important synthetic structures that can be used to mimic and study the properties and functions of cellular membranes, as well as to perform various bioassays which involve membrane bound receptors. The fusion of phospholipid vesicles and formation of a supported lipid bilayer can be detected in real time with high sensitivity by carbon nanotube field effect transistors which have been patterned on the same substrate. The properties of different vesicles, such as fusion rates and phospholipid composition can be distinguished by the conductance change of carbon nanotube field effect transistors. Fluorescence is used to verify the formation of a supported lipid bilayer, although the detection scheme is label-free. This demonstrates that electrical detection with carbon nanotubes can provide a powerful tool for study of lipid membranes.   

Structure and morphology of carbon nanotube AFM probes fabricated by dielectrophoresis

Carbon nanotube probes with diameter of 1$\sim $100 nm and large aspect ratios have been demonstrated both theoretically and experimentally that they are quasi-one-dimensional solids with many unique electronic and mechanical properties. We have recently demonstrated the feasibility of fabricating carbon nanotube AFM probes by a solution based dielectrophoresis process. CNT AFM probes can be readily assembled on the apexes of commercial AFM probes with controlled and predetermined length and orientation. In this talk we discuss the effects of the structure and morphology of the cnt AFM probes, the dispersion and stability of the cnt suspension on the quality and reliability of the cnt probes fabricated. The structure and mechanical stability of the probes were also investigated.   

Metal-coated carbon nanotube tips for scanning probe microscopy

Metal coating has been introduced in order to improve the resolution and capabilities of carbon nanotube scanning probe microscopy tips. We demonstrate magnetic force microscopy of magnetic recording tracks using Co coated nanotube tips. The resolution achieved is better than 20 nanometers. We also use Au coated nanotube tips to perform electrostatic force microscopy of a cut single nanotube with a narrow gap. The metal coating on nanotubes is found to enable the use of micrometers long nanotubes as scanning probes for topographic imaging of high aspect ratio structures. The metal-coated nanotube tips are shown to significantly decrease the convolution effects from the pyramidal silicon tip in these force microscopy techniques.   

Alignment of metal-coated nanotube tips and application in AFM imaging of cells

We demonstrate a reliable method to precisely control the direction of metal-coated nanotubes by exposing them to Ga ions in a Focused Ion Beam. With this method, many metal-coated carbon nanotubes on AFM tips are aligned to a desired direction. Equipped with high aspect-ratio nanotube tips, which are almost perpendicular to the sample surface, we imaged basal cell membrane of polarized Madin Darby Canine Kidney cells in an AFM. Very steep (greater than 80 degree to sample surface) and high (more than 300nm) features in these images illustrate the ability to image high aspect-ratio features with well-aligned nanotube tips.   

Singlewall Carbon Nanotubes as Torsional Springs in a Nanoelectromechanical Device

Nanoelecromechnical devices have been fabricated that utilize an individual singlewall carbon nanotube as a torsional spring for a fully suspended, lithographed metal platform. The torsional properties of the structure were measured through repeated deflection with a scanning probe tip. We discuss results of such measurements as well as progress towards high Q oscillator behavior and integrated device arrays.   

Growth of carbon nanofibers on tipless cantilevers: process development and applications in scanning probe microscopy

Carbon nanofibers (CNFs) are grown on tipless cantilevers as probe tips for scanning probe microscopy. A catalyst dot pattern is formed on the surface of the tipless cantilever using electron beam lithography and CNF growth is performed in a direct-current plasma enhanced chemical vapor deposition reactor. Because the CNF is aligned with the electric field near the edge of the cantilever during growth, it is tilted with respect to the cantilever surface, which compensates partially for the probe tilt introduced when used in scanning probe microscopy. CNFs with different shapes and tip radii can be produced by variation of experimental conditions. The tip geometries of the CNF probes are defined by their catalyst particles, whose magnetic nature also imparts a capability for imaging magnetic samples. We have demonstrated their use in both atomic force and magnetic force surface imaging. These probe tips may provide information on magnetic phenomena at the nanometer scale in connection with the drive for ever-increasing storage density of magnetic hard disks.   

Rapid and reproducible fabrication of nanotube/nanowire AFM probes by dielectrophoresis

Atomic force microscopes (AFM) are commonly used to map the surface structure and topography of different objects and devices. The resolution, sensitivity, and probing depth of an AFM depend on the geometry of the probe. Here, we demonstrate an efficient method to fabricate nanotube/nanowire based AFM tips by dielectrophoresis. Under dielectrophoretic force, the processed CNT bundles can be readily assembled to the apexes of conventional AFM tips with controlled and pre-determined length and orientation. Both the lateral resolution and probing depth have been studied. The fabrication process can also be utilized to assemble other 1-D nanostructures. The research was supported by NASA URETI on Bio Inspired Materials (NCC-1-02037).   

Room temperature in-situ growth of single Ag2Ga needles on AFM tips

We have found that single metallic nanowires can be grown in various directions, including sticking straight out from the end of a sharp (or for that matter blunt) tips, e.g. atomic force microscope (AFM) tips or even tipless AFM cantilevers. This is done by coating a silicon cantilever with a thin film of silver (with an underlying chrome flash to promote adhesion). Then the tip is dipped in a small drop of melted gallium (at or near room temperature) for 5 minutes. The tip is removed at one micron per second, and a single nanoneedle is found formed on the tip in at least 50{\%} of the experiments. Faceted nanoneedles have been formed from 1 to 20 microns in length and 45 to 300 nm in diameter. In-situ scanning electron microscopy is used to observe the growth and mechanical properties of the needles and transmission electron microscopy shows the needles to be single crystal. Contact and non-contact mode AFM imaging and voltage lithography with these needle-tipped cantilevers is reported.   

Composite Nanowire-Based Probes for Magnetic Resonance Force Microscopy

We will present a nanowire-based methodology for the fabrication of ultra-high sensitivity and resolution probes for atomic resolution magnetic resonance force microscopy (MRFM). The fabrication technique combines electrochemical deposition of multi-functional metals into nanoporous polycarbonate membranes and chemically selective electroless deposition of optical nanoreflector onto the nanowire. The completed composite nanowire structure contains all the required elements for ultra-high sensitivity and resolution MRFM sensor with: (a) magnetic nanowire segment providing atomic resolution magnetic field imaging gradients as well as large force gradients for high sensitivity, (b) noble metal enhanced nanowire segment providing efficient scattering cross-section from a sub-wavelength source for optical readout of nanowire vibration, and (c) non-magnetic/non-plasmonic nanowire segment providing the cantilever structure for sensitive mechanical detection of magnetic resonance.   

Scanning Probe Microscopy of Semiconducting Nanowires

A liquid He cooled scanning probe microscope (SPM) with a conducting tip has been used to image conduction through InAs and InP nanowires. The nanowires, grown using a vapor-liquid-solid technique, have diameters between 50 nm and 100 nm and resistances on the order of 10 k$\Omega $. Ti/Al electrodes were defined using e-beam lithography to form source and drain contacts with a spacing of 1 to 3 $\mu $m. The charged SPM tip is scanned in an area above the nanowire; the resulting change in nanowire conductance is recorded to form the image. These conductance images are used to study the behavior of electrons in the nanowire on a local scale.   

Photoconductivity, High-resolution AFM, and Scanning Conductance Microscopy of Porphyrin Nanorods

We have shown$^{1}$ that the diacid form of the porphyrin tetrakis(4-sulfonatophenyl) porphine (TPPS$_{4})$ self assembles into nanorods with well-defined height and width. Upon illumination, their conductivity grows over hundreds of seconds. They also produce a zero-bias photocurrent with trainable polarity.$^{2}$ We present measurements as a function of illumination wavelength and intensity, which support a model of charge hopping along paths of previously photoionized porphyrin molecules. We also give results from Scanning Conductance Microscopy experiments; these are designed to clarify the role of the contacts in the DC measurements. Our high-resolution AFM images support the model of a hollow tube$^{3}$, which collapses on contact with the substrate. $^{1}$A.D. Schwab \textit{et al.}, J. Phys. Chem. B \textbf{107}, 11339 (2003). $^{2}$A.D. Schwab \textit{et al.}, Nano Letters \textbf{4}, 1261 (2004). $^{3}$S.C.M. Gandini, E.L. Gelamo, R. Itri, and M. Tabak, Biophys. J. \textbf{85}, 1259 (2003).   

Integration of carbon nanotubes into atomic resolution UHV-STM lithography and nanofabrication schemes on H-passivated Si(100) surfaces

Nanoscale patterning of the Si(100)-2x1:H surface with the UHV-STM [1] is leveraged to chemically modify the Si substrate acting as a pristine semiconducting support for isolated single-walled carbon nanotubes (SWCNTs). By intercepting an isolated SWCNT with a sub-5-nm-wide pattern of Si dangling bonds [2], we can reproducibly strengthen the SWCNT-Si interaction which is directly manifested as an enhanced mechanical stability of the SWCNT in the presence of the rastered STM tip and a concomitant topographic depression. Spatially-resolved tunneling conductance maps have been generated for individual SWCNTs spanning both depassivated and unperturbed domains on the Si(100)-2x1:H surface. We have also demonstrated the controlled manipulation of SWCNTs with the STM tip, including the reversible actuation of a 13-nm-long segment intentionally cut from a longer SWCNT and the splitting of two SWCNTs originally in wall-to-wall contact. [1] J.W. Lyding et al., APL 64, 2010 (1994). [2] P.M. Albrecht and J.W. Lyding, Superlattice Microst. 34, 407 (2003).   

Session J27: Focus Session: Carbon Nanotubes: Electronic Properties II

Suppression mechanism of inter-tube transfer in double-wall carbon nanotubes

Double-wall carbon nanotubes have incommensurate lattice structure and are quasi-periodic[1,2]. Therefore, inter-tube transfer of electrons between incommensurate tubes is the key to understanding of double-wall tubes. Although some theoretical studies reported suppression of inter-tube transfer in multiwall tubes[3], the mechanism has not been well understood. The purpose of this paper is to clarify effects of inter-tube transfer in double-wall tubes. Using a tight-binding model length-dependence of conductance due to inter-tube transfer is calculated. The conductance is negligibly small in comparison to the conductance quantum and oscillates around an average which is approximately independent of the length. It is revealed based on the first-order perturbation theory that the result is attributed to quasi-periodic oscillation of position dependence of small local effective inter-tube coupling. [1] M. Kociak, K. Suenaga, K. Hirahara, Y. Saito, T. Nakahira, and S. Iijima, Phys. Rev. Lett. {\bf 89} (2002) 155501. [2] J. M. Zuo, I. Vartanyants, M. Gao, R. Zhang, and L. A. Nagahara, Science {\bf 300} (2003) 1419. [3] Y.-G. Yoon, P. Delaney, and S. G. Louie, Phys. Rev. B {\bf 66} (2002) 073407.   

Electrical Properties of Defects in Carbon Nanotubes

Unmodified single-walled carbon nanotubes are generally considered to be defect free conductors, even though most synthesis and fabrication techniques introduce non-zero defect densities. Primarily using scanned probe microscopies, we have investigated a number of electronic devices in which defects play important, if not primary, roles. For example, defects in otherwise metallic nanotubes can lead to switching behaviors in a field-effect transistor geometry, leading to the misidentification of such tubes from transport measurements alone. Furthermore, defect sites also contribute disproportionally to the device resistance of a metallic nanotube. In semiconducting nanotubes, field-induced switching has different signatures depending on whether the contacts, the bulk nanotube, or a defect site dominates the behavior. We will present data and measurements from a variety of samples demonstrating the characteristics and frequency of such effects in typical nanotube devices. This work has been supported by NSF-DMR.   

Effects of Disorder on the Conductance of Semiconducting Carbon Nanotubes

The single parameter graphene sheet model for single-wall carbon nanotubes has been used to successfully explain many of their fundamental properties. However, even within this simple tight-binding approach calculations of nanotube conductance are typically restricted to the achiral armchair and zig-zag tubes because of the relatively small number of atoms in their translational unit cells. By taking advantage of helical symmetry we have overcome this limitation. This Green function based approach allows ready treatment of the effects of disorder on conductance without regard to the tube's chirality. Results for the effects of residual disorder on the conductance of a series of chiral semiconducting nanotubes will be presented.   

Probing Scattering in Single-Walled Carbon Nanotubes

Transport measurements and atomic force microscopy were used to study electron scattering rates in metallic single-walled carbon nanotubes. From scaling of the resistance of the same nanotube with length in the low and high bias regimes, the mean free paths for both regimes are inferred. The observed scattering rates are consistent with calculations for acoustic phonon scattering at low biases and zone boundary/optical phonon scattering at high biases. We have also developed techniques to probe the high frequency transport properties of nanotube transistors. We have used the nanotube transistor as a microwave mixer operating at frequencies up to 50 GHz. The long-term goal is to directly measure the fundamental excitations and scattering rates. The author would like to acknowledge Ji-Yong Park, Yuval Yaish, Vera Sazonova, Xinjian Zhou, Hao Lin, Hande Ustunel, Stephan Braig, T.A. Arias, Piet W. Brower, Sandip Tiwari and Paul L. McEuen of Cornell University for their contributions to this work.   

Velocity Saturation in Semiconducting Carbon Nanotubes

Charge transport in individual semiconducting single-walled nanotubes (SWNTs) with Schottky barrier contacts has been studied at high bias voltages. We observe nearly symmetric ambipolar transport, and find that both electron and hole currents may significantly exceed 25 $\mu $A, thought to be the limiting current in metallic SWNTs due to optical phonon emission. The current for a ballistic ambipolar nanotube field-effect transistor has been calculated carefully, treating the potential and the charge of the nanotube self-consistently, and including electron-hole recombination. The result is directly compared with the experimental transport data, and it is found that the current may be as high as one-fifth that expected for a ballistic nanotube field-effect transistor, even for nanotubes with lengths of tens of microns. The high-bias behavior in semiconducting nanotubes is better explained by velocity saturation, rather than current saturation. We propose a charge-controlled current model of transistor operation, with maximum saturation velocity $v_{s}$ of 1.8 $\times $ 10$^{7}$ cm/s, which explains the magnitude of both the differential conductance under symmetric bias and the transconductance.   

Electrical generation and absorption of phonons in carbon nanotubes

We have performed low temperature scanning tunneling spectroscopy on individual single-wall carbon nanotubes freely suspended over trenches. Spatially resolved spectroscopy shows a Coulomb-staircase behavior superimposed on the local density of states. In addition to the Coulomb peaks from the addition of electrons, side peaks appear due to phonon-assisted tunneling. Electrons inelastically tunneling into the nanotube cause a non-equilibrium phonon occupation, leading to both emission and absorption of phonons by successive tunneling electrons. The addition of a gate electrode into our STM configuration allows further validation of this interpretation. These observations represent a new class of electrical transport phenomena, namely a current induced non-equilibrium phonon distribution and its influence on transport through a molecule.   

Adsorbed monolayers on individual single-walled carbon nanotubes

We have built devices and apparatus to search for the effects of adsorbed monolayers on electrical transport through single-walled carbon nanotubes. The nanotubes, grown by chemical vapor deposition, are contacted by shadow evaporation for cleanliness, and placed in a controlled pressure and temperature environment where an external magnetic field can be applied. We concentrate on atoms and molecules which are likely to influence the electronic properties. Our eventual aim is to extend earlier work on the two-dimensional phases of matter on pyrolytic graphite to the nearly one-dimensional regime presented by cylindrical monolayers on a nanotube surface. We report preliminary results on oxygen, which is thought to dope nanotubes, and which exhibits magnetic order in low temperature 2D monolayers.   

Electrical Breakdown of Carbon Nanotubes in Ultrahigh Vacuum

It has been shown$^{1}$ that the current-induced breakdown of multiwall carbon nanotubes occurs at a higher voltage in high vacuum than in air, and that the size of the resulting gap is smaller. It is believed that the breakdown is due to joule heating and oxidation. Therefore, we expect that the maximum voltage and current would be higher in an oxygen-free environment, and that these conditions would allow study of the fundamental limits of current density in nanotubes. Further, it is reasonable to expect that the resulting gap would be smaller, and perhaps more suitable for making electrical contacts to other target molecules.$^{2}$ We present measurements on single wall nanotubes taken in a vacuum better than 1 x 10$^{-10}$ Torr, so that less than one oxygen molecule impinges on a nanotube over a several hour experiment. $^{1}$P.G. Collins \textit{et al}, Phys. Rev. Lett. \textbf{86}, 3128 (2001)$.$ $^{2}$ K. Tsukagoshi, I. Yagi, and Y. Aoyagi, Appl. Phys. Lett. \textbf{85}, 1021 (2004).   

High Temperature Conductivity and Reactivity of Carbon Nanotube Electronic Circuits

At sufficiently high temperatures, carbon nanotubes (CNTs) begin to react with their immediate environment. For example, adsorbates first desorb, then the carbon may react with connective electrodes, and ultimately Stone-Wales defects become mobile and can be annealed. We have designed and built an apparatus to study electronic transport in individual CNTs under these extreme conditions. Our apparatus provides continuous, four probe measurements of impedance and transimpedance from room temperature to 1500 K in an ultrahigh vacuum (UHV) system. By heating the devices to such temperatures, we are able to study the onset and progress of reactions, and the UHV environment allows for precise control of the local surface chemistry. Furthermore, the devices can be heated either resistively or radiatively at rates exceeding 100 K/min, allowing for pulsed thermal processing and an investigation of photoinduced chemistries. We will present results on the high temperature resistance of CNT devices in a UHV environment, and preliminary results indicating irreversible chemical changes which occur at high temperatures.   

Equivalent Circuit Modeling for Carbon Nanotube Schottky Barrier Modulation in Polarized Gases

We study the carbon nanotube Schottky barrier (SB) at the metallic electrode interface in polarized gases using an equivalent circuit model. The gas-nanotube interaction is often weak and very little charge transfer is expected [1]. This is the case with oxygen, but the gas- electrode interaction is appreciable and makes the oxygen molecules negatively charged. In the closed circuit condition, screening positive charges appear in the nanotube as well as in the electrode, and the SB is modulated due to the resultant electrostatic effects [2]. In the case of ammonia, both the gas-nanotube and gas-electrode interactions are weak, but the SB can still be modulated since the molecules are polarized and align in the preferred orientation within the gap between the electrode and nanotube in the open circuit condition (dipole layer formation). In the closed circuit condition, an electric field appears in the gap and strengthens or weakens the preferred dipole alignment reflecting the nanotube Fermi level. The resultant dipole field modulates the SB. The modulation is visible when the nanotube depletion mode is involved, and the required dipole density is as low as 2 x 10 $^{13}$ dipoles/cm$^{2}$, which is quite feasible experimentally. [1] Bauschlicher and Ricca, Phys. Rev. B 70, 115409 (2004). [2] Yamada, Phys. Rev. B 69, 125408 (2004).   

Two-dimensional Percolation Effects of Transparent, Conductive Carbon Nanotube Films

Ultra-thin, uniform single-walled carbon nanotube films of varying densities have been made at room temperature by a vacuum filtration method. Measurements of the sheet conductance as a function of nanotube network density show 2D percolation behavior. In addition, the network transparency in the visible spectral range was examined and the results are in agreement with a standard thin-film model: fits to the standard theory at 550 nm. Transparency measurements also indicate the usefulness of nanotube network films as a transparent, conductive coating.   

Measurements of nonlinear transport and interactions in single-walled carbon nanotubes

It has recently been emphasized that the nonlinear two-terminal conductance of a piece of material can be used to measure the strength of electron-electron interactions in that material. More precisely, contributions to the current which are quadratic in voltage bias and proportional to the applied magnetic field should be proportional to the interaction constant at low temperatures and to the interaction constant squared at high temperatures. The question of what role interactions and correlations play in one-dimensional conductors such as single-walled carbon nanotubes remains open, although it is generally agreed that the degenerate electron system in a pure nanotube behaves as a Luttinger liquid. With this in mind we have measured these nonlinear terms in single-walled carbon nanotubes, in both semiconducting and metallic regimes, and at high and low temperatures.   

Tomonaga-Luttinger liquid related superconductivity in end-bonded carbon nanotubes

Is it possible to find superconductivity in one-dimensional (1D) systems? It is well known that 1D systems have some obstructions that prevent emergence of superconductivity, e.g. Tomonaga-Luttinger liquid (TLL), spin fluctuation, van-Hove singularity, Peierls transition, and charge-density waves. Carbon nanotube (CN) is a good candidate to investigate this possibility. Although a variety of intriguing quantum phenomena has been reported in CNs, only two groups reported intrinsic superconductivity without reproducibility by other researchers. As well, the transition temperature (Tc) was as low as 0.2K in suspended ropes of SWNTs. Although Tc of 15K was found in thin SWNTs, it was identified only from the Meissner effect. No correlation with 1D phenomena, in particular with TLL arising from 1D electron-electron interaction, was also clarified. Here, we report superconductivity with the onset Tc as high as 12K and T=7.8K, at which resistance drops to zero ohm, for the highest case in end-bonded CNs, which were packed into nanopores of alumina templates. The transition temperatures were approximately 25-times and 40-times larger than those in a past report, respectively. We find that end-bonding the CNs by an electrode is the crucial factor for realizing superconductivity that overcomes TLL.   

Suppressed Conductance of Individual Single Walled Carbon Nanotube/Polypyrole Composite Nanowires and Their Sensing Applications

We present synthesis of individual single walled carbon nanotube/polypyrrole composite nanowire by chemical vapor deposition followed by electrochemical deposition for the first time. The transport properties of the composite nanowire were studied and suppression in conduction through carbon nanotube channels was discovered and discussed. Moreover, we also demonstrated the composite nanowire devices can serve as chemical sensors, which responses to oxidizing and reducing gases. The studies on the transport of the composite and their sensing applications shed light on the interaction between the nanotubes and the electrochemically coated polymers and also opens the way toward high performance chemical/bio sensors with high selectivity.   

Session J28: Focus Session: Metallic Glasses and Liquids II

Effect of shear-band formation and structural relaxation on mechanical properties of an Al-based metallic glass

The effect of cold rolling on the mechanical behavior of amorphous Al$_{86.8}$Ni$_{3.7}$Y$_{9.5}$ has been investigated by nanoindentation. This alloy does not crystallize in response to plastic deformation at room temperature. While significant pile-ups are observed around indentations in the as-spun alloy, they are small in the rolled sample. It is also found that rolling reduces the hardness. Deformation of the as-spun alloy occurs by nucleation and propagation of shear bands, whereas the cold-rolled alloy deforms by propagation of pre- existing shear bands. Annealing leads to a recovery of the pile-ups, with the hardness increasing to above its value for the as-spun sample. Using high-resolution transmission electron microscopy, nanovoids are observed to be uniformly distributed in the shear bands formed due to rolling. Annealing does not appear to affect these nanovoids. This work was funded by the National Science Foundation, Grant DMR-0314214   

Shear Processes in Pd$_{40}$Ni$_{40}$P$_{20}$ Bulk Metallic Glasses

Depending on the imposed deformation rate, plastic deformation in metallic glasses can be either Newtonian or non-Newtonian. To investigate the influence of deformation history on non-Newtonian plastic flow in bulk Pd$_{40}$Ni$_{40}$P$_{20}$ glass we deformed the same volume of the glass specimen along differently oriented glide planes. We found that the glass has a memory of its previous plastic deformation, but this memory is largely independent of the previous glide direction. Loss of the memory follows first-order kinetics with a time constant of 1260 s at 553 K. The transition from Newtonian to non-Newtonian flow is rather abrupt and occurs at a Deborah number, \textit{De = }$\dot {\gamma }\,\cdot \tau $ = 0.5, where $\dot {\gamma }$ is the plastic shear strain rate and $\tau $is the time constant for the exponential annihilation of the flow defects. This value of \textit{De} is consistent with the value of \textit{De} $\approx $ 1 observed at the onset of flow instabilities in liquids. The abruptness of the transition, together with the strong stress-sensitivity of the viscosity in the non-Newtonian regime, suggests that the plasticity agents in the Newtonian and non-Newtonian flow regimes are not the same.   

A resonant ultrasound spectroscopy determination of the elastic constants of bulk metallic glasses

It has been shown recently that the fragility of a glass-forming liquid is closely related to the elastic constants, and in particular to Poisson's ratio, of the corresponding glass phase.$^{1}$ Resonant ultrasound spectroscopy yields simultaneously the bulk and shear moduli of millimeter-sized samples, and thus provides a convenient and non-destructive way to determine Poisson's ratio of bulk metallic glasses. The elastic constants and Poisson's ratio of several bulk metallic glasses have been measured as a function of temperature between 5 and 400 K, and the obtained data will be compared to fragility measurements. [The support from NSF IGERT EEC-9984548 and DMR 0206625, and of DARPA SAM Program under ONR Grant N00014-01-1-0961 is acknowledged.] $^{1}$ Novikov V. N. and Sokolov A. P., \textit{Nature} \textbf{431}, 961-963 (2004)   

NMR Observation of Dynamics in Metallic Supercooled Liquids and Glasses

Dynamic crossover at temperature $T_{c} \quad > T_{g}$ in the supercooled liquid is an important issue for understanding the nature of glass transition. In a liquid of densely packed atoms, any given atom is temporarily trapped by the transient cage formed by neighboring atoms. Mode-coupling theory (MCT) predicts that such cage trapping of atoms undergoes a dynamic arrest below $T_{c}$ leading to a liquid to solid transition in the absence of hopping. Here we show that NMR could detect selectively different types of atomic motions based on their different spatial characteristics and timescales. The discussion will focus mostly on results obtained from the bulk metallic glass Pd$_{43}$Ni$_{10}$Cu$_{27}$P$_{20}$, one of the best glass formers with extremely high accessibility to the supercooled liquid region. We will show that the Knight shift of $^{31}$P is an NMR parameter sensitive to the Debye-Waller factor caused by local vibrations and cage rattling. Experiment shows that the mean-squared amplitude of such local motions depends linearly on $k_{B}T$ above a certain crossover temperature $T_{c}$ as well as below $T_{g}$ as expected by the equipartition theorem for harmonic vibrations. From $T_{c}$ down to $T_{g}$, the mean-squared amplitude decreases much more rapidly. Such crossover behavior is shown to be in good agreement with the prediction of the MCT with regard to motions of cage rattling. We will also show that NMR of quadrupolar nuclei $^{65}$Cu and $^{63}$Cu can probe the time correlation function of the local electric-field gradient (EFG) in the supercooled liquid. We measured the temperature dependence of the EFG correlation time which is closely related to atomic diffusion. Once again, the result is shown to be consistent with the prediction of the MCT.   

Fast in-situ, high-resolution PDF analysis studies of glasses and nanocrystalline materials

We are increasingly interested in complex materials for their unique functional properties. Complex materials often exhibit nanoscale local structures that are important in determining their properties. These come about from defects but often are intrinsic, coming from competing interactions in the materials. It is important to characterize these ``nanostructures'' but this is difficult because they are not, by their nature, long-range ordered and cannot be studied using traditional crystallographic methods. The atomic pair distribution function (PDF) analysis method has, for a long time, been used to study the structure of glasses and liquids. I will describe recent developments in both data collection and analysis that make this method a powerful quantitative probe of nanostructures. In particular, intense high energy x-rays from third generation synchrotron sources now make it possible to study materials in-situ under extreme conditions while studying their local atomic structure. The experiments are quick opening the way to time-resolve studies.   

Role of Yttrium in Glass Formation of Bulk Metallic Glasses

It was found experimentally that appropriate additions of yttrium in both Fe-based and Zr-based bulk metallic glasses can improve their glass-forming ability (GFA) dramatically$^{1,2}$. In this talk, focuses will be placed on understanding the beneficial effects of yttrium on glass formation in these alloy systems. Our studies indicated that the striking enhancement of the GFA in these systems is attributed to two factors: \begin{enumerate} \item Yttrium additions can successfully suppress the formation of competing crystalline phases, thus adjusting the composition to be closer to the eutectic and lowering the liquidus temperatures of the alloys (i.e., destabilizing the competing crystalline phases). \item Yttrium can also scavenge oxygen to form innocuous yttrium oxides from the undercooled liquid, thereby stabilizing the liquid phase. \end{enumerate} The current work demonstrates clearly that the GFA of bulk metallic glasses can be increased by either enhancing the liquid phase stability or suppressing the formation of competing crystalline phases, which further confirms and verifies our previous analysis$^{3}$. \newline \newline [1]. Z. P. Lu {\&} C. T. Liu, Phys. Rev. Lett., 92(2004)245503. \newline [2]. Z. P. Lu et al, Appl. Phys. Lett., 83(2003)2581. \newline [3]. Z. P. Lu {\&} C. T. Liu, Phys. Rev. Lett., 91(2003)115505. \newline \newline \newline This research was sponsored by the Division of Materials Sciences and Engineering, Office of Basic Energy Sciences, U.S. Department of Energy under contract DE-AC05-00OR-22725 with UT-Battelle, LLC. .   

Pair Distribution Function Study of Zr$_{55}$Cu$_{35}$Al$_{10}$ Bulk Metallic Glasses

Despite strong interests in bulk metallic glasses for a variety of engineering applications, details of their structures still remain uncertain. During annealing bulk metallic glasses show heat release prior to the crystallization, which corresponds to the structural relaxation. To investigate the changes in local atomic structure (short-range to medium-range order) associated with the structural relaxation and partial crystallization, we carried out neutron-scattering measurements on the Zr$_{55}$Cu$_{35}$Al$_{10}$ bulk metallic Glasses subjected to three different heat treatment conditions: (1) as-quenched, (2) annealed at 703 K for 2.1 ks to induce structural relaxation, and (3) annealed at 703 K for 4.2 ks to partially crystallize. The neutron-scattering results were studies by pair distribution function analysis. In addition, high-resolution transmission electron microscopy and x-ray diffraction have been performed to investigate the as-cast structure and crystallization processes to complement the neutron-scattering structural studies. \textit{This work is supported by the NSF International Materials Institutes Program under DMR-0231320 with Dr. C. Huber as the Program Director.}   

The Genesis Mission Metallic Glass Solar Wind Collector

NASA's Genesis mission continuously exposed materials to the solar wind, and brought them back to Earth for analysis. Despite the hard impact landing of the sample return capsule in Sept./2004, some of the solar wind collectors were recovered in pristine condition; one was a metallic glass, with target composition Zr$_{58.5}$Nb$_{2.8}$Cu$_{15.6}$Ni$_{12.8}$Al$_{10.3}$. In this talk, we will describe the glassy alloy, the mission critical-properties, and expected science returns. Metallic glasses are well suited to measure solar wind components: 1) the disordered structure reduces fractionation during solar wind ion-implantation and loss of solar wind ions due to diffusion; and 2) an absence of grain boundaries eliminates high-diffusion-rate pathways. The glass will be analyzed for high-energy elements; e.g., He and Ne, with beyond solar wind energies. It is hoped that the glass will help elucidate the origin of solar energetic particles, a solar wind component with controversial origin.   

Structural and energetic properties of nickel clusters

The four most stable structures of Ni$_N$ clusters with $N$ from 2 to 150 have been determined using a combination of the embedded-atom method in the version of Daw, Baskes and Foiles, the {\it variable metric/quasi-Newton} method, and our own {\it Aufbau/Abbau} method. A systematic study of energetics, structure, growth, and stability of also larger clusters has been carried through without more or less severe assumptions on the initial geometries in the structure optimization, on the symmetry, or on bond lengths. We present and apply different analytical tools in studying structural and energetic properties of such a large class of clusters. These include means for identifying the overall shape, the occurrence of atomic shells, the similarity of the clusters with, e.g., fragments of the {\it fcc} crystal or of a large icosahedral cluster, and a way of analysing whether the $N$-atom cluster can be considered constructed from the $(N-1)$-atom one by adding an extra atom. In addition, we compare in detail with results from chemical-probe experiment. Maybe the most central result is that first for clusters with $N$ above 80 general trends can be identified.   

Session J30: Elastomers and Gels

Mechanical and swelling properties of end-linked polydimethylsiloxane networks with hydrogen bonding or ionic interactions

We have synthesized a series of endlinked polydimethylsiloxane networks in which the polydimethylsiloxane chain has carboxyl groups at regular intervals along the polymer backbone. The spacing between these carboxyl groups and the number of these carboxyl groups/chain has been varied. Modulus studies at room temperature when carboxyl groups have very weak interactions show moduli similar to conventional endlinked PDMS networks. At high temperatures such as 150 C, carboxyl groups undergo intermolecular interactions and act as additional cross-link points thereby strengthening the networks. On bringing the networks back back to room temperature a number of these intermolecular bonds are retained. Swelling data on networks annealed at 150 C for several days as well as those not annealed will be presented. Carboxyl based networks have also been converted to Gallium and Cobalt networks by treatment with appropriate salts. Swelling and moduli studies at room temperature and on thermally annealed samples of these ionomer networks with both covalent endlinks and physical ionic domains will be presented.   

Nonaffinity and nonlinearity in random elastic networks

We develop a general framework for the elasticity of networks with spatially varying elastic constants. We consider spring networks with randomness in either the spring constants, node position and connectivity, or internal stresses (through frustrated bond lengths). The non-affine component of the elastic response and corrections to bulk elasticity are calculated as a function of the magnitude and spatial correlations of the effective local elastic constants. Our calculations are verified through extensive numerical simulations. We believe this framework will apply to stiff polymer gels, foams, and random bead packings.   

MD simulations of chemically reacting networks

The aging of polymeric networks is difficult to investigate in the laboratory. Chemical control is at the mercy of nature, and resulting network structures aren't easily observable. Molecular dynamics simulations, without these limitations, are used to perform virtual stress relaxation and permanent set measurements. Tobolsky's Independent Network Model (INM) and Flory's modifications are tested. The INM states that crosslinks added to an existing network can be treated as forming a second, parallel network. Simulations of these two-stage systems support this model. Flory hypothesized that the effect of removing first stage crosslinks is reduced by an effective switching of second-stage crosslinks. This is supported by simulations in which all first-stage crosslinks are removed. Tobolsky and Flory assumed Gaussian network segments and, more problematically, affine deformation in the derivation of quantitative relationships for the INM. The simulation results match their predictions only qualitatively. More advanced models of elasticity are discussed.   

Thiol-Vinyl Photopolymerizations: Controlled Network Evolution

Presently, the photopolymerization field is dominated by acrylic systems. Thiol-vinyl photopolymerizations have many advantages over acrylic polymerizations, like reduced oxygen inhibition, initiatorless polymerization, delayed gelation, and greater control over network structure. However, currently there are no theories for predicting and thereby controlling network evolution of these kinetically controlled mixed step-chain thiol-vinyl polymerizations. Here, a combined kinetic and statistical modeling framework is developed to predict network properties including molecular weight, gel point, and crosslinking density. Further, non-mean field kinetic modeling is used to include network non-ideality, cyclization, and ensuing network predictions are successfully contrasted with experiments. Then, network control aspect of these polymerizations is employed to design degradable materials with tunable degradation kinetics that are suitable for controlled drug delivery and tissue engineering. Both the network properties and concomitant degradation kinetics are adjusted by changing thiol functionality or stoichiometric ratio.   

Traveling Waves in a Reactive Polymer Gel

We consider a theoretical model of a polymer gel, which exhibits a swelling-deswelling behavior in response to the Belousov-Zhabotinsky (BZ) reaction. The BZ reaction generates periodic redox changes of a metal catalyst, and a wide variety of spatiotemporal structures have been observed in the course of the BZ reaction in solution. If the catalyst is covalently bonded to a responsive hydrogel soaked in a solution containing the rest of the BZ reagents, then the metal redox changes may cause variations in the gel volume. The self-oscillation of the gel volume and the traveling chemical waves accompanied by the local swelling have been experimentally observed by Yoshida and co-workers. Here, we present a simplified theoretical description of a hydrogel with the BZ reaction. The description is based on the Oregonator model of the BZ reaction, and on the two-fluid model of the gel dynamics. The formulated model is applied to studying one-dimensional wave trains in the reactive gel. We focus on the dispersion law as it reflects the inherent coupling between the chemical and mechanical degrees of freedom.   

Scaling of entropic shear rigidity

The scaling of shear modulus near the gelation/vulcanization transition is explored heuristically and analytically [1]. It is found that in a dense melt the effective chains of the infinite cluster have sizes that scale{\it sub-linearly\/} with their contour length. Consequently, each chain contributes $k_{\rm B} T$ to the rigidity, which leads to a shear modulus exponent $d\nu$. In contrast, in phantom elastic networks the scaling is {\it linear\/} in the contour length, yielding an exponent identical to that of the random resistor network conductivity, as predicted by de Gennes. For non-dense systems, the exponent should cross over to $d\nu$ when the percolation correlation length is much larger than the density-fluctuation length. [1] X. Xing, S. Mukhopadhyay and P. M. Goldbart, Phys. Rev. Lett. 93, 225701 (2004).   

A cavity approach to the heterogeneity of the random solid state

Replica statistical mechanics has been invoked to explore a wide collection of properties of the random solid state of matter, as formed, e.g., by vulcanized macromolecular systems. Even at the mean-field level, this approach yields a rich and illuminating description of the state in terms of a fraction of localized particles and the statistical distribution of their localization lengths. An application of the cavity method -- successfully used to analyze a wide range of spin glass models and other statistical-mechanical systems with quenched disorder -- allows replica-based results for random solids to be recovered in a way that sheds light both on their physical origin and their limitations. An extension of this cavity approach, involving an assembly of Ornstein-Uhlenbeck processes, points towards a strategy for addressing certain aspects of the dynamics of the random solid state, at least at the mean-field level.   

Structural changes in polymer gels probed by Fluorescence Correlation Spectroscopy

We apply fluorescence correlation spectroscopy (FCS), a non-invasive optical technique, to measure the diffusion of small fluorescent probes (TAMRA, Mw = 430 Da; TAMRA-labeled dextran, Mw = 10 kDa) in semidilute, non-fluorescent --hence invisible-- poly(vinyl alcohol) (PVA, Mw = 85 kDa) solutions and cross-linked PVA gels. The measurements indicate that for the same polymer concentration, the diffusion of the particles slows down when the polymer solution is cross-linked. Further, the more the polymer chains are cross-linked, the slower the particles diffuse. We attribute this effect to the formation of large-scale structural changes caused by cross-linking of the PVA chains. These results suggest that the cross-link density, which is often ignored in the analysis of probe diffusion data in gels, must be taken into account. Measurements of the elastic modulus support this conclusion, as indicated by the linear correlation between the diffusion time of the particles and the elastic modulus of the gels.   

The elasticity of smectic liquid crystal elastomers

The elasticity of smectic elastomers is remarkable: response is solid-like along the layer normal and rubbery in the plane. These 2-D elastomers have extreme Poisson ratios and a reversible threshold above which the elastic modulus is drastically reduced. This behavior can be understood from a microscopic model based on the anisotropic chain shape of liquid crystalline polymers that couple to the smectic layers through the crosslinks. This coupling constrains the layer normal to deform affinely with the rubber matrix. Additionally chain shape distribution anisotropy colors the complex shears occurring after the mechanical instability occurs. Results fit well with experimental elastic and X-ray scattering data. \newline Smectic C elastomers are predicted to display soft elasticity, accompanied by an intricate microstructure to accommodate the strains required for softness. They are anticipated to be of great technological importance because their symmetry permits piezoelectricity, ferroelectricity and second harmonic generation.   

Self-Adhesion of uncrosslinked elastomers using a probe method

Relatively few studies have been carried out on the adhesion between uncrosslinked elastomers. A key experimental obstacle for the understanding of this problem is the separation of the surface and the bulk contributions to adhesion for such highly deformable and viscoelastic materials. A modification of the probe tack experiment used in the Pressure Sensitive Adhesive industry allows us to study the self-adhesion of elastomers with a very good control of the experimental parameters (contact time down to 1s, pressure of contact, debonding velocity). A video acquisition also allows the detailed analysis of the debonding mechanisms. We present here results on the self adhesion of three SBR Rubbers with the same microstructure (20{\%} styrene, 42{\%} vinyl, 19{\%} cis and 19{\%} trans, Mw/Mn lower than 1.1) but with different molecular weights (80~000, 160~000 and 240~000 g/mol). We observed different debonding mechanisms depending on the time of contact, the debonding velocity and the polymer used. We found that these different fracture behaviours are directly related to the bulk rheology of the polymers, especially their reptation time. Finally, we propose a map of the mechanisms as a function of two reduced parameters, the ratio of contact time to reptation time and the Deborah number.   

Effects of Substitutes on the Self-Assembling of Rigid Polymers

Beyond the basic understanding of the assembly process of highly conjugated rigid polymers, these macromolecules are inherently semiconductors and may be used as molecular wires with the appropriate doping. In device applications, the molecules have to be in contact with either an interface or other molecules. In an effort to correlate the elctro-optical properties of substituted poly(para phenyleneethynylene) (PPE), we have recently shown that PPE substituted by short alkyl chains, associate to form flat aggregates in solutions of toluene as well as at interfaces. With increasing concentration in solution, these aggregates form fragile gels. The shape and size of the PPE aggregates as well as the overall phase diagrams are strongly dependent on molecular parameters and also on the environment in which the association takes place. AFM and small angle neutron scattering studies have shown that a bulky substitutent triisopropylsilyloxy changes the structure of the assembly, the nature of the gels and the morphology at interfaces.   

Ultrasound Devulcanization of Natural Rubber, Studied by NMR Relaxation and Diffusion

In support of recycling of industrial rubbers, we have studied the effect of intense ultrasound on unfilled natural rubber networks using proton and carbon transverse NMR relaxation and diffusion, sol extraction, and bulk characterization. At 70.5$^{\circ}$ C the proton echo decay exhibits three components, due to entangled sol and crosslinked network; unentangled polymeric sol plus dangling chain ends; and oligomer remnants. Contrary to the 13C results which indicate backbone mobilities decreasing with sonication (hence with increasing sol fraction), all 1H component T2 values, hence intermolecular mobilities, increase by similar modest factors; the network component amplitude decreases strongly. Diffusion of unentangled sol is sharply bimodal, arising from intermediate fractions and unreactive oligomers. Both diffusion rates decrease slightly with sonication in spite of plasticization by sol, because degradation adds sol primarily of intermediate molecular weights. We compare these results with our earlier work in SBR. Although ultrasound devulcanization does not recover many properties of the virgin melt, recyclability is not compromised.   

Local Conversion model for Phase Diagrams and Calorimetric properties of gel-forming LCST-type polymers.

Assuming a sharp internal conversion of the properties of monomers in a polymer system, we derive the phase diagrams and calorimetric properties of gel-forming LCST-type (lower critical solution temperature) polymers. The transition models the release of water molecules associated with the polymer backbone. After the release, monomers are free to associate and form gel structures. We compare these results with properties of experimentally studied systems.   

Studies on Phase Separation in a-PMMA/PEG Gels

Stereo-irregular atactic poly(methyl methacrylate) ($a$-PMMA) is known incapable of forming gels in common solvents, irrespective of the solvent quality. However, we recently found a rigid opaque thermal-reversible $a$-PMMA gel in the solvent of the polyethyl glycol oligomer (PEG) (the PEG molecule mass differ from 400 to 4000 were used). FT-IR, dynamic mechanical temperature analysis and Solid state NMR measurements were used to study the gel properties and gelation mechanism. The \textit{in situ }IR studies in $a$-PMMA/PEG gel suggested that some $a$-PMMA segments were in the aggregated state in solution, which became a node in the solution. With decreasing temperature, the fraction of aggregated $a$-PMMA in solution increases, resulting in the formation of physical network finally. Spin diffusion was used to determine the size (\textit{$\xi $}) of domains in the gels. We found that, $a$-PMMA/PEG4000 was miscible (\textit{$\xi $ }$\sim $ 9nm), while $a$-PMMA/PEG1000 was micro phase separated (\textit{$\xi $ }$\sim $ 57nm) and $a$-PMMA/PEG400 was macro phase separated (\textit{$\xi $} $>$ 300nm). The $a$-PMMA self-aggregation was attributed to the depletion interaction that becomes important in the case of middle-sized solvents.   

Stretching Networks of Helical Polymers

We present a computational study of the stress-strain behavior of a network of helical polymers. For this study, we employ a combination of Monte Carlo simulations based on the Wang-Landau algorithm and the traditional three-chain model of polymer networks. The helical polymers are described with a recently developed model (V. Varshney \textit{et. al}., \textit{Macromolecules} \textbf{2004}, $37$, 8794) that has proven to capture the configurational, conformational and thermodynamic properties of single helical polymers correctly. In this talk, we will focus on the mechanical and thermodynamic properties of the network as a whole together with the conformational and configurational characteristics of a single chain in the network. We find that the mechanical response of the network is strongly dependent on temperature. For example, for the same amount of strain the network is stiffer at temperatures below the helix-coil transition temperature than at temperatures above it. In addition, we find that the stress-strain curve is \textit{non-monotonic}, indicating that the network \textit{softens} at high enough elongations due to the \textit{force-induced melting} of the helical structures.   

Normal stresses and elastic modulus in sol gels polyester blends

Visco-elasticity and normal stresses in gels can be analyzed using scaling arguments. In our study, polyester gels were made and melt mixed in an extruder at different volume concentrations. Normal stresses and elastic modulus as functions of volume concentration and fractals were analyzed and compared to existing models.   

Session J31: Frank J. Padden Award Symposium

Local Dynamic Mechanical Properties of Model Free-Standing Polymer Thin Films

We present results that strongly support a heterogeneous distribution of dynamic mechanical properties (DMP) in model free-standing polymer thin films. While a number of groups are investigating the mechanical properties of glassy polymer thin films on a substrate, enhanced molecular-level understanding will be required to further rationalize their observations. This study highlights the free-standing geometry; we eliminate any convolution of effects arising from the presence of a free surface and a substrate. The local DMP are calculated from nonequilibrium MD simulations with a coarse-grained polymer model. Our simulation results suggest that the soft surface layers coexist with the bulk-like rigid regions in the thin films below the glass transition temperature (Tg). By increasing the temperature, we show that the thickness of the soft surface layers substantially increases. The overall stiffness of the free-standing films is thereby shown to be much smaller than that of the bulk. The molecular mechanism of surface softening is revealed by a normal mode analysis of the glassy thin films; large, cooperative motions of polymer segments are enhanced near the free surfaces in the vicinity of Tg.   

Self-assembly and cross-linking of nanoparticles at liquid-liquid interfaces

The fabrication of functional nanostructured materials for sensing, encapsulation and delivery requires practical approaches to self-assembly on multiple length scales and the synthesis of tough yet permeable structures. Here, ligand-stabilized nanoparticles assembled into three-dimensional constructs at fluid-fluid interfaces driven by the reduction in interfacial energy were investigated. Studies on the dynamics of the nanoparticles and the self-assembled structures formed at the interface, using fluorescence photobleaching methods and in-situ grazing incidence small angle x-ray scattering, suggest a liquid-like behavior and ordering at the interfaces. Cross-linking of the nanoparticle assembly using functional ligands, affords robust membranes that maintain their integrity even when they are removed from the interface. These composite membranes, nanometers in thickness, are elastic yet permeable. The assembly of virus and other biological complexes at fluid interfaces was also investigated where interfacial assembly rendered an easy route to direct and assemble the bioparticles into 2-D and 3-D constructs with hierarchical ordering. These assemblies enable the potential use of the bioparticles as a natural supramolecular building block to obtain materials with well-defined bio-functionalities.   

Predicting the Viscosity of a Miscible Polymer Blend

The composition-dependent rheological properties of polymer mixtures have been a topic of longstanding interest. We present a roadmap for viscosity predictions for the simplest binary polymer mixture, a miscible polymer blend. Drawing on the Lodge-McLeish model to predict the composition dependences of segmental dynamics, and the double reptation model to account for the chain dynamics, we present new comparisons of predictions with model blend viscosity data. The results suggest that some outstanding issues regarding the prediction of component chain dynamics remain. A modified constraint release model improves the prediction of component longest relaxation times and is able to resolve some of the observed discrepancies between experiment and theory.   

Host polymer influence on dilute polystyrene segmental dynamics

We have utilized deuterium NMR to investigate the segmental dynamics of dilute (2{\%}) d$_{3}$-polystyrene (PS) chains in miscible polymer blends with polybutadiene, poly(vinyl ethylene), polyisoprene, poly(vinyl methylether) and poly(methyl methacrylate). In the dilute limit, we find qualitative differences depending upon whether the host polymer has dynamics that are faster or slower than that of pure PS. In blends where PS is the fast (low T$_{g})$ component, segmental dynamics are slowed upon blending and can be fit by the Lodge-McLeish model. When PS is the slow (high T$_{g})$ component, PS segmental dynamics speed up upon blending, but cannot be fit by the Lodge-McLeish model unless a temperature dependent self-concentration is employed. These results are qualitatively consistent with a recent suggestion by Kant, Kumar and Colby (Macromolecules, 2003, 10087), based upon data at higher concentrations. Furthermore, as the slow component, we find the segmental dynamics of PS has a temperature dependence similar to that of its host. This suggests viewing the high T$_{g}$ component dynamics in a miscible blend as similar to a polymer in a low molecular weight solvent.   

Tailoring Protein and Cell Adsorption Using Surface-grafted Polymer Gradients

We report on tuning the adsorption of proteins and cells on surfaces using grafted polymer gradients. Specifically, we manipulated the amount of adsorbed fibronectin (FN) by adsorbing it onto surface-anchored poly (2-hydroxyethyl methacrylate) (PHEMA) with orthogonal variation of PHEMA molecular weight and grafting density. The amount of FN adsorbed on the surface decreased as molecular weight and/or grafting density of the PHEMA increased. Incubating cells that recognize FN resulted in cell density gradient on PHEMA gradient surface. The number of adsorbed cells decreased as FN concentration decreased along PHEMA gradient.   

Experimental Investigation of Entangled Polymer Flow Behavior

In this presentation, we report additional features of entangled polymer solutions and melts undergoing either continuous shear at high stresses comparable to the dynamic plateau modulus or step shear at large strain amplitudes, following the initial observations of a yield-like bulk flow transition in entangled polybutadiene solutions, made of ultra-high molecular weight PBD and lower molecular weight PBD (as the ``solvent'') [1]. Specifically, the dynamics of the flow transition have been first examined as a function of the chain length of the solvent. In a separate investigation, the stress responses after large step-strain deformation have been studied in detail for several PBD melts and solutions. In both cases, flow birefringence observations using white light were made to delineate the spatial distribution of the optical retardation and to provide further information on the microscopic origins of the observed flow behavior, with the polarized light incident onto either 1,2 plane or 1,3 plane. [1] Tapadia, P.; Wang, S. Q. \textit{Phys Rev. Lett}, \textbf{91}, 198301 (2003); Tapadia, P.; Wang, S. Q. \textit{Macromolecules }\textbf{37}, 9083 (2004).   

The distribution of Tgs in bulk and nanoconfined polymer films measured by a novel fluorescence method

We have recently developed (Nature Mater., 2, p695, 2003) fluorescence approaches that have allowed the direct measurement of Tg in specific layers 10-15 nm in thickness near interfaces and in between. This novel approach has made possible the unique characterization of the distribution of Tgs in bulk and ultrathin polymer films where it has been observed that the distribution in Tgs is highly dependent on the type of interfacial interaction (as at the free surface or substrate) and the degree of nanoconfinement. These measurements have revealed that interfacial effects almost entirely explain Tg deviations observed in ultrathin polymer films and that these interfacial effects have the ability to impact the properties (Tg and associated cooperative segmental dynamics) of material that is even tens of nanometers away from an interface. For example, these measurements have indicated that the 14 nm nearest the free surface of a relatively thick polystyrene (PS) film has a Tg that is reduced by more than 30 K and that this Tg reduction at the free surface influences material even 30 nm away from that interface.   

Growth of the Cooperative Length Scale Below the Caging Temperature of Glass-forming Liquids

A cooperative mechanism is invoked to explain the acute property changes observed in glass-forming liquids near the glass transition temperature $T_g$. This theory implies the existence of cooperatively rearranging regions (CRR) as characterized by a dynamic length scale $\xi$, which is present in both experiments and simulation. Armed with the temperature dependence of this length scale and the fractal dimension of the CRR (from simulations) a simple scaling model for glassy behavior can be constructed. This scaling model has been applied to measurements of viscosity, the $\alpha$-relaxation and probe dynamics to estimate the $\xi(T)$ for numerous glass forming liquids.   

Conjugated Polymer Nanowires: Preparation, Morphology, Optical Properties and Field-Effect Transistors

Nanowires of conjugated polymers are ideal system for studying 1-D confinement effects on optical and electrical properties and hold promise as building blocks for nanoelectronics. We have prepared high quality nanowires of conjugated polymer semiconductors, such as poly(3-hexylthiophene) (PHT), poly(4-hexylquinoline) (PHQ), poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEHPPV), poly(9,9-dioctylfluorene) (PFO) and their blends, using self-assembly and electrospinning techniques and investigated their morphological, optical and electrical properties. Self-assembled crystalline nanowires of binary blends of regioregular PHT and PHQ showed good ambipolar charge transport with hole and electron mobilities of 0.012 and 0.004 cm$^{2}$/Vs, respectively. Electrospun fibers of MEH-PPV and its blends with PHT and PFO had diameters of 30-500 nm and tunable optical and charge transport properties. $p$-Channel field-effect transistors based on the MEH-PPV/PHT blend nanofibers had hole mobility of up to 1$\times $10$^{-4}$ cm$^{2}$/Vs.   

Session J32: Focus Session: Superconductivity: Theory and Computation (Mostly Triplet)

Spin-Orbit Coupling and Symmetry of the Order Parameter in Strontium Ruthenate

Determination of the orbital symmetry of the pairing state in the spin triplet superconductor Sr$_2$RuO$_4$ is a challenge of considerable importance. Most of the experiments show that a chiral state of the $\hat{z} (k_x \pm ik_y)$ type is realized and remains stable on lowering the temperature. Here we have studied the stability of various superconducting states of Sr$_2$RuO$_4$ in the presence of spin-orbit coupling. Numerically we found that the chiral state is never the minimum energy. Alone among the five states studied it has $<{\bf \hat L}.{\bf \hat S} >=0$ and is therefore not affected to linear order in the coupling parameter $\lambda$. We found that stability of the chiral state requires spin dependent pairing interactions. This imposes a strong constraint on the pairing mechanism.   

APRES lineshape analysis of the Fermi liquid Sr$_2$RuO$_4$

ARPES spectra of correlated materials, shown by other measurements to be Fermi liquids, have yet to show a linewidth that is narrower than the binding energy of the quasiparticle -- a key prediction of Fermi liquid theory. We show that although the effects of energy and momentum resolution can be a significant factor in the quantitative description of the scattering rates from APRES, it is possible to effectively account for them. We find that the ARPES linewidth in Sr$_{2}$RuO$_{4}$, a correlated 2-dimensional Fermi liquid, is narrower than its binding energy, and decreases with the expected functional form as the Fermi energy is approached. In combination with the previously determined Fermi surface, these results give the first complete picture of a Fermi liquid via ARPES.   

Effects of disorder in parity violating superconductors

In materials without an inversion center of symmetry the spin degeneracy of the conducting band is lifted by spin orbit coupling (SOC). If the splitting of the non-degenerate quasi-particle energy spectrum is larger than the energy scale of the superconducting gap, i.e., $\alpha > k_{B}T_{c}$, only one spin-triplet pairing channel survives to the lack of the inversion symmetry. In the limit of larger $\alpha$, i.e, $\alpha >> k_{B}T_{c}$, the spin susceptibility in case of spin-singlet and protected spin-triplet paring are similar, making it difficult to determine the pairing type through Knight shift measurements. However, Anderson's theorem concerning non-magnetic impurities still holds. This should make possible to discriminate experimentally at least between conventional and unconventional pairing. If the density of states at the Fermi level is different for the two non-degenerate bands, the spin-triplet and the spin-singlet pairing channels mix, modifying further the response of the system on disorder. The impurity dependence of the critical temperature, the spin susceptibility, and the paramagnetic limiting field have been studied using the Born approximation for various combinations of the pairing channels, and different distributions of the conduction electrons on the two non-degenerate bands.   

Topological Terms in the effective action of the two dimensional $p_x + i p_y $ superfluid/superconductor

The effective action of a two dimensional $p_x + i p_y $ superfluid/superconductor, such as the layered Sr$_2$RuO$_4$ superconductor or a thin film of $^3$He-A is known to contain a Hopf term, which is responsible for non abelian statistics. We study the applicability of the Eilenberger formalism which is used to reduce higher dimensional problems to effective one dimensional ones, in computing the effective action. We also investigate the Wess Zumino Witten model in this connection.   

Role of Magnetic Fields on Superconducting Sr$_2$RuO$_4$

Nearly all theoretical descriptions of the spin-triplet pairing symmetry in Sr$_2$RuO$_4$ are characterized by a ${\bf d}({\bf k})$ vector aligned along the four-fold symmetry axis. Two well known consequences of all such theories are that the in-plane anisotropy of the upper critical field should exist up to $T_c$ and that there must be multiple vortex phases corresponding to changes in the order parameter structure. Neither of these two predictions appear to have strong experimental support. We examine these predictions within an approximate analytic solution of the quasiclassical equations for the vortex phase to provide tenative explanations for this inconsistency.   

Triplet-Singlet Mixed Phases in High-Tc Superconductors

We reveal experimental conditions, where a triplet component of a superconducting order parameter appears in layered singlet d-wave superconductors. We describe exotic properties of such singlet-triplet mixed phases and discuss experimental methods to detect the triplet component in d-wave high-Tc compounds.   

Symmetry of the order parameter in non-centrosymmetric superconductors: Implementation for CePt$_3$Si and Cd$_2$Re$_2$O$_7$

In noncentrosymmetric metals, the spin degeneracy of the electronic bands is lifted by spin-orbit coupling. We consider general symmetry properties of the pairing function $\Delta(k)$ in noncentrosymmetric superconductors with strong spin-orbit coupling (NSC). We find that $\Delta(k) = \chi(k) t(k)$, where $\chi(k)$ is an even function which transforms according to the irreducible representations of the crystallographic point group and t(k) is a model dependent phase factor. We consider tunnelling between a NSC and a conventional superconductor. It is found that, in terms of thermodynamical properties as well as the Josephson effect, the state of NSC resembles a singlet superconducting state with gap function $\chi(k)$. We propose the gap functions which may account for the experimental properties of the heavy fermion compound CePt$_3$Si and the distorted pyrochlore Cd$_2$Re$_2$O$_7$.   

The FFLO state in two-dimensional D-wave superconductors

We studied the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state for two-dimensional D-wave superconductors with magnetic field parallel to the superconducting planes. This state occurs at high fields near the Pauli-Clogston limit and is a consequence of the competition between the pair condensation and Zeeman energy. We used the quasiclassical theory to self-consistently compute the inhomogeneous order parameter and the free energy. We mapped out the phase diagram and found that the transitions from the FFLO phase to either the normal state or uniform superconducting state are of second order, analogous to the S-wave case. Contrary to the S-wave case, we find that the FFLO state of a D-wave order parameter breaks translational symmetry along prefered directions. The orientation of the nodes in real space is pinned by the nodes of the basis function in momentum space. We will discuss the experimental consequences of this effect.   

Assessment of Possible FFLO States in Real Materials.

Very recently a few mateirals have been suggested to show realizations of the Fulde-Ferrell-Larkin-Ovchinnikov superconducting phase with nonzero Q pairs: $\kappa$-(BEDT-TTF)$_{2}$Cu(NCS)$_{2}$, CeCoIn$_5$, UBe$_{13}$, and conceivably ZrZn$_2$. Occurrence of this phase is highly dependent on the degree to which finite pair momentum can nest spin split Fermi surfaces. Histograms of the "projected Fermi velocity density of states" provides a useful method of measuring the degree of nesting possible for a given material, and hence its suitability for supporting an FFLO phase. We present and discuss the nesting properties for a number of materials using projected velocity methods, and critically evaluate the suggestions for FFLO phases.   

First-principles study of the pressure effects on (BEDT-TTF)2XCl2 (X= I or Au)

We have calculated the lattice parameters, internal coordinates, and electronic structures of (BEDT-TTF)$_{2}X$Cl$_{2}$ ($X$ = I or Au) using the density functional theory within the generalized gradient approximation (GGA). Although these two salts both show non-metallic behaviors at ambient pressure, their transport properties at high pressures are completely different. ICl$_{2}$ salt shows nonmetal-metal and superconducting transitions under the pressure around 8 GPa, while the AuCl$_{2}$ salt keeps its non-metallic behavior even when the pressure of 10 GPa is applied. We first show that the calculated structures at ambient pressure agree well with the experimental ones for both materials, implying that GGA is reliable for the theoretical determinations of the crystal structure. Then we report that the inter-molecule interactions exhibit various and complex pressure dependence, contrary to the simple suggestions made so far. Although we cannot find sensible differences in the pressure effects on the crystal structures, we have found that the pressure dependence of their band structures is qualitatively different between these two salts. We will show the dimensional change of Fermi surfaces, increase of the band width, and the pressure dependence of TB parameters which reproduces our present DFT results.   

Phase diagram of $\beta'$-BEDT-TTF salts

We present theoretical studies on the phase diagram of layered organic charge transfer salts, $\beta'$-(BEDT-TTF)$_2$ICl$_2$ and $\beta'$-(BEDT-TTF)$_2$AuCl$_2$. The former shows the highest superconducting transition temperature ($T_{\rm c} $14.2~K under a high hydrostatic pressure) among the organic superconductors. We study an effective model using the fluctuation-exchange (FLEX) approximation based on the results of the first-principles calculations in DFT/GGA under applied pressures. In the obtained phase diagram of $\beta'$-(BEDT-TTF) $_2$ICl$_2$, the superconductivity with $d_{xy}$-like symmetry is realized next to the antiferromagnetic phase. The calculated $T_{\rm c}$ quantitatively coincides well with the experimental one.   

Pairing and superconductivity driven by strong quasiparticle renormalization in two-dimensional organic charge transfer salts

We introduce and analyze a variational wave function for quasi two-dimensional kappa-(ET)2 organic salts containing strong local and nonlocal correlation effects. We find an unconventional superconducting ground state for intermediate charge carrier interaction, sandwiched between a conventional metal at weak coupling and a spin liquid at larger coupling. Most remarkably, the excitation spectrum is dramatically renormalized and is found to be the driving force for the formation of the unusual superconducting state.   

The Gap Function for a 2-leg t-J Ladder

The wave vector and frequency dependence of the gap function $\phi(k,\omega)$ for a doped 2-leg t-J ladder is obtained from a Lanczos calculation. Based upon this information, we argue that the pairing interaction is (1) short range in space, (2) retarded, and (3) dominated by the S=1 channel near its onset.   

Weak-Coupling BCS Theory of Skutterudite PrOs$_4$Sb$_{12}$

A weak-coupling theory of the A- and B-phases of the recently discovered heavy-fermion superconductor PrOs$_4$Sb$_{12}$ is developed. The BCS gap equation is solved assuming a p+h-wave pairing symmetry with an A-phase nodal gap function $\Delta ({\bf k}) \sim e^{\pm i\phi_{i}} (1-\hat{k}_{x}^4 - \hat{k}_{y}^4 - \hat{k}_{z}^4)$ and with a B-phase nodal gap function $\Delta({\bf k})\sim e^{\pm i\phi}(1-\hat{k}_{y}^4)$, where $e^{\pm i\phi_{i}} \sim \hat{k}_ {z} \pm i\hat{k}_{x},\,\, \hat{k}_{x} \pm i\hat{k}_{y}$ or $\hat {k}_{y} \pm i\hat{k}_{z}$ and $e^{\pm i\phi} \sim \hat{k}_{z} \pm i\hat{k}_{x}$. The B-phase order parameter has similar thermodynamic properties as the recently proposed spin-singlet s+g-wave superconductor [Maki et al, Europhys. Lett. {\bf 64} 496 (2003)]. However, there is accumulating evidence for spin- triplet pairing for superconductivity in PrOs$_4$Sb$_{12}$. Analytic limiting case results as well as numerical solutions for the whole temperature range $T \leq T_{c}$ are presented for the order parameter, the specific heat, the thermodynamic critical field, and the superfluid density. We compare these results with data from a few recent experiments.   

Weak coupling theory of multicomponent superconductors.

We consider the pairing state due to the BCS mechanism in substances of cubic, hexagonal, or tetragonal symmetry, where the superconducting parameter is multicomponent, i.e., transforms according to either a 2-dimensional or a 3-dimensional representation of the crystal point group. We show that the superconducting phase always breaks the time-reversal symmetry for singlet multicomponent superconductors. For triplet superconductors, the superconducting order parameter turns out to be always non-magnetic in the cubic group, ferromagnetic in tetragonal, and either ferromagnetic ($E_{1u}$ representation) or non-magnetic ($E_{2u}$ representation) in hexagonal group. According to these results, for example, the superconducting phases in both Sr$_2$RuO$_4$ and Na$_x$CoO$_2$-yH$_2$O should be ferromagnetic.   

Spin-Wave Excitations and Superconductivity

We study in terms of the effective two-particles $T$-matrix pairing by exchanging spin-wave excitations near the ferromagnetic or antiferromagnetic instability. We show that near the ferromagnetic instability the paring due to the exchange the paramagnetic excitations can occur in the $p$-wave state (\textit{$\ell $} = 1) and $m=\pm $1. We show that near the antiferromagnetic instability the pairing due the exchange the paramagnetic excitations occurs also in the $p$-wave state (\textit{$\ell $} = 1) but with $m$ = 0 and that there is no pairing in $d$-wave state.   

Session J34: Environmental Interfaces V

Chemical Reactivity at Metal Oxide-Aqueous Solution Interfaces

The chemical reactivity of metal oxide surfaces in contact with aqueous solutions, with respect to cations and anions, is controlled by the composition, structure, and charging properties of the surface, the dielectric properties of the bulk oxide, and the stability of the aqueous cation or anion complex versus its sorption complex. These points will be illustrated for selected cations, anions, and metal oxides using macroscopic uptake and EXAFS spectroscopy results, x-ray standing wave data, and crystal truncation rod diffraction data. The reactivity of metal oxide surfaces with respect to low molecular weight (LMW) carboxylic acids is also dependent on the types of ring structures formed between surface functional groups and the LMW organic molecules. These types of interactions will be illustrated using ATR-FTIR data and dissolution measurements as a function of pH for oxalate, maleate, phthalate, and pyromellitate interacting with boehmite (AlOOH). \newline \newline Co-Authors are Tae Hyun Yoon, Stephen B. Johnson, Dept. of Geological \& Environmental Sciences, Stanford University, Stanford CA 94305-2115; Thomas P. Trainor, Dept. of Chemistry and Biochemistry, University of Alaska Fairbanks, Fairbanks, AK 99775; Anne M. Chaka, National Institute of Standards and Technology, Gaithersburg, MD 20899   

Water dissociation on TiO$_{2}$(110): application of the reactivity mapping method

We have recently developed a method to calculate the chemical reactivity of a surface with respect to water dissociation. The method is based on one calculation of the electronic states and provides a map of the activity of different sites at the surface. We present an application to the water physisorption and chemisorption process on the (110) surface of TiO$_{2}$ in the rutile structure. The predictions of this chemical reactivity mapping method are in excellent agreement with more conventional method of calculation chemical reactivity, including ab-initio molecular dynamics. As a result of its simplicity it is possible to study larger surface areas including the effect of correlations and catalytic effects.   

Two-dimensional hydration shells of alkali metal ions at a hydrophobic surface

We study the hydration shell formation of alkali metal ions at a graphite surface. Two-dimensional (2D) shell structures are found in the initial stage of hydration, in contrast to the 3D structures in bulk water and clusters. Comparison of vibrational spectra with experiments identifies the shell structures and the thermally induced transition from the first to the second shell. We also found intriguing competition between hydration and ion-surface interaction, leading to different solvation dynamics between K and Na. Implications of these results in ionic processes at interfaces are elaborated.   

Surface Chemistry of Environmentally Important Minerals

Motion of surface ions is integral in the dissolution and growth dynamics of carbonate minerals. The present study investigates the density and the mobility of surface ions and the structure of the adsorbed water layer with changes in relative humidity (RH). The time evolution of the polarization force, which is induced by an electrically biased tip of an atomic force microscope, shows that the density and the mobility of surface ions increase with rising humidity, a finding which is consistent with increasing surface hydration. A marked change in the observations above 55{\%} RH indicates a transition from a water layer formed by heteroepitaxial two- dimensional growth at low RH to one formed by multilayer three-dimensional growth at high RH. A comparison of the results of several rhombohedral carbonates (viz. CaCO$_{3}$, FeCO$_{3}$, ZnCO$_{3}$, MgCO$_{3}$, and MnCO$_ {3})$ shows that a long relaxation time of the polarization force at high RH is predictive of a rapid dissolution rate. This finding is rationalized by long lifetimes in terrace positions and hence greater opportunities for detachment of the ion to aqueous solution (i.e., dissolution). Our findings on the density and the mobility of surface ions help to better constrain mechanistic models of hydration, ion exchange, and~dissolution/growth. \\ \\Co-authors are Young-Shin Jun, Treavor Kendall, and Owen Duckworth.   

A Theoretical Study of Copper Adsorption on the (110) Surface of TiO2

Strong interactions between metal catalysts and their supporting oxide substrates give rise to enhanced catalytic properties. Using Density Functional Theory, we are exploring this phenomenon for the prototypical system of copper on the (110) surface of rutile TiO2. In this talk, we will present our results for the first stages of this investigation, including predictions of the optimal binding site and binding energy for Cu at various coverages and on both stoichiometric and reduced TiO2 (110) surfaces. Connections will be drawn to recent experiments, and an overview of our ongoing investigations will be presented.   

A theoretical study of adsorption of NO on RuO$_2$(110)

Experiments suggest that while RuO$_{2}$(110) is a highly reactive surface for CO oxidation, the same is not the case for NO adsorbed on this surface [1]. In order to understand the rationale for this difference, we have carried out \textit{ab initio} electronic structure calculations to examine the idiosyncrasies in the adsorption, dissociation, and oxidation processes for CO and NO on RuO$_{2}$(110). Our calculated geometry and adsorption energies for NO adsorption are in good agreement with experiments available. Our results confirm that while CO can easily oxidize on this surface [2], NO does not bind easily to the bridging O atoms to produce NO$_{2}$. Detailed analysis of the surface electronic structure, including the charge transfer, charge density distribution and local electronic density of states together with its hybridization is pesented to compare and contrast the energetics and dynamics of CO and NO on RuO$_{2}$(110). [1] Y. Wang, K. Jacobi, and G. Ertl, J. Phys. Chem. B \textbf{107}, 13918 (2003). [2] Z. Liu, P. Hu, A. Alavi, J. Chem. Phys. \textbf{114}, 5956 (2001).   

The Unusual Adsorption and Reaction Chemistry of NOx on Oxide Surfaces

First-principles atomistic simulations based on density functional theory have reached a state of development that they now provide a powerful complement to experiment in the effort to understand, control, and optimize heterogeneous catalytic processes. While these methods have been extensively applied to metal surface reactions, metal oxides have received less attention. In this work two examples of current research in oxide surface chemistry relevant to the catalytic reduction of NO$_{x}$ ($x $= 1, 2) to N$_{2}$ in the presence of a large excess of interfering O$_{2}$ will be discussed. We first consider the nature and origins of NO$_{x}$ ``cooperative'' adsorption on basic metal oxides like the alkaline earths, the surprisingly strong adsorption enhancement arising from electron transfer between neighboring surface adsorbates. We then discuss the catalytic oxidation of NO to NO$_{2}$ on transition metal oxides, in particular contrasting with the superficially similar oxidation of CO to CO$_{2}$. In both cases, the combination of NO$_{x}$ with metal oxide is found to lead to novel and unanticipated behavior---behavior that could be exploited for improved catalytic function.   

Thermodynamics of toxic gas molecules on metal oxide nano powders

Studies on physical/chemical reactions of gas molecules on nano sized metal oxide powder surface is interesting because the results from the research can directly be applied to the investigation of the surface mediated catalytic reactions aimed at reducing atmospheric pollutants. Thermodynamic properties of nitrous oxides (NO$_{x})$ on varies nano sized metal oxides such as MgO and ZnO powders were studied using a computer controlled gas adsorption isotherm apparatus, which has good temperature stability within 0.01K in a wide rage and good pressure accuracy with a resolution better than 10$^{-5}$ torr in 100 torr. A set of isotherms below the triple points of the nitrous gas was measured. A specific surface area of particles was also obtained from calculations of a molecular area of adsorbate gas on the surface of the adsorbents. The thermodynamic results including compressibility and isosteric heat of adsorption of nitrous gases on various metal oxide surfaces will be presented.   

Density-functional study of chemical reactions at a solid-fluid interface: passivation of the (0001) surface of Cr$_2$O$_3$

Using a new form of density functional theory for the {\em ab initio} description of electronic systems in contact with a dielectric environment, we present the first detailed study of the impact of a solvent on the surface chemistry of Cr$_2$O$_3$, the passivating layer of stainless steel alloys. Compared to vacuum, we predict that the presence of water has little impact on the adsorption of chloride ions to the oxygen-terminated surface but a dramatic effect on the binding of hydrogen to that surface. These results indicate that the dielectric screening properties of water are important to the passivating effects of the oxygen-terminated surface.   

Session J35: Energy Landscapes in Clusters, Materials, and Biology IV

What Must We Know to Gain Useful Knowledge from a Complex Surface?

Complex potential surfaces can be analyzed, in principle, to yield virtually any desired information about their topographies. However for a system of more that about 20 particles, the surfaces are so complex that the only feasible ways to work with them involve the extraction of some minimal set of data that can provide the information we most want. Molecular dynamics and Monte Carlo searches, especially with long-range exploration algorithms or coarse-graining, are two well-studied approaches. Kinetic methods based on master equations provide another with the power to reach long time scales. Their largest difficulty is finding a way to construct a master equation based on a suitably small subset of the information that will yield, with reliability, the important rates we need, specifically the rates of the slow but attainable processes. We review several approaches that we have been pursuing: reduction of the kinetics to interbasin passage, reduction of the number of coordinates to just the most important, by principal component analysis, and sampling of pathways on the surface.   

The potential energy landscape of glass-forming materials: a computer study

Glass-forming systems show remarkable properties in the supercooled liquid phase (e.g. non-Arrhenius temperature dependence or strong connections between thermodynamics and dynamics) as well as in the glassy phase (e.g. the emergence of tunneling centres at very low temperatures). Another aspect deals with the surprisingly fast dynamics of ions in some glassy materials. In this talk a concept is presented which allows one to describe all these features in a coherent way. It is based on the study of the potential energy landscape. Using appropriate numerical tools as well as physical insight it is possible to elucidate the underlying nature of the many relevant features in the supercooled as well as in the glassy regime. Among other things the difference between strong glasses and fragile glasses (Arrhenius vs. Non-Arrhenius temperature dependence) can be expressed in terms of specific properties of the potential energy landscape.   

The role of electrosn in the structure of liquid water

The structure of liquid water is analyzed by using DFT-based Ab initio molecular dynamic simulations. The essential role that the electronic degrees of freedom play in the the hydrogen bond (HB) interaction is described and a new HB definition based in electrons and not in geometric parameters is proposed. Using this HB probe the structure and dynamics of the HB network in liquid water will be presented, providing new insights interestingly different from the picture that emerges from simulations based on geometrical criteria.   

Air-liquid interface of ionic liquid-water binary system studied by surface tension measurement and sum-frequency generation spectroscopy

Surface of room-temperature ionic liquid (RIL)+water mixture is investigated using surface tension measurement and surface sum- frequency generation (SFG) vibrational spectroscopy. Results indicate the liquid surface is mostly covered by the cations at a very low bulk concentration (less than 0.02 bulk mole fraction). Increase of surface tension from 0.016 up to $\sim$0.05 mole fraction suggested that the anions start to appear at the surface from $\sim$0.016 mole fraction until the anions and cations are equally populated at c$\sim$0.05 or higher. From the analysis of the SFG spectra, the terminal CH$_3$ group of the butyl chain is polar-oriented with its symmetry axis aligning rather vertical to the surface for the whole range of concentration.   

Water condensation in proximity of a nanoscale asperity: A density functional description for isotropic fluid

A quantitative understanding of water condensation phenomena in proximity of nano- asperities under ambient humidity is important for different applications from scanning probe microscopy to macroscopic adhesion and friction. A non-local density functional formalism is used to describe an equilibrium distribution of the water-like fluid in the asymmetric nano-scale junction. The model system contains spherically curved and planar surfaces presenting an atomic force microscope (AFM) tip dwelling above the surface. The hydrogen bonding dominated in intermolecular attraction of the fluid is modeled as a square well potential with two adjustable energy and length parameters characterizing well's depth and width. The size and shape of the liquid meniscus formed between the surfaces with a given affinity to the fluid are determined for the different values of the ambient humidity. This model can be easily generalized for more complex geometries and effective intermolecular potentials. The results of our study establish a basic framework for the density functional description of the system with orientational anisotropy in the fluid induced by the non-uniform external electric field.   

Dynamics of Water in AOT Reverse Micelles Probed Using Ultrafast IR Vibrational Echo and Pump-Probe Spectroscopies

Water is used extensively as a solvent in chemistry and is ubiquitous in biological systems. Water's unique properties are intimately related to its dynamic hydrogen bond network. In addition to the bulk, water is often found in nanoscopic environments. Therefore, it is important to understand the dynamics of water that is nanoscopically confined and to compare it with bulk water dynamics. Nanoscopic pools of water (1.7 -- 4.0 nm diameter) in Aerosol-OT reverse micelles were directly probed using ultrafast vibrational echo and pump-probe spectroscopies on the OD hydroxyl stretch mode of water (5{\%} HOD in H$_{2}$O). The data are compared with experiments conducted on bulk water as well as 6M NaCl solution. Fits of the vibrational echo data demonstrate that the dynamics of water slow down substantially with decreasing reverse micelle size. The fastest dynamics ($\sim $50 fs) which reflect local hydrogen bond fluctuations, are similar to bulk water. The longer time scale dynamics are attributed to the dissociation and reformation of hydrogen bonds, and slow significantly ($\sim $10 times) as the nanopool size is reduced. Lifetime and anisotropy measurements using pump-probe spectroscopy also display similar size dependences. The vibrational echo and pump-probe data clearly distinguish the dynamics of bulk water and concentrated NaCl solutions from the dynamics of nanoscopic water confined in reverse micelles.   

Nuclear Quantum Effects on Molecular Packing in Light and Heavy Water

Light water shows a temperature of maximum density (TMD) at 4$^{\circ}$C, below which the liquid begins to expand upon cooling. Hydrogen bonding is thought to be responsible for the anomalous density behaviour below the TMD: molecules in the liquid must move apart in order to form hydrogen bonds which acts to drive down the density. The density maximum is thought to mark the point during cooling at which formation of an open hydrogen-bonded tetrahedral network becomes energetically preferred over contraction familiar in simple liquids. In heavy water, the TMD leaps upwards to 11$^{\circ}$C. The lower TMD in light water might be supposed to be a result of breakdown in the open hydrogen-bonded network due to NQD: orientational delocalisation of molecules weakens hydrogen bonding and allows a higher density at a lower temperature. We have analysed first- and second-nearest-neighbour distances in relation to molecular orientation using results from path integral molecular dynamics simulation. A simple breakdown in tetrahedral order upon density increase due to orientational delocalisation is not found in light water. Rather, NQD in light water permits hydrogen bonds to become \emph{straighter} at higher density. The result is to increase the size of cavities in the tetrahedral network and thus provide accommodation for a greater number of interstitials which find homes in those cavities. A qualitative explanation for the phenomena is given in terms of the energetics of the water dimer.   

Molecular dynamics behind the principle relaxation of water: Observation of inertia and memory effects

We report an analysis of the single dipole to collective dipole moment time correlation function (TCF) calculated from the molecular dynamics simulations of liquid water at room temperature with a rigid and nonpolarizable TIP5P water model [1]. The single to collective dipole moment TCF is especially important because it allows us to investigate the local relaxation processes in a cluster of molecules and it can be directly related to the dielectric constant via the Fatuzzo-Mason equation. We have calculated the collective dipole moment associated with the first through fourth solvation shells and observed retarded relaxation processes for outer shells. We have also discovered a long-term and long-range rephasing process of about 40 ps involving more than 500 molecules. [1] M. W. Mahoney and W. L. Jorgensen, J. Chem. Phys. 112, 8910 (2000).   

Direct Observation of Heterogeneous Translational Motion at Tg

Recent experiments have provided direct evidence of heterogeneous translational motion in a supercooled small molecule organic liquid, tris-naphthylbenzene. The early stages of diffusion were measured using neutron reflectivity, and indicate that translational motion near the glass transition temperature T$_{g}$ is qualitatively different than diffusion in ``normal'' liquids. The diffusion coefficient was found to be wave-vector dependent, D(q) $\propto $ q$^{-2}$, with a crossover to a q-independent value, D(q $\to $ 0), at a length scale of $\sim $22 nm at T$_{g}$. These results demonstrate that translational motion on the nanometer length scale can be extremely heterogeneous in a single component system near T$_{g}$, giving rise to large jumps of roughly 20 molecular diameters. This observation explains the unusually fast diffusion coefficients found in many materials near T$_{g}$, and also the unusually rapid crystallization of supercooled liquids.   

Study of Short-Range Order in Supercooled Liquid Silicon by Beam-Line Electrostatic Levitation (BESL)

Previous studies of the liquid structure of supercooled Si by x- ray diffraction using electromagnetic and aerodynamic levitation have produced conflicting results. We describe a BESL technique that obtains complete diffraction patterns in 0.1 s using high- energy synchrotron x-rays, allowing the evolving structures of supercooled liquids to be measured continuously. Contrary to some molecular dynamic simulation studies, no first order liquid- liquid phase transition was observed in supercooled liquid Si over the measured temperature range (1100 $^{\circ}$C to 1600 $^{\circ}$C). The coordination number remained constant, in conflict with earlier measurements. Modeling suggests that the A5 structure of liquid Si distorts continuously toward cubic diamond structure with decreasing temperature.   

Computer Simulations, Nucleation Rate Predictions and Scaling

Computer simulations which generate small molecular cluster size distributions for use in steady state nucleation rate predictions and direct molecular dynamics simulations of the nucleation process depend crucially on the properties of the effective pair potentials and on the modeled system's metastable conditions. The latter conditions often differ significantly from those under which the experimental data are taken. For argon, in particular, simulations using both truncated and full Lennard-Jones potentials have generated a range of results for the nucleation rate, J, which, in magnitude, appear to be inconsistent with the limited experimental rate data. We propose that log J should be plotted \textit{versus} a ``universal'' function of the scaled supersaturation, lnS/[T$_{c }$/T -- 1]$^{3/2}$, which incorporates scaling of metastable system conditions with potential model (or experimental) properties, as appropriate. This plot, which provides a more realistic comparison of predicted and experimental rates, is applied to water and to argon data. For argon, this plot emphasizes the limited range of simulation results at large scaled supersaturations and suggests that none of these predicted rates are inconsistent with the much smaller experimental nucleation rates taken at reduced scaled supersaturations. For water, the plot indicates that experimental nucleation rates are likewise well characterized by the scaled supersaturation function.   

Heat Capacities and Magic Numbers in Metal Clusters

Using our own {\it Aufbau/Abbau} method in performing umbiased structure optimization of isolated M$_N$ clusters (with M being a metal) together with the {\it Embedded-Atom} method for the calculation of the total energy of a given structure, we have optimized the structure of Ni$_N$, Cu$_N$, and Au$_N$ clusters with $N$ up to 150. By analysing the total energy as a function of $N$ particularly stable clusters, corresponding to the so-called magic numbers, are identified. Subsequently, we use the harmonic approximation in calculating the vibrational frequencies for each value of $N$. These are finally using in calculating the molar vibrational heat capacity $\bar C_V$ as a function of temperature $T$. It is been discussed whether, for a given $T$, $\bar C_V$ shows a dependence on $N$ that correlates with the occurrence of magic numbers. An explanation for the observations is being offered.   

Session J36: Trapped Fermi Gases and Feshbach Resonances

Superfluid Density of an inhomogeneous Fermi Gas

We present a microscopic calculation of the superfluid density for a Fermi gas with a spatially varying density profile. By imposing an infinitesimal twist on the phase of the order parameter and calculating the resulting strain energy perturbatively, we find the helicity modulus, which is directly tied to the density of the superfluid component. By self consistently solving the Bogoliubov-de Gennes equations for a harmonically trapped gas we compare the exact expression to that obtained by using the homogeneous gas result in a local density approximation. In addition, it is shown that as the critical temperature is approached the superfluid density acquires the same spatial profile as the squared norm of the order parameter.   

Quench Dynamics of a Superfluid Fermi Gas

With an eye toward the interpretation of so-called `cosmological' experiments performed on the low temperature phases of $^{3}$He, in which regions of the superfluid are destroyed by local heating with neutron radiation, we have studied the behavior of a Fermi gas subjected to uniform variations of an attractive BCS interaction parameter $\lambda$. In $^{3}$He the quenches induced by the rapid cooling of the hot spots back through the transition may lead to the formation of vortex loops via the Kibble-Zurek mechanism. A consideration of the free energy available in the quenched region for the production of such vortices reveals that the Kibble-Zurek scaling law gives at best a lower bound on the defect spacing. Further, for quenches which fall far outside the Ginzburg-Landau regime, the dynamics on the pair subspace, as initiated by quantum fluctuations, tends irreversibly to a self-driven steady-state with a gap $\Delta_{\infty} = \epsilon_{C}(e^{2/N(0)\lambda} - 1)^{- 1/2}$. In weak coupling this is only half the BCS gap, the extra energy being taken up by residual collective motion of the pairs.   

Density and spin response functions in ultracold fermionic gases

We study the two-body correlation functions in a two-component ultracold fermionic gas governed by an attractive short-range interaction. Based on a zero-temperature mean-field analysis we suggest that considerable insight in the properties of the ground-state can be gained by measuring the density and spin response functions, and predict differences between the properties of a $^{40}$K ultracold fermionic gas and the properties of a $^6$Li ultracold fermionic gas.   

Ferromagnetism in Fermi Gases

We investigate the possibility of itinerant ferromagnetism in a two-component Fermi gas with strongly repulsive short-range inter-particle interactions. Interestingly, unlike the case of an electron gas with long-range Coulomb interactions, we find that the Hartree-Fock theory underestimates the tendency towards ferromagnetism. A Fermi gas with strongly respulsive interactions may be realized in experiments with ultracold fermionic alkali atoms near a Feshbach resonance. We investigate the prospects for observing the ferromagnetic state in these cold atoms systems. One possible detection scheme is based on a spectroscopic method for measuring the pseudospin susceptibility which we expect to be strongly enhanced when repulsive interactions are strong. The same method can be used to look for supressed pseudospin susceptibility when the singlet superconductor state is approached on the attractive interaction side of the Feshbach resonance.   

Fast rotating clusters of fermions with general half integer spin

We have studied the ground states of fast rotating Fermi gases with half integer spins. Remarkable correlations between spin and orbital angular momentum are found. Because of the similarity of scattering lengths in different angular momentum channels, density-density interaction dominates and the system has close to SU$(2f+1)$ symmetry. We have found general solution to the SU$(2f+1)$ symmetry case for both repulsive and attractive density interaction, and we have constructed simple ``Hund's Rules" to describe the correlation between spin and orbital angular momentum in the ground state. Residual interactions split the degeneracy of the SU$(2f+1)$ symmetry ground state which leads to fine structures with regular patterns.   

Acoustic attenuation in atom traps

We study the damping rate of a BEC phonon excitation in a Bose-Fermi mixute gas. We show how the damping varies in the presence of the BCS superfluid phase in the Fermi gas. We, furthmore, point out some substantial differences with the condensed matter analogue in normal superconductors.   

Ultracold fermion cooling cycle using heteronuclear Feshbach resonances

Ideal gas models have given much insight into the physics of dilute fermion gases that can form into bosonic molecules via a Feshbach resonance, and have even given good quantitative agreement with the molecular and Bose-Einstein condensate fractions observed in recent experiments. [1] We develop such a model for a harmonically -trapped ideal gas with three components: bosonic atoms, fermionic atoms, and a fermionic diatomic molecule produced by a Feshbach resonance involving the two atomic species. Such systems have been produced in recent experiments. [2,3] We map out the phase diagram for this three-component mixture in chemical and thermal equilibrium. Considering adiabatic association and dissociation of the molecules, we identify a possible cooling cycle, which in ideal circumstances can yield an exponential increase of the phase-space density. \newline \newline [1] J. E. Williams, N. Nygaard and C. W. Clark, {\it New J. Phys.} {\bf 6}, 123 (2004) \newline [2] C. A. Stan {\it et al., Phys. Rev. Lett.} {\bf 93}, 143001 (2004) \newline [3] S. Inouye {\it et al., Phys. Rev. Lett.} {\bf 93}, 183201 (2004)   

Nonequilibrium response of a molecular Bose-Einstein condensate near a Feshbach resonance.

We investigate the response of a molecular Bose-Einstein condensate near a Feshbach resonance to magnetic-field sweeps and resonant laser probes. For magnetic-field sweeps across the resonance, we study the process of dissociation and determine the energy distribution of the atoms produced after the sweep. We present both exact and numerical results. For laser couplings to excited molecular states, we explore the possibility of probing features of the molecular density of states that would not be accessible in equilibrium or in linear-response situations.   

Decoupling transition in a one-dimensional fermionic gas interacting via a Feshbach resonance

We study a fermionic atomic gas confined to move in one dimension and interacting via an s-wave Feshbach resonance. At low energies the system is characterized by a model of two Josephson-coupled Luttinger liquids, corresponding to fermionic atoms and their diatomic molecules. In contrast to higher dimensions, we find that this system exhibits a quantum phase transition from a phase in which the two superfluids are strongly coupled to one where the Feshbach resonance coupling becomes irrelevant and the two types of superfluid decouple.   

Damping of amplitude modes in a neutral BCS superfluid

In contrast to Bose-Einstein condensates, fermionic superfluids exhibit an amplitude collective mode related to the internal dynamics of fermion pairs, as well as the phase -- or Bogoliubov -- mode. Recent work has proposed the existence of a collisionless regime in which highly nonlinear dynamics of the amplitude mode may be observed in systems of fermionic atoms pairing via a Feshbach resonance. Here we examine the damping of this dynamics by the phase mode and the general issue of collisionless dynamics versus the time-dependent Ginzburg-Landau description.   

Phase slip in a superfluid Fermi gas near a Feshbach resonance

The properties of a phase slip in a superfluid Fermi gas is studied near a Feshbach resonance. The phase slip can be generated by the phase imprinting method. Below the superfluid transition temperature, it appears as a dip in the density profile, and becomes more pronounced when the temperature is lowered. Therefore the phase slip can provide a direct evidence of the superfluid state. The condensation energy of the superfluid state can be extracted from the density profile of the phase slip, due to the unitary properties of the Fermi gas near the resonance. The width of the phase slip is proportional to the inverse of the square root of the difference between the transition temperature and the temperature. The signature of the phase slip in the density profile becomes more robust across the BCS-BEC crossover.   

Bosonic and Fermionic Cold atoms in optical lattices.

We examine the properties of coupled one dimensional tubes of Bosons or Fermions, in an optical lattice. For bosons, we find that the intertube coupling induces a deconfinement transition where the system goes from a one dimensional Mott insulator to a three dimensional superfluid. We compute the phase diagram and physical properties and discuss the results in connection with experiments on cold atoms. For fermions, the presence of the optical lattice allows for a richer phase diagram than for standard interacting fermions in one dimension. We analyse the resulting phases and discuss how to observe them for cold atoms   

Engineering Superfluidity in Bose-Fermi Mixtures of Ultracold Atoms

We investigate many-body phase diagrams of atomic boson-fermion mixtures loaded in the two-dimensional optical lattice. Bosons mediate an attractive, finite-range interaction between fermions, leading to fermion pairing phases of different orbital symmetries. Specifically, we show that by properly tuning atomic and lattice parameters it is possible to create superfluids with $s$-, $p$-, and d-wave pairing symmetry as well as spin and charge density wave phases. These phases and their stability are analyzed within the mean-field approximation for systems of $^{40}$K-$^{87}$Rb and $^{40}$K-$^{23}$Na mixtures. For the experimentally accessible regime of parameters, superfluids with unconventional fermion pairing have transition temperature around a percent of the Fermi energy.   

Dirac Fermions in Optical Lattices

Two dimensional interacting Dirac fermions arise in many different contexts in condensed matter physics and are simmultaneously of great interest in elementary particle physics. We propose methods of constructing Dirac fermions in atomic gas systems in the presence of optical lattices. At the mean field level, the effective Hamiltonian admits a `chiral symmetry breaking' phase transition between a gapped antiferromagnet and a gapless semimetal when the on-site Hubbard interaction is varied. We show that this transition will have an epxerimental signature in the density-density correlation spectrum. Close to the criticality, the nontrivial exponents of this quantum phase transition can be experimentally probed.   

Exotic $p$-wave superfluidity of single hyperfine state Fermi gases in optical lattices

We consider $p$-wave pairing of single hyperfine state ultracold atomic gases trapped in quasi-two-dimensional optical lattices. We discuss superfluid $p$-wave (triplet) states that break time reversal, spin and orbital symmetries, but preserve total spin-orbit symmetry. We calculate the atomic compressibility, and spin susceptibility as a function of band filling for tetragonal (e.g., trapping potentials with same field intensity and same wavelengths) and orthorhombic (e.g., trapping potentials with same field intensities but different wavelenths) optical lattices. In the case of tetragonal lattices, we show that the atomic compressibility (or spin susceptibility) has a peak at low temperatures exactly at a half-filling, but this peak splits into two as the wavelength of the optical lattice is changed in one direction. These peaks reflect the $p$-wave structure of the order parameter for superfluidity and they disappear as the critical temperature is approached from below. We also calculate the superfluid density tensor, and show that for the orthorhombic case there is no off-diagonal component, however in the tetragonal case an off- diagonal component develops below the superfluid critical temperature, and becomes a key-signature of the exotic $p$-wave state.   

Adiabatic construction of d-wave resonating valence bond states of ultracold fermionic atoms in optical lattices

We discuss a controlled experimental setup to adiabatically construct superconducting $d$-wave resonating valence bond (RVB) states of fermionic atoms confined in a 2D optical lattice. The key idea is to start from an already cold initial state, in our approach from fully filled 1D tubes of atoms. The adiabatic transformation then allows to reach ultralow temperatures of a few percent of the Fermi temperature which is required to observe the $d$-wave RVB states. We discuss hole doping techniques and describe a simple experimental measurement to study the $d$-wave pairing of such vacancies. Our experimental setup can be used to effectively probe ground state properties of the doped and undoped half-filled Hubbard model on (coupled) plaquettes, ladders and the 2D lattice.   

Session J37: Fluid Turbulence and Instabilities

Concentration and Velocity Structure of Self-Preserving Steady Round Buoyant Turbulent Plumes in Crossflow

The structure of self-preserving steady round buoyant turbulent plumes in uniform crossflows were studied. The experiments involved a round salt-containing (sodium phosphate) water source jet injected into an ethyl-alcohol/water crossflow to match the refractive indices of the two flows in a water channel. Planar-Laser-Induced Fluorescence (PLIF) was used to measure mean and rms fluctuating concentrations of source fluid whereas Particle-Image-Velocimetry (PIV) was used to measure mean and rms fluctuating velocities, both over cross-sections of the flow. Self-preserving behavior was reached when the plumes were deflected into nearly the crossflow direction, yielding counter-rotating vortex systems similar to self-preserving line thermals. As a result, achieving self-preservation was strongly affected by the source/crossflow velocity ratio, e.g., the self-preserving region was observed for (x$_{c}$-x$_{os})$/d greater than the following values: 25 (u$_{o}$/v$_{\infty }$=5), 110 (u$_{o}$/v$_{\infty }$=50) and 170 (u$_{o}$/v$_{\infty }$=100). At self-preserving conditions, it was possible to construct contour plots of concentration and velocity properties over the flow cross section using scaled self-preserving variables. Another interesting property of these flows is their surprisingly rapid mixing; this was indicated by a flow width for plumes in crossflows that was 2.7 times larger than that of plumes in still surroundings.   

Particle-Generated Isotropic Turbulence in the Final-Decay Period

Isotropic turbulence generated by uniform fluxes of mono- and poly-disperse particle moving through gases was studied theoretically and experimentally. Measurements involved mean and rms fluctuating velocities, velocity PDF's, energy spectra, and integral and Taylor length scales. Particle-generated turbulence proved to be isotropic turbulence in the final-decay period that had several unusual features compared to conventional isotropic turbulence in the initial-decay period: rates of dissipation of turbulence kinetic energy were enhanced, ratios of integral/Taylor length scales were unusually large, and ratios of integral/Taylor length scales decreased with increasing turbulence Reynolds number, which is just opposite to conventional isotropic turbulence in the initial-decay period. Finally, the properties of the energy spectra of isotropic turbulence in the final-decay period yielded an effective correlation between the rates of turbulence production by the stirring action of the dispersed particles and the relative turbulence intensities of the particle-generated turbulence.   

Dynamic PIV measurement on the wake of thin plate

Dynamic PIV (Particle Image Velocimetry) with high-speed camera and high repetition laser is focused as quantitative measurement of the transient phenomenon. This system is possible to expand to time resolution of high frequency at 1kHz or 10kHz and it can be widely applied as time serial measurement of PIV which has remained the two-dimensional velocityThe configuration of high-speed camera is 512×256 pixels and 10kHz in frame rate. The repetition rate of double pulse laser is 5kHz for each rod. Double pulse interval is 20$\mu$s using frame straddling technique. The wake of the thin plate has been clearly visualized using the Dynamic PIV system. The vortex dynamics of the turbulent boundary layer has been investigated using the velocity data.   

A Structural-based Interpretation of the Strouhal-Reynolds Number Relationship

We propose a new Strouhal-Reynolds number relationship for shedding of vortices from circular cylinders. This new relationship is motivated by the observations that (i) for a fixed mean velocity U, the vortex street travels at a constant velocity v$_{st}$=cU independent of the rod diameter D, and (ii) the spatial periodicity \textit{$\lambda $} of the street is linearly proportional to D with $\lambda =\lambda _{0}+\alpha $D, where c, $\lambda _{0}$ and $\alpha $ are constants. It follows that the non-dimensional frequency or the Strouhal number St(=fD/U) should scale with the Reynolds number Re as St=1/(A+B/Re), where A and B are functions of $\lambda _{0}$, $\alpha $, and c. For the laminar wake, our result outperforms the classical relation, proposed by Lord Rayleigh, while it is comparable to other postulated relations in terms of accuracy in fitting experimental data. More significantly it describes remarkably well the two-dimensional (2D) film experiments over a broad range of Re (10$<$Re$<$3,000), where vortex shedding is unaffected by 3D instabilities encountered in all 3D measurements. We note that while the new relation converges to the classical result in the limit of a large Re, the 1/Re expansion, required for such a convergence, is not in general valid as originally proposed by Rayleigh.   

Global modes in forced wakes

We are studying the B\'{e}nard-Von Karman instability near the threshold under forcing conditions in the wake of a cylinder performing rotary oscillations around its axis.of the lock-on region, the vortices are shed at the forcing frequency in the near wake and persist on a characteristic length which depends on the forcing conditions. Downstream of this region, the system always selects a different frequency in the far wake, which is close to the natural one.complete study of the linear stability in the case of forced wakes has shown a modification in the nature (magnitude and length) of the absolute instability region typical of synchronised open flows. For frequencies bigger than the unforced one, this absolute-convective instability transition is pushed back as a function of the forcing amplitude downstream of the cylinder. present experiments in order to study the shape of this new global mode. We will show that these different global modes, due to the effect of the forcing, present scaling laws as a function of the intensity and the frequency of the forcing. These scaling laws are presented as a function of the effective growth rate modified by the mean flow perturbation induced by the forcing.is the first time that a full explanation for the behaviour of forced flows is provided and which includes the understanding of the limits of the lock-on regions.   

Reynolds number measurements in Rayleigh-Benard convection

We determined the Reynolds number $R_e$ 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 for Rayleigh numbers $R$ from $10^9$ to $10^{11}$. It is well known that the main flow structure in this system is a collection of hot and cold plumes and an associated large-scale circulation (LSC). We measured the temperature of the cell side-wall as a function of time at eight azimuthal locations on the horizontal mid-plane. The cross-correlation functions of temperatures on opposite sides of the cell indicate that localized hot or cold volumes associated with the LSC survive for a time comparable to the turnover time $\tau$ as they follow the LSC. >From maxima of the cross-correlation functions we find $\tau$, and from it the Reynolds number $R_e \equiv (4L/\tau)(L/\nu)$ ($\nu$ is the kinematic viscosity), of the LSC. The results are consistent with measurements by others \footnote{X.-L. Qiu and P. Tong, Phys. Rev. E {\bf 66}, 026308 (2002).} for $R \alt 10^ {10}$ and with the prediction of Grossmann and Lohse footnote{S. Grossmann and D. Lohse, Phys. Rev. E {\bf 66}, 016305 (2002).}.   

Plume Dynamics in Two-Dimensional Thermal Convection

We investigated single-point velocity statistics of convective turbulence driven by a thermal gradient in a freely suspended soap film in the plume dominant regime in the absence of the large scale circulation. The velocity probability density function (pdf) measured at the center of the convective region is strongly non-Gaussian and is asymmetric along the vertical direction. For positive fluctuations (velocity against gravity), the pdf tail is close to an exponential form with P(v$_{y})\sim $exp(-v$_{y}$/v$_{yrms})$ whereas for negative fluctuations (velocity parallel to gravity), the pdf is super-Gaussian with P(v$_{y})\sim $exp(-$\vert $v$_{y}$/v$_{yrms}\vert ^{3/2})$. The pdf of horizontal velocity component v$_{x}$ is more symmetric and shows a similar super-Gaussian form with P(v$_{x})\sim $exp(-$\vert $v$_{x}$/v$_{xrms}\vert ^{3/2})$. These observations can be understood in terms of plume dynamics and are consistent with the instanton formulation that relates input and output statistics of turbulence.   

LES Simulations of Turbulent Combustion in a Type Ia Supernovae

We propose a 2D axisymmetric model of a successful type $I_a$ supernova explosion, based on a front tracking sharp flame model. The calculation is free of adjustable turbulent transport parameters, and in this sense is in the spirit of LES turbulence simulations. Since the mixing is dominated by the largest eddies, resolving these and not the smaller ones results in a tolerable error. Both the 2D and the LES nature of the model greatly simplify parameter identification. The 2D model allows multiple simulations and an exploration of unknown parameters, while the LES model removes parameters from the simulation. We take first steps in the parameter identification problem, in observing that the initial conditions (initial radius and initial perturbation amplitudes, for example) are senstitive in determining the success of the explosion.   

Intermittency in two dimensional turbulence

Intermittency of the velocity difference $\delta v_{l}$ and the energy dissipation rate $\varepsilon_{l}$ on scale of $l$ is investigated on the inverse energy cascade range in the forced 2D turbulent flow. Measurements are performed on the freely-suspended horizontal soap film using particle tracking velocimetry. We use the multifractal method to analyze the energy dissipation rate $\varepsilon_{l}$ and calculate the scaling exponent $\tau_{q}$ and the intermittency parameter $\mu_{\varepsilon}$. From high order structure function $\left<\left(\delta v\right)\right>^{p}\sim l^{\zeta_{p}}$, We obtain the scaling exponent $\zeta_{p}$ with integer $p$ and estimate the intermittency parameter $\mu_{v}$. The Komogorov refined hypothesis suggests the relation $\zeta_{p}=\tau_{p/3}+p/3$. This relation agrees with the experimental data up to $p=5$. The deviation for larger $p$ may be due to linear damping in the system that also contributes to the energy flux on large scales.   

Instabilities in Bubble Pinch-Off

When gas is released from a submerged nozzle, the bubbles must separate and pinch-off before rising to the surface. This process involves a singularity in the flow as the diameter of the neck shrinks to zero. We present high-speed videos (100,000 fps) of bubble pinch-off in a variety of fluids with viscosities ranging from .01 Poise (water) to 120 Poise (silicone oil). The forces involved in the pinch-off come from surface tension, inertia and viscous dissipation. In viscous fluids, surface tension and viscosity are balanced and the neck shrinks linearly in time to beyond optical resolution. When the viscosity of the exterior fluid is sufficiently small (i.e. water), inertia dominates the flow and the neck diameter shrinks with $\tau^{1/2}$, where $\tau$ is the time remaining until pinch-off. In the low viscosity regime, we find that the flow becomes unstable, and the neck ruptures at a typical length scale of 100 $\mu$m and a time scale of $\tau$=10 $\mu$s. The acoustic signature of this instability will also be presented.   

Conformation-triggered flow instability in monolayer thick polymer films

Here we have report on a new type of flow instability triggered by conformational changes of brush-like macromolecules as they spread on a solid substrate. By tracing the movement of individual molecules by atomic force microscopy, we were able to follow the evolution of the instability pattern on the molecular level enabling a microscopic understanding of the underlying physical mechanism. The instability is an analog of the Saffman-Taylor instability in thin films. However, the instability is driven by a variation in flow velocity controlled by molecular conformation instead of a viscosity gradient.   

A Dynamical Model of Molecular Monolayers: Why Tethers Don't Snap

A bola-shaped domain in a Langmuir monolayer at the air/water interface relaxes towards a circular shape under the influence of line tension. The ``tether'' thickens continuously in this process, in marked contrast to the Hele-Shaw and the three-dimensional cases, where hydrodynamic instabilities lead to the tether snapping. A simplified dynamical model allows us use lubrication theory to explain this without incorporating repulsive forces to stabilize the tether in 2D. The model also allows us to give a better estimate of line tensions from the relaxation rate of such monolayer domains. This material is based upon work supported by the National Science Foundation under Grant No.9984304.   

Marangoni Convection and Deviations from Maxwell's Evaporation Model

We investigate evaporation and natural convection from thin pools of volatile liquids. We find that evaporation rates do not always follow the classical Maxwell evaporation model; deviations become larger with increasing liquid volatility. Thermal imaging shows that the liquids are not always stable to Marangoni convection; surface flows grow in strength with increasing volatility. A highly sensitive thermal imaging camera allows us to characterize the Marangoni patterns as a function of volatility, time, and liquid pool height. To help explain our results, we develop a heat balance model to connect the evaporation rates to the convective dynamics, and show that the convective flows are the source of the deviations from Maxwells' evaporation model.   

Session J38: Metal-Insulator Phase Transitions - Experiment II

The Electronic Structure of (La, Ce)Te2 System: Interplay between different degrees of freedom

Rare earth tellurides (La,Ce)Te$_{2}$ are interesting layered materials showing Charge Density Wave (CDW) formation with transition temperature of the order of 1000K. By substituting La with Ce, coexistence between magnetism and CDW is observed. Angle resolved photoemission spectroscopy (ARPES) is an ideal tool to study the competition and coexistence of these two phases, since it directly probes quasiparticles and many-body interaction. Here we report a detailed ARPES study as a function of momentum, temperature and composition of the (La, Ce)Te$_{2}$ system. The Fermi surface geometry in the CDW phase, the formation of the CDW gap and its anisotropy, as well as the ARPES lineshapes will be presented. The competition and cooperation between different degrees of freedom will be discussed.   

Field Effect Devices for Controlling the Conductivity of Ultrathin Films

An electric-field effect device geometry that uses a mechanically-thinned strontium titanate dielectric substrate has been used to affect large charge transfer into ultrathin elemental films. A large, non-ambipolar effect of the electric field on the conductance in a 10 {\AA} thick film of amorphous bismuth was found, as evidenced by shifting its superconducting transition temperature of 446 mK higher by as much as 56 mK with positive gate voltage, and shifting it lower by as much as10 mK with negative gate voltage. This work is supported in part by the National Science Foundation under grant NSF/DMR-0138209.   

Temperature/frequency scaling of conductivity near the M-I transition in doped semiconductors

Doped semiconductors undergo a metal-insulator transition (MIT) at T = 0, i.e. a quantum phase transition and can be probed by varying the dopant concentration for a set of samples through a critical concentration. In crystalline systems such as Si:P, this occurs at a dopant concentration near 10$^{18}$, but in amorphous systems such as a-NbSi and a-GdSi, it occurs closer to 10-15 at.{\%}$^{1,2}$. Doping with Gd introduces the possibility of examining this transition with concentration as well as magnetic field tuning, as it has been shown that a magnetic field tunes a sample of a-GdSi through the MIT$^{3}$. We present scaling results for families of DC conductivity and optical conductivity curves for both the concentration tuned as well as the magnetic field tuned MIT. Probing the same quantum critical point using two separate tuning parameters allows for a unique and separate determination of the success of dynamic scaling as a function of the tuning parameter. A comparison of the concentration tuned vs. the magnetic field tuned transition shows distinct differences. [1] Hertel et al., \textit{Phys. Rev. Lett. }\textbf{50}, 743 (1983) [2] Hellman et al., \textit{Phys. Rev. Lett.} \textbf{77}, 4652 (1996) [3] Teizer et al., \textit{Phys. Rev. B} \textbf{67}, 121102(R) (2003)   

Transport Measurement of Ultra-dilute 2D-Holes in High Quality (100) GaAs HIGFETs

We have studied transport properties of high purity p-channel GaAs HIGFETs and observed a surprising temperature dependence of conductivity ($\sigma$) at ultra-low densities from $p=2\times10^{10}cm^{-2}$ to $p=8\times10^{8}cm^{-2}$. For $p>4\times10^{9}cm^{-2}$, as the temperature is decreased $\sigma$ first decreases linearly with T, then exhibits a clear upward bending which signifies a metallic behavior. The temperature at which the bending occurs shifts to a lower value as $p$ is decreased. However, the up-turning of $\sigma$ weakens as the density goes lower until, at $4\times10^{9}cm^{-2}$, it becomes almost independent of T between 80mK and 40mK. As $p$ is decreased from $4\times10^{9}cm^{-2}$ to $2\times10^{9}cm^{-2}$, $\sigma$ continues to show little temperature dependence and tend to saturate at some finite values below $80mK$. Meanwhile, the high temperature linear regions become almost parallel to each other. As $p$ is further decreased below $2\times10^{9}cm^{-2}$, the results are even more striking: the linear T-dependence persists even though the slope of the linear region starts to decrease. Remarkably, $\sigma$ maintains the weak T-dependence below $80mK$. At $8\times10^{8}cm^{-2}$, $\sigma$ retains a finite value of $0.045e^{2}/h$ at $40mK$. Consistent results were obtained through measuring three different samples. These observations contrast sharply with the metal-insulator-transition model which would predict a low-density insulating state, whereas we found no evidence of localization all the way down to the lowest density of $8\times10^{8}cm^{-2}$.   

Temperature and Magnetic Field Enhanced Hall Slope of the Dilute 2D Holes in GaAs in the Ballistic Regime

We report the temperature($T$) and perpendicular magnetic field ($B$) dependence of the Hall resistivity $\rho_{xy}(B)$ of dilute metallic two-dimensional(2D) holes in high mobility GaAs quantum wells over a broad range of temperature(0.02-1.25K). The low $B$ Hall coefficient, $R_H$, is found to be enhanced when $T$ decreases. Strong perpendicular magnetic fields further enhance the slope of $\rho_{xy}(B)$ at all temperatures studied. Coulomb interaction corrections of a Fermi liquid in the ballistic regime ($k_BT>\hbar/\tau$ with $\tau$ being the scattering time) can not explain the enhancement of $\rho_{xy}$ which occurs in the same regime as the anomalous metallic longitudinal conductivity. In particular, although the metallic conductivity in 2D systems has been attributed to electron interactions in a Fermi liquid, these same interactions should reduce, {\it not enhance} the slope of $\rho_{xy}(B)$ as $T$ decreases and/or $B$ increases. Preprint available at cond-mat/0411391.   

Observation of abrupt first-order metal-insulator transition in GaAs-based two-terminal devices

An abrupt first-order metal-insulator transition (MIT) as a jump of the density of states is observed for Be doped GaAs, which is known as a semiconductor, by inducing very low holes of approximately $n_p\approx$5$\times$10$^{14}$~cm$^{-3}$ into the valence band by the electric field; this is anomalous. In a higher hole doping concentration of $n_p\approx$6$\times$10$^{16}$~cm$^{-3}$, the abrupt MIT is not observed at room temperature, but measured at low temperature. A negative differential resistance are also observed as further evidence of the MIT. The upper limit of the temperature allowing the MIT is deduced to be approximately 440K from experimental data. The abrupt MIT rather than the continuous MIT is intrinsic and can explain the ``breakdown'' phenomenon (unsolved problem) incurred by a high electric field in semiconductor devices.(ref:NJP 6 (2004)52:(www.njp.org))   

2D Metal-insulator transition behavior in a high mobility strained Si quantum well

The apparent metal-insulator transition is observed in a high quality two-dimensional electron system (2DES) in the strained Si quantum well of a Si/Si$_{1-x}$Ge$_{x}$ heterostructure with mobility $\mu$=1.9$\times$10$^{5}$cm$^{2}$/Vs at $n$=1.45 $\times$10$^{11}$cm$^{-2}$. The critical density $n_c$, where the thermal coefficient of low T resistivity changes sign, is 0.32$\times$10$^{11}$cm$^{-2}$, much smaller than the $n_c$ of $\sim$0.8$\times10^{11}$cm$^{-2}$ seen in clean Si-MOSFET's (usually with a peak $\mu$$\sim$4$\times$10$^{4}$cm$^{2}$/Vs). This result is consistent with previous observations in the GaAs systems that $n_c$ decreases with increasing 2DES quality. Moreover, in low $n$ range, for 0.27$\times$10$^{11}$cm$^{-2} $$<$$n$$<$0.35$\times$10$^{11}$cm$^{-2}$, close to the transition region, the conductivity increases roughly linearly with $T$ around the Fermi temperature and, surprisingly, all the curves of different densities are parallel to each other for $T>1.2K$. In the higher density range where the 2DES shows metallic-like behavior, the in-plane magnetoresistance $\rho$(B) first increases $\sim$B$^{2}$ and then saturates to a finite value $\rho$(B$_C$) for B$>$B$_C$. The full spin-polarization field B$_C$ decreases monotonically with $n$ but appears to saturate to a finite value as $n$$\rightarrow$0. We find $\rho (B_C)/\rho(0)$$\sim$1.8 for all the densities ranging from 0.35 to 1.45$\times10^{11}$cm$^{-2}$ and, when plotted versus B/B$_C$, collapse onto a single curve.   

Shear strain profile of a driven CDW probed by X-ray microbeam diffraction

We have probed charge density wave (CDW) structure in stepped, whisker-like NbSe$_{3}$ samples with lateral variations of pinning strength by X-ray microbeam diffraction using 2-ID-D beamline at APS-ANL. The rotation of the CDW \textbf{\textit{q}}-vector on the depinned side appears above the threshold field for CDW depinning, consistent with the picture of inhomogeneous pinning. The corresponding shear strain profile is determined with a resolution of 300 nm. The results demonstrate how the magnitude of shear strains changes with the DC bias applied along the direction of CDW motion. This profile is compared with finite element modeling.   

Forced localization in thin K films, investigated with the superconducting proximity effect

Thin films of alkali metals are forced into an insulating state by being covered with sub-mono-layers of Pb. The superconducting proximity effect is used to investigate the electronic change in the alkali film. On the length scale of the film thickness the electronic properties of the alkali film do not change noticeably during the metal-insulator transition.   

Revealing the local structure of the charge-density-wave material, CeTe$_{3}$, using atomic pair distribution function analysis

CeTe$_{3}$ is a layered charge density wave material whose Te square nets undergo a Peierls distortion. Formation of the CDW in this material has been validated through observing superlattice reflections by single crystal x-ray diffraction and Fermi surface nesting by ARPES. However, this electronically driven structural distortion is difficult to measure and has not been previously elucidated. We were recently able to solve this using single crystal x-ray diffraction for the first time. In addition, the $local$ structure of CeTe$_{3}$ has been studied using the atomic pair distribution function analysis of x-ray powder diffraction data. The study shows that the local structure of CeTe$_{3}$ is more distorted than the average structure. Interestingly, a bimodal Te-Te bond-length distribution of Te nets is found from the local structural model whereas a Gaussian like distribution from the average structural model. We will discuss how the local and average structures can be reconciled by the existence of structural disorder in the Te network.   

Fabrication and characterization of end current injection contacts to the quasi-1D conductor NbSe3

We have successfully fabricated end current injection contacts to the CDW conductor NbSe$_{3}$ through a combination of electroplating and standard lithographic procedures. These contacts allow direct carrier injection along the direction of CDW motion and produce uniform transverse current and electric field profiles in this highly anisotropic material. The differential conductance of these end-contacted samples shows some quantitative differences from previous measurements [1,2] using side contacts, which are particularly evident in measurements of the condensate current versus phase-slip voltage. This may indicate differences in how phase dislocation loops convert normal carriers to condensate when shear components to the driving force are eliminated. [1] M. P. Maher \textit{et al.}, Phys. Rev. B \textbf{52}, 13850 (1995). [2] S. G. Lemay \textit{et al.}, Phys. Rev. B\textbf{ 57}, 12781 (1998).   

Electrodynamics of the 2D superconductor-insulator transition

Using microwave cavities and a novel cryogenic system we have probed the evolution of the low frequency electrodynamics of thin InO$_x$ films across the nominal 2D field-tuned superconductor insulator quantum phase transition. Such a finite study allows us, at least in principle, to access the true phase coherent ($\hbar \omega > k_B T$) quantum critical behavior. A number of other interesting items are found including evidence for significant finite frequency superfluid density well into the ``insulating" regime of the phase diagram. Various scenarios for frequency dependent scaling are also investigated.   

Charge Density Wave Gap in $ZrTe_{3}$

The transition metal trichalcogenides ($MX_{3}, M= Ta, Nb$) have been widely studied as prototype examples of linear chain conductors exhibiting novel charge density wave (CDW) phenomena. The title compound is semimetallic with linear chain structure but quasi-two-dimensional conductivity. At $T_{CDW}$=63 K it undergoes a phase transition, which most strongly affects its conductivity components perpendicular to the conducting chains, and it becomes superconducting at 2 K. We have measured the optical reflectivity from the far-infrared up to the ultraviolet spectral range as a function of temperature and with light polarized both along and perpendicular to the chains. Through Kramers-Kronig transformation we have extracted the optical conductivity. We found optical evidence for the CDW gap, which opens over a tiny amount of the Fermi surface. The temperature dependence of the gap follows the BCS behaviour for an order parameter. The role played by fluctuation effects will be also discussed.   

Conductivity of granular metals

The conductivity of granular metals, which consist of isolated metallic regions embedded within an insulating matrix, is often experimentally seen to obey the `soft-activation' law $\sigma\propto e^{-\sqrt{T_0/T}}$ over a very large temperature range, as opposed to the Arrhenius `hard-activation' law $e^{-E_c^*/T}$ that would have been expected from basic Coulomb blockade theory. Our extensive perturbative and path-integral Monte-Carlo analysis of the Ambegaokar-Eckern-Sch{\"o}n (AES) model for a regular array of grains gives an Arrhenius law and does not reveal a soft-activation behaviour even at the lowest temperatures we considered. This result is in agreement with recent experiments on silver nanoparticle arrays with controllable disorder, and suggests that the soft-activation law should be attributed to disorder.   

Session J39: Phase Transitions in Surfaces and Films: Ferroelectrics and Multiferroics

Thickness dependent ferroelectric properties of BaTiO3 ultra-thin films

For decades, ferroelectricity in ultra-thin ferroelectric films has attracted much attention because of high-density nonvolatile memory application. Recently, first-principle calculation on the thickness limit of ferroelectricity in BaTiO$_{3}$(BTO) thin films was reported [1]. Experimental evidence of thickness limit and macroscopic polarization behaviors in the ultra-thin ferroelectric films were rarely studied. In this work, we investigated thickness dependence of polarization retention properties and coercive field in BTO films. SrRuO$_{3}$/BTO/SrRuO$_{3}$ capacitors were prepared with BTO layer thickness from 30 nm to 5 nm. From the electrical measurements, we observed that the upper limit of the critical thickness should be less than 5 nm. All the capacitors showed rapid relaxation of polarization in 1 s, and coercive field is independent of thickness. We would discuss these thickness-dependent polarization relaxation and coercive field in connection with depolarization field, which plays an important role in ultra-thin ferroelectric capacitor [2]. [1] J. Junquera and P. Ghosez, Nature (London) \textbf{422}, 506 (2003). [2] A. M. Bratkovsky and A. P. Levanyuk, Phys. Rev. Lett. \textbf{84}, 3177 (2000).   

Effective Hamiltonian modeling of ferroelectric ultra-thin films

We have further extended first principles Hamiltonian approaches [1,2] that are applicable to the bulk systems (i.e., 3D systems periodically repeated in all three Cartesian directions) to study ferroelectric properties of ultra-thin films. The main feature of our new approach is that we treat the dipole-dipole interactions for the systems with 2D periodicity {\it exactly}, based on the symmetrized Green's function $\mathcal{G}(\mathbf{r'},\mathbf{r})$ of the Laplace equation. Although essentially microscopic, our model nevertheless accurately reproduces macroscopic characteristics such as depolarization and Lorentz fields in the limit of thick films. Within this approach, the finite-temperature behavior of different ferroelectric ultra-thin films have been simulated under different boundary conditions. Our results (1) are compared with those obtained in the framework of a 3D-like approach [3] that uses thick vacuum gaps between the periodic replicas of the films within an atomistic Hamiltonian, (2) provide a deep microscopic understanding of ferroelectric thin films. This work is supported by NSF grants DMR-0404335 and DMR-9983678 and by ONR grants N 00014-01-1-0365, N 00014-04-1-0413 and N 00014-01-1-0600. [1] Zhong {\it et al}, Phys. Rev. Lett. {\bf 73}, 1861 (1994); Phys. Rev. B {\bf 52}, 6301 (1995). [2] L. Bellaiche {\it et al}, Phys. Rev. Lett. {\bf 84}, 5427 (2000). [3] I. Kornev {\it et al}, Phys. Rev. Lett. {\bf 93}, 196104 (2004).   

Ferroelectricity in Ultrathin Perovskite Films in Metal-Ferroelectric-Metal Junctions

We present first-principles studies of electronic and ferroelectric properties of Metal/Ferroelectric/Metal junctions. We consider KNbO$_{3}$ as the ferroelectric, while strontium ruthenite, SrRuO$_{3}$, and platinum, Pt, as metals. These materials have a match in the lattice constants, which eliminates the appearance of stresses and misfit dislocations. We utilize the projector augmented plane wave method to perform the first principles calculations. The fully relaxed geometry of the structure is obtained assuming that the substrates have bulk lattice parameters away from the interface. We use the Berry phase formalism to obtain the polarization of the ferroelectric layer. The calculation of the electric polarization requires a proper subtraction of the Berry phase for the reference paraelectric structure from that for a distorted structure. We show the existence of ferroelectricity in these heterostructures at ferroelectric thickness of 0.22nm. We find that for the case of asymmetric leads, i.e. for the SrRuO$_{3}$/KNbO$_{3}$/Pt junction, the electrostatic potential profile across the ferroelectric layer is asymmetric with respect to the direction of the electric polarization suggesting a change in transport properties with switching the electric polarization. This work is supported by Nebraska Research Initiative and National Science Foundation.   

UV Raman study of ferroelectric BaTiO3/SrTiO3 superlattices

The results of the first experimental study of ferroelectric short-period superlattices (SLs) by ultraviolet (UV) Raman spectroscopy will be presented. The high quality (BaTiO$_{3})_{m}$/(SrTiO$_{3})_{n}$ SLs (m, n are between 4 and 10 unit cells) with atomically smooth interfaces were grown by molecular beam epitaxy on SrTiO$_{3}$ substrates. Raman spectroscopy with UV excitation (351.1 nm) made possible the observation of superlattice phonons without overwhelming substrate signal in the spectra. Raman data on the BaTiO$_{3}$ phonons show that BaTiO$_{3}$ layers in SLs remain in the tetragonal phase in the entire temperature range studied (80-400K), and the low-temperature phases characteristic for bulk BaTiO$_{3}$, are suppressed. Biaxial compression of BaTiO$_{3}$ layers in SLs is likely cause for such a behavior, according to the calculated phase diagram for BaTiO$_{3}$ as a function of temperature and strain. Features attributed to the first-order Raman scattering in SrTiO$_{3}$ layers indicate that the inversion symmetry is broken, and the SrTiO$_{3}$ layers in the SLs are polar. Supported in part by DOE (Grant {\#} DE-FG02-01ER45907).   

Domain Size Dependence of Piezoelectric Properties of Ferroelectrics

The domain size dependence of piezoelectric properties of ferroelectrics is investigated using a continuum Ginzburg-Landau model that incorporates the long-range elastic and electrostatic interactions. Microstructures with desired domain sizes are created by quenching from the paraelectric phase by biasing the initial conditions. Three different two-dimensional microstructures with different sizes of the $90^{o}$ domains are simulated. An electric field is applied along the polar as well as non-polar directions and the piezoelectric response is simulated as a function of domain size for both cases. The simulations show that the piezoelectric coefficients are enhanced by reducing the domain size, consistent with recent experimental results of Wada and Tsurumi (Brit. Ceram. Trans. {\bf 103}, 93, 2004) on domain engineered $BaTiO_{3}$ single crystals.   

Atomistic View of Manganite Thin Films by Scanning Tunneling Microscopy

Perovskite manganites Ln1-xMxMnO3(Ln=La, Pr, Nd, M=Ca, Sr, Ba, Pb) have been extensively studied due to the famous CMR effect. However, the underlying mechanism is not yet well understood. It is generally accepted that polaron plays a very important role. Scanning tunneling microscopy and spectroscopy (STM/S) capable of obtaining atomic structure and electronic information near the Fermi level at atomic resolution has been extremely successful in studying layered transition metal oxide superconductors. To study the manganites with STM/S, we have developed an in-situ Laser MBE system. Atomic resolution has been successfully achieved in manganite thin films. Results pertaining to phase separation and polaron effects will be discussed in this talk. Research sponsored by the U. S. Department of Energy under contract DE-AC05-00OR22725 with the Oak Ridge National Laboratory, managed by UT-Battelle, LLC   

Control of La$_{0.5}$Ca$_{0.5}$MnO$_3$ superstructure through epitaxial strain release

Intergranular variations of superlattice periodicity in polycrystalline La$_{1-x}$Ca$_{x}$MnO$_3$ have been attributed to variations in strain. Here we control the superlattice periodicity within a continuous crystal. A focussed ion beam microscope (FIB) was used to pattern an electron transparent window in an untwinned coherently strained epitaxial thin film of La$_{0.5}$Ca$_{0.5}$MnO$_3$ grown on NdGaO$_3$ by pulsed laser deposition. It was found that the wavenumber could be reduced by 3\% in regions isolated by cuts from the rest of the window. We attribute this variation to the release of epitaxial strain beyond the resolution of the electron microscope.   

Intrinsic Character of the 3x3 to R3xR3 Phase Transition in Pb/Si(111): True variable Temperature STM Experiments

We have studied the (3x3) to (root 3 x root 3) reversible phase transition in Pb/Si(111) by means of variable temperature scanning tunneling microscopy and density functional first-principles calculations. This phase transition consists on a lowering of the symmetry of the system produced by a change of the surface periodicity that evolves from a (root 3 x root 3) at room temperature to a (3x3) at low temperature. We have been able to prepare extremely large domains completely free of defects. These large free of defects regions, together with our ability to track the same area with atomic resolution in a temperature range between 40 K and 200 K have allowed us to detect the intrinsic character of the phase transition at temperatures around 86 K. This intrinsic character is in full agreement with our first-principles calculations. Moreover, our results show that the hypothesis that point defects play a fundamental role as the driving force, reported for similar systems, can be discarded for Pb/Si(111).   

Observing a Wetting Transition for Water

A liquid is said to wet a solid surface if it spontaneously spreads out uniformly over the surface. It is said to not wet the surface, if it remains a droplet on the surface, exhibiting a finite contact angle. A wetting transition is defined as an abrupt change from non-wetting to wetting or vice-versa. Water does not wet most semiconductor and metal surfaces near room temperature. However, recent state-of-the-art theoretical calculations [1] predict that water should exhibit a wetting transition on these surfaces, completely wetting them at high temperatures. The search for this wetting transition is difficult because at high temperatures, water can be quite oxidizing and has a very high vapor pressure. An experimental cell has been constructed suitable for careful thermodynamic optical studies of liquid water droplets on solid surfaces. Preliminary experimental results will be presented. [1] M. Gatica, X. Zhao, J. K. Johnson and M. W. Cole, J. Phys. Chem. B 108, 11704 (2004).   

Surface and Interface studies of Au overlayers on 4H-SiC(000-1)

The effects of Au overlayers prepared on the SiC(000-1) surface have been investigated. A 2x2 reconstructed surface was prepared by Si deposition at substrate a temperature of 800$^{o}$C. A 3x3 reconstructed surface was prepared by in situ heating to ca 1050$^{ o}$C. By utilizing high resolution photoemission and synchrotron radiation together with low energy electron diffraction, the formation of ordered overlayer structures was identified and features developing in the Si 2p, C 1s and Au 4f core level spectra were investigated in detail. After Au deposition on the 2x2 reconstructed surface and annealing a new stable $\surd $7x$\surd $7 R 19$^{o}$ surface reconstruction was identified by LEED and surface related components were observed in Si 2p and Au 4f core levels. After Au deposition on the 3x3 reconstructed surface and annealing no new stable reconstruction was observed to form. In the C 1s core level no significant changes were possible to detect after Au deposition and annealing. These findings will be presented and discussed.   

Session J40: Focus Session: Transport Properties of Nanostructures III: Semiconductors & Surfaces

Charge transport in adiabatically driven ratchets - Waveform and phase dependence -

We report on the adiabatic pumping of electrons in a driven lateral superlattice (LSL) with broken symmetry. The device is fabricated from a two-dimensional electron gas (2DEG), located $54.8\:\mathrm{nm}$ below the surface in a GaAs/Al$_x$Ga$_{1-x}$As heterostructure, grown by molecular beam epitaxy. The LSL is realized by two transducer gates, each comprising 75 stripes of 160 nm width and 1 $\mu$m period. The gates are interlaced off center by two thirds of the period and can thus induce a ratchet-like potential modulation in the 2DEG when appropriately biased. When the transducer is driven by two identical but phase-shifted ac signals, a lateral pumping current $I(\phi)$ results, which strongly depends on both the phase shift $\phi$ and the wave form $V(t)$. Surprisingly, we find that for different periodic signals, the phase dependence $I(\phi)$ closely resembles $V(t)$. A simple model of adiabatic pumping in 2DEGs is presented, which can reproduce our experimental findings. Possible applications for waveform sampling are discussed.   

Cotunneling-mediated observation of excited states in the Coulomb blockade regime

We present finite bias transport measurements on a few-electron quantum dot. In the Coulomb blockade regime, we observe strong signatures of inelastic cotunneling which can directly be assigned to excited states observed in the non-blockaded regime. In addition, we observe structures related to sequential tunneling through the dot, occuring after it has been excited by an inelastic cotunneling process.   

Imaging quantum interference patterns on a quantum point contact

Scanning gate microscopy (SGM) images have been taken inside the constriction of a quantum point contact (QPC) fabricated on an In$_{0.53}$Al$_{0.47}$As/In$_{0.53}$Ga$_{0.47}$As heterostructure. Shubnikov-de Haas measurements revealed the occupation of two sub-bands on this sample with carrier concentrations of 7.24x10$^{11}$cm$^{-2}$ and 2.42x10$^{11}$cm$^{-2}$, respectively. The images show the behavior of the wavefunction interference at different points of the transmission curve of the QPC. It is believed that these images correspond to different resonance peaks observed on the curve. Additional images have been taken at different temperatures indicating a reduction of the clear interference patterns, which is attributed to a decrease of the phase coherent area of the sample.   

Scanning Probe Microscopy for Atomic-scale Silicon Device Fabrication

Over the past three decades the driving force behind the expansion of the microelectronics industry has been the ability to pack ever more features onto a silicon chip, achieved by continually miniaturising the size of the individual components. However, after 2015 there is no known technological route to reduce device sizes below 10nm. In this talk we demonstrate a complete fabrication strategy towards atomic-scale device fabrication in silicon using a combination of scanning tunneling microscopy and high purity crystal growth. In particular we overcome one of the major obstacles to making functional semiconductor devices with an STM -- connecting macroscopic leads to the device once it is removed from the vacuum environment [1]. We demonstrate key steps of the fabrication process, including the ability to place individual phosphorus atoms in silicon at precise locations [2] and encapsulate them in epitaxial silicon with minimal diffusion and segregation of the dopants [3]. We present magnetoresistance data showing the cross-over from 2D to 1D transport in nano-scale quantum wires and arrays. Finally we discuss the implications of these results for the construction of more sophisticated atomic-scale devices in silicon such as a silicon based quantum computer. [1] F.J. Ruess, L. Oberbeck, M.Y. Simmons, K.E.J. Goh, A.R. Hamilton, T. Hallam, N.J. Curson and R.G. Clark, ``Fabrication of quantum wires using scanning probe microscopy'', Nano Letters 4, 1969 (2004). [2] S. R. Schofield, N. J. Curson, M. Y. Simmons, F. J. Ruess, T. Hallam, L. Oberbeck and R. G.Clark, ``Atomically precise placement of single dopants in silicon'', Physical Review Letters 91, 136104 (2003). [3] L. Oberbeck, N. J. Curson, T. Hallam, M. Y. Simmons and R.G. Clark, ``Measurement of phosphorus segregation in silicon at the atomic-scale using scanning tunneling microscopy'', Appl. Phys. Lett. 83, 1359 (2004).   

Silicon nanoscale 2D donor devices fabricated by UHV-STM lithography

We developed a scheme to fabricate nanoscale electronic devices by patterning 2D shallow donors into single crystal silicon. The goal of this approach is to seamlessly integrate nano- and microelectronics. In this approach, we pattern the devices on H terminated Si(100)-2x1 surfaces via UHV-STM. Phosphine molecules selectively adsorb onto the patterned areas to define conduction pathways. Low temperature Si MBE is used to encapsulate the dopants in the Si lattice. Two-terminal electrical connection to the outside-world is provided by a template structure formed by conventional microfabrication. A third terminal used for gate modulation of the device is formed by silicon nitride jet vapor deposition and metallization. Low temperature electrical characterization of conducting wires show significant departure from Ohmic conduction for width $<$ 50nm. Electro and magnetotransport properties will be discussed. Tunnel junction and single electron transistor fabrication are currently underway. The low charged-defect density provided by complete encapsulation could allow the fabrication of a solid state quantum computer.   

Single electron transistors at high temperature

The tunneling current through a germanium (Ge) quantum dot (QD) embedded in SiO$_{2}$ matrix is studied theoretically. The energy levels and Coulomb interactions of electrons in a nanometer Ge QD are calculated using an effective mass model. In small Ge QDs, the effect of electron correlation is significant and hence, both the interlevel and intralevel Coulomb interactions are important in electron transport properties. The tunneling current of a Ge-QD single electron transistor (SET) is calculated using the Keldysh Green function method and two-level Anderson model. In addition to four peaks arising from the intralevel Coulomb interactions, extra differential conductance peaks are found due to the interlevel Coulomb interactions and the statistical nature of the open system.   

Resistance Noise and Morphology in Percolating Films of Ag on Si(111)

Structure, conductance and noise in submonolayer Ag films on Si(111)-(7x7) near the onset of electrical conduction have been measured in situ in a UHV growth chamber. Noise measurements characteristic of the film morphology can be accomplished only when transport through the substrate is prevented. In this case, extremely low doping ($\sim $10$^{13}$ cm$^{-3})$ and low temperature measurements were used to accomplish this. Measurement of the exponent of the variation of the noise level with Ag coverage near the percolation threshold yields a value of 1.06 $\pm $ 0.09, inconsistent with lattice percolation at the interface with the Si substrate. The inverted random void model is consistent with this result, yielding the possibility that a broad distribution of degree overlap of adjacent Ag clusters is the origin of the noise. This model will be discussed in context of the detailed observations of film morphology near the percolation threshold using low-temperature STM and simultaneous conductivity measurements. Implications for the role of the Si substrate in electrical transport are also discussed.   

Phonon Scattering by Molecular Dynamics: Temperature Dependence and Effect of Structural Disorder

We use molecular dynamics to simulate individual phonon-grain boundary scattering events in Silicon as a function of temperature and grain-boundary disorder. The temperature dependence of the scattering is investigated by varying the lattice parameter and incident phonon's properties to match the equilibrium bulk crystal's lattice spacing and dispersion relation at the prescribed temperature. For a given twist angle, different grain boundary structures are formed by a simulated growth process wherein cooling from the melt permits different grains to grow towards one another and eventually impinge, resulting in a boundary. The effect of boundary disorder on the reflected and transmitted phonons is then characterized. The temperature dependence incorporated in this manner may be compared with existing scattering models in the continuum limit while the effects of disorder can be used to estimate uncertainties in scattering models for larger--scale methods such as direct-simulation Monte Carlo.   

Heat transfer experiments on micro- and nanoscale: Interface and size effects

We present heat transfer experiments using a heated silicon cantilever/tip at or near contact with a variety of surfaces. Under ambient conditions cooling of the cantilever results from conduction through the air, through the cantilever beam and through the tip-surface contact, as well as from radiation cooling. By varying the ambient conditions, the sample material and the tip-sample distance we can quantify the various contributions. Under ambient conditions the heat transport is dominated by conduction through the air and the cantilever. At distances of a few times the mean free path of air molecules the heat transfer is accompanied by surprisingly a large momentum transfer. In vacuum, with conduction excluded, heat transport through the nm-sized tip-surface contact can be measured. The heat transport is found to depend decisively on the size of the tip, the size of the mechanical contact and the surface material. Radiative heat transport becomes significant under vacuum conditions. At small separations we observe a strong distance dependence due to near field effects. This deviation from Stefan-Boltzmann’s law also exhibits a strong material and temperature dependence.   

Stochastic Evolution of Nano-Structures in the Continuum Step Model

Stochastic Evolution of Nano-Structures in the Continuum Step Model * M. Degawa, F. Szalma and E.D. Williams, Department of Physics and MRSEC University of Maryland College Park MD, 20742 Technological demands of the fabrication of nano-structures and quantum dots provides renewed motivation for understanding the atomistic properties that control crystal shapes. With decreasing structure size, the issues of finite size and shape effects become non-negligible and also the increasing sensitivity to external perturbations, such as the substrate interface. We have previously shown that the effects of curvature, which cannot be neglected in nanoscale structures, yield a family of crystal shapes with constant surface chemical potentials. The member of this family that represents an absolute minimum in the total free energy follows the Pokrovsky-Talapov ECS (PT-ECS), which is also the result obtained in the limit of zero curvature. The remaining members of the family represent metastable states. Here, we extend the continuum results to include the discreteness of steps using the continuum step model. The metastable states now represent the structures, which are formed due to a barrier for the peeling (and also nucleation) of layers necessary for the evolution of the crystal shape. We relate the barrier height to the crystallite parameters, including volume and interface interactions, and discuss its consequences for the kinetics of shape evolution. *This work has been supported by the DOE-NNI and NSF-MRSEC.   

Electrostatic interactions between atomic force microscope tip and mostly dielectric surface near glass transition point

Behavior of thin dielectric near glass transition point is a mystery. A strong non-uniform electric field (10$^{9}$ Vm$^{-1})$ induced by a biased atomic force microscope tip creates nanoscopic mass transport resulting in nanostructure formation in a broad class of polymers near GTP. Similar trend under same experimental conditions is observed in iridovirus shell composed of proteins folded in capsomers. It is suspected that structural re-arrangement of polar amino acids is the reason. In all cases an AFM tip is a major player in the technique, we name, atomic force microscopy electrostatic nanolithography (AFMEN). This experimental technique produces very similar nanostructural changes in polymers, SAM, and biological cells. We attempt to describe this behavior.   

Session J41: Correlated Electrons: Manganites

Temperature-Dependent X-Ray Absorption Spectroscopy of Colossal Magnetoresistive Perovskites

We have measured the temperature dependence of the O K-edge pre-edge structure in the x-ray absorption spectra of the perovskites La$_{1-x}$A$_{x}$MnO$_{3}$, (A = Ca, Sr; x = 0.3, 0.4). Our measurements reveal a correlation between the disappearance of the splitting in the pre-edge region and the presence of Jahn-Teller distortions. The different magnitudes of the distortions for different compounds are proposed to explain some dissimilarity in the line shape of the spectra taken above the Curie temperature.   

Resonant x-ray diffraction study of orbital and magnetic order in half-doped manganites

Resonant x-ray diffraction performed near a transition metal $L$ absorption edge is directly sensitive to the chemical and magnetic environment of the important 3d electrons in transition metal oxides. This technique can be used to directly probe and compare the orbital and magnetic correlations which form the ground state in half-doped, insulating manganites [1]. In this talk, we compare resonant diffraction measurements as a function of temperature, energy and scattering geometry for three half or near-half doped insulating manganites, $Pr_{1-x}Ca_{x}MnO_{3}$ ($x$ = 0.4 and 0.5) and $Nd_{1-x}Sr_{x}MnO_{3}$ ($x$ = 0.5). In all three samples, the orbital scattering exhibits a characteristic resonant (energy dependent) line- shape and dependence on azimuthal angle. However, subtle changes in the resonant line-shapes measured in different samples may indicate different interactions with the surrounding lattice. Interestingly, both x=0.4 and 0.5 $Pr_{1-x}Ca_{x}MnO_{3}$ samples show a smaller correlation length associated with orbital order compared to magnetic order. [1] K. J. Thomas et al, Phys. Rev. Lett. 92 237204 (2004).   

Resonant Soft X-ray Scattering on Layer Manganite: Observation of Orbital Ordering Effect

We have developed an endstation to perform resonant soft X-ray scattering on highly correlated electron systems. With this instrument, we are able to study the photon energy and temperature dependence of superlattice reflection originating from orbital ordering on bilayer manganite. The intensity of superlattice reflection closely correlates to the resistivity behavior across phase transition boundary, indicating the intimate relationship between them. The correlation length is consistent with results reported previously in hard X-ray studies.   

Glassy response to gate and magnetic fields in ultrathin manganite films

We have studied the low temperature behavior of thin films of La$_{0.8}$Ca$_{0.2}$MnO$_{3}$ in a field effect geometry. The films exhibit the usual insulator-metal transition near the Curie temperature, but below 30K there is a re-entrant insulating regime. In this regime, we observe glassy dynamics and a hierarchical response in the resistance to both electronic and magnetic perturbations. In addition, the magnetization of the film responds to electrostatic gating. We interpret these results in a framework where the system dynamics are governed by strain relaxation. This work was supported by the National Science Foundation under grant NSF/DMR-0138209 and the Univ. of Minnesota MRSEC (NSF/DMR-0212032).   

Resonant Higher Order Scattering in Double Perovskites

Resonant and non-resonant higher order Raman scattering in the double perovskites La$_{2-x}$Sr$_{x}$FeCrO$_{6-}$ (x = 0, 0.33, 0.66,1) and Ba$_{2}$(Sr$_{2})$FeWO$_{6}$ is investigated. The B-site disordered compound La$_{2}$FeCrO$_{6}$ displays an exceptional series of resonant higher order excitations for =514 nm (2.42 eV). This feature is attributed to defects, in the form of oxygen vacancies, resulting in a localized resonant electron-phonon coupling effect similar to the Franck-Condon effect predicted in the perovskite structured manganites. The resonant state is critically sensitive to Sr doping and . Higher energy excitations in the compounds Ba$_{2}$(Sr$_{2})$FeWO$_{6 }$are shown to be of non-resonant multiphonon character.   

Dynamical mean field study of Manganite superlattices

A theoretical study of 001 manganite superlattices $[La Mn O_3]_n [Sr Mn O_3]_m$ is presented. The superlattice is defined by different charges of $La$ and $Sr$; the conduction band is modelled via a nearest neighbor tightly binding model. The interaction between conduction electrons and localized $t_{2g}$ spins is treated by the dynamical mean field approximation, while the long range Coulomb interaction is taken into account by the Hartree approximation. The magnetic phase diagram and physical properties including charge distribution, dc and optical conductivity are calculated at different layer geometries. A range of physical parameters is explored. The research is supported by Columbia University MRSC (CL) and DOE-ER46169(AM)   

First-principles study of x-ray linear dichroism at the Mn $K$ edge of LaMnO$_3$

LaMnO$_3$ shows orbital ordering of Mn $3d$ ($3x^2-r^2/3y^2-r^ 2$) states associated with the cooperative Jahn-Teller distortion. Resonant x-ray scattering measured at the Mn $K$ edge by Murakami {\it et al.}\ shows peculiar azimuth-angle dependence, indicating that the degeneracy of the Mn $4p$ states should be lifted. For its mechanism, two models have been proposed. One is the Coulomb mechanism, that is, the $3d$ orbital ordering causes the level splitting of the $4p$ states through the intra-atomic $3d$-$4p$ Coulomb interaction. The other is the Jahn-Teller mechanism: the $4p$ bands are strongly influenced by the Oxygen displacements, not by the anisotropic $3d$ charge distribution resulting from the orbital ordering. The important point to note is that these two models yield opposite way of the $4p$ splitting. To resolve this argument, very recently, Maruyama {\it et al.}\ have carried out x-ray photoabsorption measurements at the Mn $K$ edge using linearly polarized light. The energy and polarization dependence of their spectra clear up the unoccupied Mn $p$ states, apparently supporting the Jahn-Teller mechanism. Furthermore, they found interesting azimuth-angle dependence as a function of photon energy. For its interpretation, detailed band-structure information is undoubtedly necessary. In this talk, using the first-principles FLAPW calculations, we will discuss the overall feature of the measured spectra and its relation to the $3d$ orbital ordering and Jahn-Teller distortion.   

Electronic structures of non-half-metallic antiferromagnetic double perovskites ALaVMoO$_6$ (A = Ca, Sr, and Ba)

Recently, double perovskites $A$LaVMoO$_6$ ($A=$ Ca and Sr) of the $Fm\bar{3}m$ space group were proposed experimentally to be half-metallic antiferromagnets.\footnote{Uehara, Yamada, and Kimishima, Solid St. Commun. \textbf{129}, 385 (2004).} The electronic structures and magnetism of the double perovskites $A$LaVMoO$_6$ ($A=$ Ca, Sr, and Ba) were determined within the generalized gradient approximation to density functional theory using the all-electron full-potential linearized augmented plane wave (FLAPW) method.\footnote{Wimmer, Krakauer, Weinert, and Freeman, PRB \textbf{24}, 864 (1981).} The $A=$ Ca case shows \emph{metallic} ferrimagnetism as the most stable phase, with magnetic moments of $1.15\;\mu_B$ for V and $-0.53\;\mu_B$ for Mo, whereas the Sr and Ba cases are calculated to be almost non-magnetic metals. Comparing the calculated density of states, we find that the heavier $A$ implies stronger hybridization between the divalent atom $sp$ states and the transition metal atom $d$ states. The stronger $sp$-$d$ hybridization is considered to be responsible for the suppression of magnetism for the Sr and Ba cases. These results, at least for the $Fm\bar{3}m$ space group, are in contrast with the recent experimental result proposing half-metallic antiferromagnetism for $A=$ Ca and Sr.   

Magneto-Optical Investigation of La$_{1-x}$Sr$_{x}$CoO$_{3}$\,($x$\,=\,0.15, 0.2)

A complete magneto-optical characterisation of the perovskite cobaltites La$_{0.8}$Sr$_{0.2}$CoO$_{3}$ and La$_{0.85}$Sr$_{0.15}$CoO$_{3}$ was performed using temperature dependent spectral generalised magneto-optical ellipsometry (SGME). The measurements cover the energy range from 1.5 to 5.5\,eV and temperatures between 175 and 25\,K. The complex diagonal and off-diagonal elements $\varepsilon_{xx}$ and $\varepsilon_{xy}$ of the dielectric tensor are determined simultaneously yielding enhanced sensitivity to the interplay between electronic and magnetic properties and thus to the electronic structure of the ferromagnetic phase. The investigated compositions are close to the phase boundary between unconventional ferromagnetism and a mixed phase displaying spin-glass as well as ferromagnetic behaviour at $x = 0.18$. While, for the 15\,\% doped sample, the amplitude of the magneto-optical response is found to be proportional to the net magnetisation, for the $x = 0.2$ sample this holds true only for temperatures $T\,\geq\, T'\,\approx\,100$\,K. Additionally, an energy shift of the order of 100\,meV of spectral features of $\varepsilon_{xy}$ is observed below $T'$.   

Ultrafast photo-induced melting of charge and orbital order in the manganite Nd$_{0.5}$Sr$_{0.5}$MnO$_3$

The physics of perovskite manganites is a subject of intense research in condensed matter physics due to the interplay between charge, spin, lattice, and orbital degrees of freedom in these materials. The manganite Nd$_{0.5}$Sr$_{0.5}$MnO$_{3}$ is ferromagnetic below its Curie temperature ($T_{c})$ of $\sim $250 K, exhibiting colossal magnetoresistance upon application of a magnetic field. Below $T_{co}\sim $150 K, it is antiferromagnetic with charge and orbital ordering, which can be ``melted'' upon application of a high magnetic field. Optical conductivity measurements revealed an optical gap below $T_{co}$, with Drude-like behavior for $T_{co} [Preview Abstract]

Instrinsic Insulators at the Surfaces of Layered Ferromagnetic Manganites

To explore loss of ferromagnetic order at the surfaces of manganites, we brought together a powerful combination of two surface probes, tunneling and polarized x-ray interactions, to study the intrinsic electronic and magnetic surface states of a layered manganite, La$_{2-2x}$Sr$_{1+2x}$Mn$_2$O$_7$, that is ferromagnetic and conducting in the bulk. These probes present clear evidence for an intrinsic insulating nonferromagnetic surface layer atop adjacent ferromagnetic subsurface layers. The presence of a nonferromagnetic surface layer of one bilayer thickness was observed with x-ray resonant magnetic scattering (XRMS), and point contact tunneling results show this layer is insulating (0.6 eV band gap), in agreement with the expectations of the double-exchange model. Temperature dependence shows the magnetization in the sub-surface bilayer retains the bulk value close to T$_c$. In addition the nonferromagnetic surface layer shows a linear response in fields up to 7T, consistent with antiferromagnetic order in the surface bilayer. This research, including the use of the Advanced Photon Source, was supported by the U.S.Department of Energy, Office of Science, under Contract No. W-31-109-Eng-38.   

Lattice Distortion due to Charge-ordering effects in single-layer La$_{0.5}$Sr$_{1.5}$MnO$_4$

We report measurements of the local structure of the single-layer manganite La$_{1-x}$Sr$_{1+x}$MnO$_4$ (x = 0.5) using polarized EXAFS at the Mn K-edge. Earlier diffraction data suggested a quadrupling of the simple perovskite unit cell, most likely from a charge/orbital ordering effect. From a careful analysis of the single crystal diffraction pattern, space group symmetry of the ordered structure is deduced to be B2mm (special space group settings employed to keep Mn-O planes perpendicular to the c-axis). In this space group symmetry there are 3 distinct Mn sites, but the diffraction data indicates that two of them are related by a pseudo-glide and the distinction between them is very subtle. For the EXAFS analysis we have imposed the glide symmetry and therefore assumed a more symmetric space group Bbmm, which contains only two unique Mn sites. We have further assumed that the predominant effect of the charge/orbital ordering is on the basal Mn-O bonds. Bbmm symmetry allows for 4 unique basal Mn-O bond distances, however, the EXAFS analysis shows that they are clustered in two distinct groups. Combining these results with analysis of single crystal diffraction data indicates that the primary mode of distortion of the basal Mn-O bonds is driven by the Jahn-Teller effect. (That is, each Mn has two close basal O neighbors and two distant basal O neighbors.) Support: NSF DMR0301971, and BES/DOE.   

Orbiton features in YTiO$_3$ and LaTiO$_3$ probed by Raman light scattering

The existence of collective orbital excitations, termed orbitons, in YTiO$_3$ has been proposed by G. Khaliullin et al. [1] and S. Ishihara [2]. We have performed Raman and inelastic neutron scattering experiments on LaTiO$_3$ and YTiO$_3$ single crystals in order to determine the excitation spectrum for energies above the range of optical phonons. There are two unexpected features in the high energy Raman spectrum of both materials. A sharp structure at 165 meV and a broad continuum like feature around 230 meV. Our theoretical calculations of the phonon dispersion relation have revealed, that the peak at 165 meV is right at the high energy cutoff of the two-phonon density of states, but it's unexpected large intensity cannot be explained with two-phonon processes alone. The temperature dependence, polarization dependence and the resonance behavior probed with different laser lines for excitation, yields that the feature at 230 meV can possibly be assigned to orbital excitations. [1] G. Khaliullin and S. Okamoto, PRB {\bf 68}, 205109 (2003). [2] S. Ishihara. PRB {\bf 69}, 075118 (2004).   

Session J42: Focus Session: Magnetic Nanoparticles, Nanostructures & Heterostructures IV

Nanomagnetism on Artificially modulated Single Crystalline Substrates

We have developed a new experimental technique under ultra-high vacuum to grow epitaxially the `compositionally wedged' alloys on single crystalline substrates, which can be used to produce single crystalline templates with continuously modulated lattice constants, as well as with gradually varied chemical compositions. As an example, it is applied to investigate Fe on Cu$_{x}$Au$_{1-x}$/Cu(100) grown at room temperature. Its structure and magnetism are determined as a function of compositions when the in-plane lattice constant is fixed (strained films) and when the in-plane lattice constants are variable (strain-released films). In this way, the structural and compositional contributions to the magnetic properties can be well separated and independently studied. The other applications of this new technique will also be discussed.   

Domain walls in antiferromagnetically-coupled multilayer films

We report experimentally observed magnetic domain-wall structures in antiferromagnetically-coupled multilayer films with perpendicular anisotropy [1, 2]. Our studies reveal a first-order phase transition from domain walls with no net moment to domain walls with ferromagnetic cores. The transition originates from the competition between dipolar and exchange energies, which we tune by means of layer thickness. The overall dipolar fields generated by such a ferromagnetic domain wall can be reduced by having the orientation of the ferromagnetic regions reverse periodically along the domain wall. This produces a one-dimensional equivalent of stripe domains that form in ferromagnetic thin films with perpendicular anisotropy. With increasing layer thickness these one-dimensional stripe domains evolve continuously into two dimensional stripe domains that fill the sample. Although observed in a synthetic antiferromagnetic system, such domains-wall structures may be expected to occur in A-type antiferromagnets with anisotropic exchange coupling. [1]. O. Hellwig \textit{et al}., Nature Materials \textbf{2}, 112 (2003). [2]. O. Hellwig, A. Berger and E. E. Fullerton, Phys. Rev. Lett. \textbf{91}, 197203 (2003).   

Nanostructural effects on the magnetic anisotropy in epitaxial thin films

We have observed singular behavior in the anisotropy properties during magnetization reversal of epitaxial thin films studied with longitudinal magneto-optical Kerr effect (MOKE). We have observed that annealed Ni films epitaxially grown on (001) MgO substrates exhibit additional uniaxial anisotropy superimposed to the expected four-fold anisotropy due to magnetocrystalline anisotropy. Non annealed films only show four-fold symmetry. We will show that this additional anisotropy can be explained in terms of nanostructural changes of the film surface via oxidation as well as interfacial changes. Further, annealed and non-annealed films exhibit peculiar high coercive values along the magnetization hard axes. This is particularly noticeable as ``spikes'' in a polar plot representation of the coercive field. We have proposed a model to explain these ``spikes'' in terms of a second order type of transition for the magnetization before reversal that implies enhanced domain nucleation during switching along these particular directions. In order to validate our model we have performed polarized neutron reflectometry (PNR) studies on these films. We will present our correlated MOKE and PNR studies.   

Anomalous Minor Loop Growth in Perpendicular Co/Pt-Multilayers

We have studied the evolution of magnetic minor loops upon multiple field cycling for Co/Pt-multilayers with perpendicular anisotropy. For this purpose, we saturated the samples in a sufficiently strong magnetic field first and then measured up to 20 successive minor loops with identical field cycles. In our experiment, the minor loop field cycles were chosen to be symmetric around zero, i.e. unbiased. While most theoretical models of magnetic materials assume the existence of a stable minor loop for multiple field cycles, many experiments show a slight drift of magnetic minor loops upon repeat cycling due to thermal excitation. For the thinnest (0.4 nm Co/0.7 nm Pt)$_{3}$-multilayers, we observe such a conventional small minor loop drift. However, somewhat thicker (0.4 nm Co/0.7 nm Pt)$_{8}$-multilayers show a very different and anomalous behavior. Here, we observe a continuous growth of the magnetization amplitude for successive minor loop measurements, revealing a multi-loop memory effect. The growth is found to be limited only by approaching the full magnetization reversal, even for field amplitudes as small as 0.8*H$_{c}$. The amplitude and cycle dependence of this effect is experimentally characterized and can be consistently explained by a strong asymmetry between the nucleation and annihilation mechanism of domains in these structures.   

Effect of disorder on itineracy and magnetism in ultra thin Fe and Fe/C$_{60}$ films

We present \textit{in situ} measurements of the temperature and field dependence of the longitudinal ($R_{xx})$ and transverse ($R_{xy})$ resistance of ultra thin (d $<$ 50 {\AA}) Fe thin films and Fe/C$_{60}$ thin-film composites. The study is motivated by the question as to what happens in a band ferromagnet when the ability of the itinerant electrons to transfer spin is compromised by disorder. We use the sheet resistance $R_{xx}$ as a measure of disorder and correlate this quantity with the saturation field, the saturated moment and the carrier concentration as derived from normal and anomalous Hall effect (AHE) measurements. We find that in the weakly disordered regime ($R_{xx} \le 5000 \quad \Omega )$ where logarithmic temperature dependences dominate, there is a suppression of the saturated moment and the scaling relations $\partial R_{xx} /R_{xx} = \quad \partial R_{xy}^{AHE} /R_{xy}^{AHE} =$ $-\partial \sigma _{xx}^{AHE} /\sigma _{xx}^{AHE} $ hold. When a monolayer of C$_{60}$ is predeposited on the substrate, stable Fe/C$_{60}$ films can be made. In the strongly disordered regime where the sheet resistance at low temperatures ($\sim $5~K) is approaching 1~M$\Omega $ (a value well above the quantum limit), the saturated moment as determined by AHE measurements is still remarkably robust.   

Ferromagnetic Switching of Nb/Ni Multilayers and Trilayers in the Superconducting and Normal States

Ni(y)[Nb(x)/Ni(y)]$_{z}$ multilayers (z = 5, 8) with x = 23, 10 nm and y = 2.5, 3.5, 5 nm, and Nb(x)/Ni(y)/Nb(x) and Ni(y)/Nb(x)/Ni(y) trilayers with x = 23, 200 nm and y = 5 nm, were investigated via SQUID magnetometry with magnetic field parallel to the film plane. The superconducting transition temperature T$_{c}$ of samples was sometimes reduced well below 8 K, depending upon the Nb layer thickness x and the total number of Ni layers. Abrupt, reproducible switching anomalies are observed for multilayers, and complex magnetization curves observed for trilayers, in the superconducting state, instead of the smooth ferromagnetic hysteresis seen in the normal state. These results suggest that a complex magnetic coupling exists between Ni layers when mediated by Nb layers in the superconducting state.   

The Dynamic Phase Transition in Ultrathin Co/Pt-Multilayer Films: Experimental Evidence and Comparison with Simulations

The dynamic phase transition (DPT), observed in numerical simulations of magnetic systems [1,2], manifests itself by the spontaneous occurrence of a non-vanishing period-averaged magnetization (the order parameter $Q$) when the frequency $f$ of an applied alternating magnetic field exceeds a critical value $f_c$. Near $f_c$, the DPT shows all common characteristics of a second-order phase transition. Our experimental studies of ultrathin Co/Pt-multilayers provide the first strong experimental evidence of a DPT. The multilayer structure results in perpendicular anisotropy and negligible demagnetizing effects [3]. We measure out-of-plane magnetization time series by the polar-Kerr effect as a function of $f$ and an applied bias field $H_b$, observing a sharp increase in $Q$ as $f$ is increased above $f_c$. In addition, we see sharp switching of $Q$ as $H_b$ is changed from positive to negative values. The data sets allow the assembly of an experimental phase diagram. Detailed comparison with simulations of a kinetic Ising model provides strong evidence that our data represent the first unequivocal experimental observation of the DPT. [1] S.W.\ Sides et al., Phys.\ Rev.\ Lett.\ {\bf 81}, 834 (1998) [2] B.\ Chakrabarti and M.\ Acharyya, Rev.\ Mod.\ Phys.\ {\bf 71}, 847 (1999) [3] Y.Yafet and E.M.\ Gyorgy, Phys.\ Rev.\ B {\bf 38}, 9145 (1988).   

Spin-reorientation transition in exchange-coupled (Pt/Co)n/Sm-Co multilayers

Exchange coupling through Pauli-paramagnetic spacer layers is a scientifically interesting phenomenon with applications in sensors and magnetic recording. We have investigated the interplay between interlayer exchange and competing anisotropies. Due to interface anisotropy, the (Pt/Co)$_{n}$ multilayer exhibits an out-of-plane easy axis (perpendicular anisotropy). The Sm-Co layer has in-plane anisotropy. The exchange between the (Pt/Co)$_{n}$ and Sm-Co layers is tuned by varying the thickness of the Pt spacer layer. In the (Pt5{\AA}/Co4{\AA})$_{n}$/Sm-Co40{\AA} system, we observe a spin-reorientation transition at a spacer-layer thickness of somewhat less than 5 {\AA}. Above this threshold, the Pt/Co retains its out-of-plane anisotropy, and a low-temperature coercivity of 3 kOe is obtained. Below the threshold, the Pt-Co film is exchange-coupled to the Sm-Co layer and the spin structure of (Pt/Co)$_{4}$ changes, with a low-temperature coercivity of 200 Oe. The transition is also seen in the hysteresis-loop shape. The samples exhibit a low-temperature loop shift after field cooling, which is not observed after zero-field-cooling. The behavior of the film is modeled and discussed as a function of the Pt/Co bilayer period n and of the spacer-layer thickness. ­ This research is supported by DOE, ARO, the W. M. Keck Foundation, INSIC, and CMRA.   

Nanofabrication as A Probe of Anisotropy Distribution in Co/Pd Multilayers

Arrays of patterned magnetic islands typically exhibit switching field distributions (SFD) which are much broader than those predicted based solely on the dipolar fields from neighboring islands. The source of the broad distributions is not understood. To determine if the source of the broad island SFD arises from intrinsic film properties or extrinsic sources we have fabricated arrays of Co/Pd islands with perpendicular anisotropy and sizes ranging from 30 nm to 5$\mu $m. The islands displayed single domain remanent states after easy axis saturation. The island array's coercivity and SFD increase with decreasing island size. The observed angle dependent switching fields closely resemble the behavior predicted by the Stoner-Wohlfarth model with a minimum at 45 degrees. This angle dependence is expected for small islands which may reverse by rotation, but is surprising for the larger islands. As expected, the continuous film exhibits the 1/cos dependence as predicted for domain wall motion controlled reversal. These data lead to a model whereby the reversal of the larger islands is controlled by a nucleation event, followed by a rapid wall motion. The observed switching field of the island is thus the switching field of the softest nucleation site in the island, and the distribution of these nucleation energies in the full grown film is the source of the island SFD.   

Ferromagnetic Phase Diagram for Nanoscale Epitaxial (Co,Mn) Alloys on GaAs(001)

A important goal of device and materials research is directed at discovering ways to combine the properties of ferromagnets and semiconductors. Our current work is aimed at synthesizing ferromagnetic materials which are compatible with GaAs. In a previous study, it was shown that the Heusler ferromagnet, Co$_2$MnAl can be grown epitaxially on GaAs and has a Curie temperature T$_C$=800-1000~K.$^{[1]}$ In the present work, nanoscale alloys of Co$_{1-x}$Mn$_x$ were grown epitaxially on GaAs(001) substrates. Nanometer thick layers of (Co,Mn) are found to be ferromagnetic over a wider range of x-values than for bulk materials --- x$\leq$0.8 for 5-10~nm thick films, compared to only x$\leq$0.32 for bulk materials. The Curie temperature of (Co,Mn) nanometer films decreases linearly for increasing x, from T$_C$=800~K at x=0.35 to T$_C$=0 at x=0.8. The extended range of x for ferromagnetism is attributed to the strained lattice which is in registration with the GaAs lattice. Work supported by NSF-DMR-0305360. [1] Y. Chen, D. Basiaga, J.R. O'Brien, and D. Heiman, \emph{Anomalous magnetic properties and Hall effect in ferromagnetic Co$_2$MnAl epilayers}, Appl. Phys. Lett. \textbf{84}, 4301 (2004).   

Magnetocapacitance and surface magnetism in Pd/Fe/Pd trilayer structures

For sufficiently thin insulator spacing in metal-insulator-metal (M-I-M) trilayer structures, the capacitance can be dominated by the interface of the dielectric and the electrodes. If one or both of the electrodes are ferro- or paramagnetic, the screening length is influenced by a difference in the spin-up and spin-down densities of states, and the resulting magnetic-field induced changes in capacitance (magnetocapacitance) become a sensitive measure of surface magnetism. We have grown Pd (200 {\AA})/Fe~(1.5 {\AA})/Pd ($x~${\AA}) trilayer structures, where $x$ was varied from 50~{\AA} to 2~{\AA}. All of the films are ferromagnetic having similar saturation magnetizations at 10~K. However, as $x$ is decreased, there is a significant increase in the coercive field ($H_{c})$ from $H_{c}\sim $7~Oe for $x$~=~50~{\AA} to $H_{c}\sim $30~Oe for $x$~=~2 {\AA}. The sensitivity of magnetic properties to the proximity of the interface reflects a cross over from bulk to surface-dominated magnetism. We will correlate this crossover with magnetocapacitance measurements on M{\-}I{\-}M capacitors where the Pd/Fe/Pd trilayer is the base electrode and $x$ is the separation of the Fe layer from the interface with the dielectric of the capacitor.   

Low temperature magnetism in Sb$_{2-x}$V$_x$Te$_3$ thin film grown on sapphire substrate by MBE

Ferromagnetic semiconductors containing transition ions have received considerable attention since they are promising materials for spintronics application. However, their Curie temperatures need to be increased in order to be useful. In this work, good quality ferromagnetic semiconductor Sb$_{2-x}$V$_{x}$T$_{3}$ thin films have been grown on sapphire (0001) substrate by non-equilibrium MBE technique. The change from diamagnetism to ferromagnetism with the increasing concentration of V in the Sb$_{2}$Te$_{3}$ matrix has been studied via temperature dependent magnetic susceptibility and magnetotransport measurements. The solubility limit of V in the Sb$_{2}$Te$_{3}$ thin films has been studied by X-ray diffraction and EPMA (Electron Probe Microanalysis). The anisotropic dependence of the magnetization in Sb$_{2-x}$V$_{x}$T$_{3}$ thin films has been studied. The underlining physical mechanism that leads to the long-range magnetic order in Sb$_{2-x}$V$_{x}$T$_{3}$ will be discussed.   

Buried-interface characterization in magnetic nanostructures using standing wave-excited x-ray emission and resonant inelastic x-ray scattering

Yang et al. (J. Phys. Cond. Matt. \underline {14}, L406 (2002)) have discussed a new method for studying buried interfaces using soft x-ray standing waves created by Bragg reflection from a multilayer mirror, combined with a wedge-shaped sample profile. Prior work has been based on photoemission, a photon-in/electron-out spectroscopy. We will here discuss the first experimental results of applying this method via more bulk-sensitive photon-in/photon-out spectroscopies: x-ray emission (XES) and resonant inelastic x-ray scattering (RIXS). We have measured XES and RIXS intensities from an Fe/Cr bilayer that is a prototypical giant magnetoresistance combination via both sample-scanning and rocking-curve methods. Magnetic circular dichroism has also been measured in Fe RIXS spectra. Combining this data with x-ray optical calculations permits determining the compositional and magnetic structure of the Fe/Cr interface. Work supported by DOE Off. of Science, Basic Energy Sciences, Mat. Sci. Div.   

Session J43: Low Dimensional Magnetism: General Topics

Proton NMR study of the local magnetic field and fluctuations in a single crystal of the quasi-2D organic conductor $\lambda$-(BETS)$_{2}$FeCl$_{4}$

Measurements of the proton NMR spectrum and spin-lattice relaxation rate 1/T$_{1}$ in a single, 3 $\mu$g crystal of the quasi-2D organic conductor $\lambda$-(BETS)$_{2} $FeCl$_{4}$ at a magnetic field B$_{0}$ = 9 T $\parallel$ $a-c$ plane over the temperature ($T$) range 2-180 K are reported. They probe the static local field and its fluctuations in both the antiferromagneic insulator (AFI) and paramagnetic metal (PM) phases. As $T$ is decreased, there is an increase in the shift and the overall width of the NMR spectrum and a jump in these features at the PM-AFI transition ($\sim$4 K). A reasonable fit to these properties in the PM phase is obtained using the dipole field of non- interacting Fe$^{+3}$ ions. These features show that the proton spectrum and 1/T$_{1}$ are dominated by the Fe$^{+3}$ spins. The work at UCLA was supported by NSF Grants DMR-0334869 (WGC) and DMR-0203806 (SEB) and that at Indiana by the Petroleum Research Fund ACS-PRF-33912-ACI.   

Far-infrared and high field electron spin resonance study of magnetic and phonon excitations in the two dimensional cuprate Na$_5$RbCu$_4$(AsO$_4$)$_4$Cl$_2$

Na$_{5}$RbCu$_{4}$(AsO$_{4})$Cl$_{2}$ is a novel low dimensional magnetic material, where the magnetic properties are determined by layers Cu$_{4}$O$_{4}$ tetramers [S.-J.~Hwu \textit{et al}., J. Am. Chem. Soc.,\textbf{ 124},~12404 (2002)]. There is a second order phase transition at 17K, that likely involves the onset of antiferromagnetic order [J.A. Clayhold \textit{et al}., Phys. Rev. B, \textbf{66}, 052403 (2002)]. We have measured the far-infrared spectra and high field electron spin resonance spectra at low temperatures. A triplet excitation is observed at 12.68 cm$^{-1}$. The magnetic field dependent triplet components with the electron g-factor $g_{b}$=1.44 are split in zero field by 0.75 cm$^{-1}$. Several temperature dependent resonances are also observed. The results are being analyzed in the framework of a four-tetramer model.   

Neutron Scattering Study on a S=1/2 Quasi-two-dimensional Magnet (CuCl)LaNb$_2$O$_7$

Magnetic properties of polycrystalline (CuCl)LaNb$_2$O$_7$ were studied by magnetic susceptibility and neutron scattering measurements. Magnetic susceptibility exhibits a broad maximum around 16 K, followed by a rapid decrease with decreasing temperature. Powder neutron diffraction measurement provides no evidence of magnetic ordering down to 1.5 K. Furthermore inelastic neutron scattering measurement shows magnetic excitation at around 2.1 meV. These experimental results suggest that this compound has a spin-singlet ground state which is separated from the excited state with a finite-energy gap.   

Magnegtic Phase Diagram and Specific Heat of the Quasi-Two-Dimensional S=1/2 Antiferromagnet Cs$_2$CuBr$_4$

Cs$_2$CuBr$_4$ is an excellent laboratory model for the S=1/2 Heisenberg antiferromagnet on a triangular lattice, with a small anisotropy. In a magnetic field, the most distingushing feature of this antiferromagnet is a magnetization plateau, which occurs at 1/3 of the saturation magnetization. We have determined the magnetic phase diagram of Cs$_2$CuBr$_4$ for magnetic fields up to 20 T, using specific-heat measurements and the magnetocaloric effect. We find that the ordering temperature in the plateau region is slightly higher than in the surrounding regions of the phase diagram. The transition to the plateau phase is first order, if it is from the adjacent ordered phase at either lower or higher magnetic fields, whereas the transition from the high-temperature disordered phase is second order. Surprisingly, the temperature dependence of the specific heat indicates that the excitations in the plateau phase are gapless.   

High field magnetization, specific heat and NMR of the two dimensional tetramer-cuprate Na5RbCu4(AsO4)4Cl2

Complex novel 2D compound Na$_{5}$RbCu$_{4}$(AsO$_{4})$Cl$_{2}$ is a unique magnetic material, which contains layers of Cu$_{4}$O$_{4}$ tetramers [S.-J.~Hwu \textit{et al}., J. Am. Chem. Soc.,\textbf{ 124},~12404 (2002)]. The ground state following a second order low entropy phase transition at 15(1) K is antiferromagnetically ordered one [J.A. Clayhold \textit{et al}., Phys. Rev. B, \textbf{66}, 052403 (2002)]. We have measured the $^{87}$Rb and $^{35}$Cl NMR spectra and high field magnetization and specific heat at low temperatures. We discuss the field dependence of T$_{N}$, hyperfine couplings at various nuclear sites, and characteristics of the ordered state as seen from Rb and Cl nuclear sites.   

Laser-controlled local magnetic field using semiconductor quantum rings

We analize theoretically the dynamics of $N$ electrons localized in a semiconductor quantum ring under a train of phase-locked infrared laser pulses. The pulse sequence is designed to control the total angular momentum of the electrons. The quantum ring can be put in metastable states characterized by a persistent current much stronger than the one generated by an Aharonov-Bohm flux. The local magnetic field created by these currents can be used for a selective quantum control of single spins in semiconductor systems.   

Spin-gap state and the structural phase transition in delta-phase NH$_4$V$_4$O$_{10}$

The magnetic properties of the delta-phase vanadium oxide NH$_{4}$V$_{4}$O$_{10}$ are studied using a SQUID Magnetometer. Temperature dependence of the magnetic susceptibility reveals a broad maximum at 200 K and a drop of susceptibility at 120 K consistent with a spin-gap behavior. The magnetic network in NH$_{4}$V$_{4}$O$_{10}$ can be described as a system of spin 1/2 Heisenberg two-leg ladders with antiferromagnetic exchange. However, a theoretical temperature dependence of the susceptibility produced by this model does not fit the experimental data, as the measured susceptibility drop is much steeper than that predicted by the model. Low-temperature X-ray studies have shown that the transition to the spin-gap state is accompanied by an increase of V-V leg distance, and decrease of V-V rung distance in the ladders. Therefore, the rung exchange interaction in the low-temperature phase becomes much stronger leading to a spin-gap state and the sharp drop of the susceptibility. Magnetic properties of other delta-phase vanadium oxides, Li$_{x}$V$_{4}$O$_{10}$ and Li$_{0.6}$(C$_{2}$H$_{8}$N)$_{0.4}$V$_{4}$O$_{10}$, are also studied and explained by linear chain Heisenberg model. The relation between the magnetic and structural parameters of the compounds is discussed. This work was supported by NSF DMR 0313963.   

The XYZ-model in a magnetic field

We present a numerical study of the one-dimensional XYZ-model in a magnetic field. In particular we discuss DMRG- and Lanczos results for the gaps, the staggered and uniform magnetisation, the phase diagram and the quantum critical point of the model. We also consider thermodynamic properties.   

The observation of lattice distortion in TiOBr by using synchrotron X-rays diffraction

TiOBr has the same structure of TiOCl, and the physical properties of TiOBr were very similar to that of TiOCl. These systems attract attention as the new candidate of spin-Peierls transition accompanying orbital ordering. We performed the specific heat measurement, magnetization measurement and $\mu $SR measurement, and checked that it was the candidate of a spin-Peierls transition. However, lattice modification of TiOBr has not been observed directly before. Therefore, in this study, the X-rays diffraction measurement was performed at BL46XU of SPring-8, Japan. It succeeded in observation of the super-lattice spot by lattice modification of TiOBr, and succeeded also in observation of the temperature change. Furthermore, we investigate the superlattice structure.   

Generic Chern Numbers for a Degenerate Multiplet: For a Characterization of Topological Orders

Chern numbers for a multiplet which is a set of low lying states near the ground states are defined and an explicit expression is obtained in a gauge dependent form. We allow intrinsic degeneracies within the multiplet where a well-known standard procedure does not work. As an example, we give expressions for a spin Hall conductance for unitary superconductors with equal spin pairing. Generic topological orders will be treated in this manner particularly with nontrivial topological degeneracies.\footnote{Y. Hatsugai, J. Phys. Soc. Jpn, 2604, (2004), cond-mat/0405551}. It can be useful for a characterizaton of some class of low dimensional quantum (spin) liquids with topological degeneracies. \footnote{Y. Hatsugai, preprint}   

Magnetochromism in the Quasi-2D Heisenberg Antiferromagnet, K$_2$V$_3$O$_8$

We present the optical and magneto-optical properties (0 - 32 T) of the S = 1/2 quasi-two-dimensional Heisenberg antiferromagnet, K$_2$V$_3$O$_8$. A large magnetochromic effect, centered at $\sim$2.6 eV, is observed at 4.2 K, and it is attributed to a change in the V$^{4+}$ on-site excitation. An additional very sharp magneto-optical feature near $\sim$1.2 eV may be associated with a vibronic excitation. Fine structure allows us to identify the coupling phonon. These field-induced color changes point toward a 12 T transition involving local distortions. \newline \newline *Work at the University of Tennessee is supported by the Materials Science Division, Basic Energy Sciences, U.S. Department of Energy (Grant DE-FG0201ER45885). Oak Ridge National Laboratory is managed by UT-Battelle, LLC, for the U.S. Dept. of Energy under contract DE-AC05-00OR22725.   

NMR studies of incommensurate quantum antiferromagnetic state of LiCuVO$_{4}$

We report $^{51}$V NMR measurements in the linear spin-chain compound LiCuVO$_{4}$ single crystals. High temperature $^{51}$V spectra exhibit a classic quadrupole-split line expected for $I$ = 7/2 nuclei (splitting disappears below 50K). Linewidth anomalies correlated with characteristic features of the resonance shift were observed near 25K, 6K, and 2K. This is attributed to increased spin correlations leading to 3D antiferromagnetic order at low temperatures. The Knight shift tracks susceptibility giving a transfered hyperfine field of A$_{hf }$ = 6.5 kOe/$\mu _{B}$ on the V nuclei. Below 2K for a field parallel to crystal $c$-axis the spectra exhibit a broad two-peak feature which is absent for field along the $b$-axis. The moment orientation determined from the spectra suggests an incommensurate AF modulation along the $b$-axis in agreement with recent neutron scattering data.$^{1}$ Spin dynamics through relaxation measurements will be discussed. $^{1}$ B. J. Gibson, \textit{et al.,} Physica B350, e253 (2004).   

Magnetic interactions and spin waves in LiCu$_2$O$_2$

Magnetic excitations in the spiral magnet LiCu$_2$O$_2$ are studied by thermal and cold neutron scattering techniques. The spin models are discussed based on coupled ladder model including four exchange parameters; rung interaction $J_1$, nearest neighbor inchain interaction $J_2$, next-nearest neighbor inchain interaction $J_4$, and inter-ladder coupling $J_ {\perp}$. The magnetic dispersion suggests that antiferromagnetic $J_1$, {\it ferromagnetic $J_2$}, and antiferromagnetic $J_4$ induce frustration in this compound. Weak inter-ladder coupling was also observed. This work was carried out under DOE Contract No. DE-AC05-00OR22725.   

Magnetic excitations in weakly coupled classical and quantum spin systems

Magnetic excitations in the ordered state of Cu$_2$Fe$_2$Ge$_4$O$_{13}$ is studied by inelastic neutron scattering technique. Low energy excitations up to 10 meV is well explained by coupled classical chains of Fe$^{3+}$. Checkerboard like intensity patterns in wide reciprocal space is reproduced by spin-wave theory including magnetic non-Bravis lattice. The distinct narrow-band excitation at 25 meV is identified as quantum dimer excitation of Cu$^{2+}$. The cooperative ordering at low temperature is a result of weakly coupling between classical and quantum entities in this compound. This work was carried out under DOE Contract No. DE-AC05-00OR22725.   

Symmetry and light coupling to magnetic excitations in SrCu2(BO3)2

We present a T = 3 K Raman scattering study in the spin-dimer compound SrCu$_{2}$(BO$_{3})_{2}$. The symmetry of the elementary triplets and magnetic bound states in the 0 - 60 cm$^{-1}$ energy range are experimentally determined. A 4-spin cluster approximation is able to reproduce the symmetry and the anisotropic dispersions in external magnetic fields of the spin gap modes. We propose a light coupling mechanism induced by intra-dimer antisymmetric interactions and discuss the observation of a distinct Raman resonance behavior of the elementary and composite magnetic modes.   

Magnetic field effect on the spin-wave gaps in the real antiferromagnets

An unusual dependence of the excitation spectrum of a 2D antiferromagnet with a weak Dzyaloshinskii-Moriya interaction $D$ on the external magnetic field $H$ is studied. While the ${\bf k}=0$ gap follows closely the ``uniform precession'' behavior, $\Delta_0\sim H$, the gap at the antiferromagnetic ordering vector $\Delta_{{\bf Q}_{AF}}$ evolves from $\sim D$ at $H=0$ to $\propto \sqrt{D}$ at $H\sim H_s/2$ and then to $\propto D^{2/3}$ at $H=H_s$, where $H_s$ is the saturation field. In small fields this gap shows a non-analytic behavior $\Delta_{{\bf Q}_{AF}}\sim \sqrt{H}$. These results are directly applicable to the 2D AF K$_2$V$_3$O$_8$. The mutual effect of the magnetic field and DM interaction on the quantum fluctuations is also studied.