Session J1: Cuprate Superconductors: Underdoping and Pseudogaps

Nernst effect, fluctuation diamagnetism and vortices above Tc in cuprates

Nernst-effect and torque magnetometry experiments have provided evidence that, in the hole-doped cuprates, long-range phase coherence vanishes at the critical temperature $T_c$, while the pair condensate survives to a much higher ``onset" temperature $T_{onset}$. In the Nernst experiment, the vortex current produced by a gradient generates a Josephson $E$-field perpendicular to the applied field $\bf H$. In cuprates, this large Nernst signal $e_N$ persists to $T_{onset}\sim$ 130 K. Extensive Nernst experiments in the cuprates LSCO, Bi 2201, and 2212 yield a 3D phase diagram $(x,T,H)$ in fields up to 45 T. This picture has been confirmed by high-resolution torque magnetometry. In a tilted $\bf H$, local planar supercurrents associated with vortices above $T_c$ produce a torque that deflects a cantilever. At each $T$, the diamagnetic magnetization inferred matches the field profile of the Nernst $e_N$. The high-resolution measurement of the diamagnetic susceptibility $\chi$ over 5 field decades uncovers an unusual, fragile ``London rigidity'' that exists in the pseudogap state of Bi 2212 and 2201. The magnetization curves below $T_c$ also provide a reliable determination of the upper critical field $H_{c2}$ which is found to scale linearly with $T_{onset}$. I will also preview evidence for pairing without phase coherence at 0.35 K in LSCO for $x < x_c$ in fields to 30-45 T. \newline \newline In collaboration with Yayu Wang, Lu Li, Joseph G. Checkelsky, Michael Naughton, Seiki Komiya, Shimpei Ono, Yoichi Ando, Shin-ichi Uchida and Genda Gu.   

Controversial Issues in High-T$_c$ Superconductivity - a Specific Heat Perspective

We briefly review specific heat data on the evolution with hole doping of HTS cuprates and discuss the results in terms of current models. We see a universal progression from insulator to overdoped metal via a states-non-conserving approximately V-shaped pseudogap in the qp DOS. The gap shrinks with p due to the accumulation of new spectral weight ($\sim$ 1 state per doped hole) on the shoulders of the pseudogap (the antinodal regions) and closes abruptly close to optimal doping accompanied by a rapid increase in superconducting (SC) condensation energy. Thermodynamic measurements show no features (even broadened) at the temperature T* at which the pseudogap is generally presumed to close, and that the spectral weight loss persists to temperatures well above T*. This suggests that the pseudogap is not due to a Fermi surface instability or precursor SC fluctuations and that the pristine Fermi surface is not restored at T*. Specific heat and NMR measurements also reveal a rather high degree of SC homogeneity, casting doubt on the popular inference of gross SC gap inhomogeneity revealed by some tunnelling studies.   

From Fermi Arcs to the Nodal Metal

The pseudogap phase in the copper oxide superconductors is a most unusual state of matter, and understanding its nature will likely resolve the issue of what interactions give rise to the superconductivity itself. Angle resolved photoemission has revealed that the pseudogap phase is characterized by a partially truncated Fermi surface, denoted as a Fermi arc. We have found that the arc length is proportional to T/T*, where T* is the pseuodgap temperature. Therefore, in the zero temperature limit, the pseudogap phase has the same nodal structure as the d-wave superconducting phase. Attempts to explain this novel behavior by a variety of theoretical models will be discussed, as well as the fate of these Fermi arcs once superconductivity sets in.   

Effect of strong correlations on transport properties of disordered cuprates

The theory of thermal transport in a $d$-wave superconductor predicts a universal $T$-linear term $\kappa_0$ at low temperatures. Measurements on several cuprate families down to the 50 milliKelvin range indicate that the linear term decreases with underdoping, from which a substantial increase of the slope of the order parameter near the nodes is usually deduced by comparison with the standard theory. We discuss ways in which low-$T$ universal transport can break down, and in particular focus on the importance of strong correlations, which can induce local magnetism in the presence of disorder or other spatial perturbations. Static magnetism coexisting with superconductivity has been detected in some but not all cuprate families, particularly at low temperatures and for strongly underdoped samples, We present an interpretation of this superconducting ``spin glass'' state as local antiferromagnetic order driven by dopant atoms, particularly in the LSCO and BSCCO systems. Within this framework, recent NMR experiments on Zn-doped YBCO can also be quantitatively explained, down to detailed description of the lineshapes. Both the strong correlations and the quantum interference of impurity states appear to be vital to understand these results. In either more disordered or more underdoped systems, the tendency towards static magnetism is enhanced. Numerical solutions of the Bogoliubov-de Gennes equations of a disordered $d$-wave superconductor with Hubbard-like correlations show that in this case $\kappa_0$ is in fact strongly suppressed, universality of quasiparticle transport is violated and $\kappa_0$ may no longer be used to extract the size of the gap near the node directly. \footnote{B.M. Andersen and P.J. Hirschfeld, cond-mat/0607682, J.W. Harter et al., cond-mat/0609721 }   

Inelastic Tunneling, Electronic Nanoscale Inhomogeneities and Local Pairing in Superconductor with Inhomogeneous Bosonic Modes

There is a growing experimental evidence that nanoscale electronic inhomogeneity plays defining role in a growing classes of materials. Recently scanning tunneling spectroscopy has reached the stage where electronic properties of materials can be imaged on a nanometer scale. Local tunneling data that indicate strong nanoscale inhomogeneity of superconducting gap in high temperature superconductors[1,2]. Strong local nanoscale inhomogeneity in the bosonic scattering mode has also been observed in the same samples. We argue that these two inhomogeneities are directly related to each other. To address local boson scattering effects, we would need to develop a local strong coupling model of pairing in a coarse grained superconducting state. I will present a simple strong coupling model that yields features that are broadly consistent with the doping and isotope substitution trends observed experimentally. Oxygen isotope substitution O16 -$>$O18 reveals nontrivial changes in boson mode energy. These changes and changes in electron-boson coupling will also be discussed. [1] A. V. Balatsky and J.-X. Zhu, Local Strong Coupling Pairing in d-wave superconductors, Phys. Rev. B \textbf{74}, 094517 (2006). A.V. Balatsky, Ar. Abanov and J.X. Zhu, Inelastic Tunneling Spectroscopy in d-wave Superconductor, Phys. Rev. B 68, 214506 (2003). J. X. Zhu et al, Effects of Collective Spin Resonance Mode on STM spectra in d-wave Superconductor, Phys. Rev. Lett. v 92, 017002, (2004). [2] J. Lee et al, Interplay of electron--lattice interactions and superconductivity in Bi2Sr2CaCu2O8 , Nature, v \textbf{442}, p 546, (2006).   

Session J2: Prize Session (DCP, DCOMP, GSNP)

Frontiers of Surface Science. Structure, Bonding and Dynamics on the Nanoscale at High Pressures and at the Buried (solid-liquid and solid-solid) Interfaces

Model surfaces from single crystals to monodispersed nanoparticles are investigated at high pressures and at liquid interfaces by sum frequency generation (SFG) vibrational spectroscopy and high pressure scanning tunneling microscopy. The phenomena discovered by surface studies at low pressures in the past, adsorbate-induced restructuring, the chemical activity of surface defects, surface mobility of adsorbates and coadsorption-induced ordering are detected at high pressures as well. Newly discovered surface phenomena include the low melting point of nanoparticles, the coadsorption of water and hydrogen at polymer and metal surfaces, respectively, and hot electron flow during exothermic processes across oxide-metal interfaces (nanodiodes). Applications of surface science expanded into nanosciences, catalysis, tribology, polymers, biointerfaces, microelectronics, energy conversion and environmental chemistry will be discussed.   

Aneesur Rahman Prize Talk

During the past decade there has been a unique synergy between theory, experiment and simulation in Soft Matter Physics. In colloid science, computer simulations that started out as studies of highly simplified model systems, have acquired direct experimental relevance because experimental realizations of these simple models can now be synthesized. Whilst many numerical predictions concerning the phase behavior of colloidal systems have been vindicated by experiments, the jury is still out on others. In my talk I will discuss some of the recent technical developments, new findings and open questions in computational soft-matter science.   

Conformation-specific spectroscopy and dynamics in the complexity gap

Studies of the spectroscopy and conformational isomerization dynamics of flexible molecules typically fall into one of two size regimes: (i) small-molecule studies in which the molecule possesses two minima and a single barrier (e.g., \textit{cis-trans} isomerization about a double bond) or (ii) large macromolecules for which it is impossible to describe the potential energy surface in exhaustive detail (e.g., protein folding). Between them is a `complexity gap' of considerable proportions. This talk will describe our group's contributions to studies of molecules that are in that complexity gap in the sense that they have potential energy surfaces containing tens to hundreds of minima, and many times that number of transition states. By employing double resonance laser spectroscopy of isolated molecules cooled in a supersonic expansion, it is possible to obtain the ultraviolet and infrared spectral signatures of the individual conformational isomers of these molecules free from interference from others present in the sample. This foundation of spectroscopic data also serves as the basis for conformation-specific studies of the dynamics of conformational isomerization. In these studies, either infrared excitation or stimulated emission pumping (SEP) is used to excite a single conformation with a well-defined internal energy, thereby initiating conformational isomerization. By re-cooling the products prior to interrogation downstream, the energy thresholds for isomerization between individual X$\to $Y reactant-product pairs can be determined. Several examples from our recent work will be given to illustrate the kinds of insight that can be drawn from these studies regarding the conformational preferences, spectral signatures, barrier heights and relative energies of minima, fractional abundances, isomerization pathways, and internal energy flow accompanying isomerization.   

Nicholas Metropolis Award Talk: Quasi-static Modeling of Plasma and Laser Wakefield Acceleration

Plasma wakefields driven by intense ultrashort charged particle or laser beams can sustain acceleration gradients three orders of magnitude larger than conventional RF accelerators. These wakefields are promising for accelerating charged particles in short distances for applications such as an energy booster of a linear collider and as a ultra-compact accelerator. In the Plasma Wakefield Accelerator (PWFA) or Laser Wakefield Accelerator (LWFA), the space charge force of an electron beam or the ponderomotive force of a laser beam expels plasma electrons away from its path, forming a bubble-like structure where the longitudinal electric field inside of it provides accelerating and the transverse Lorentz force provides focusing forces on electrons. Recently, quasi-monoenergetic beams from self-trapped plasma electrons in wakefields driven by intense laser beamd have been observed in experiments in many laboratories around the world, and a PWFA experiment performed at Stanford Linear Accelerator Center (SLAC) successfully demonstrated that the energy of particles at the tail of the driving electron can be doubled from $\sim$40 GeV to $\sim$80 GeV in just 80 cms. However, to fully understand these experiments requires a particle-based computer model because the interaction between the plasma and the driver is highly nonlinear. We have developed a highly efficient, fully parallelized, fully relativistic, three dimensional particle-in-cell code, QuickPIC, for simulating plasma wakefield acceleration. The model is based on what is called the quasi-static or frozen field approximation, which assumes that the driver does not evolve during the time it takes for it to pass a plasma particle and reduces a fully three-dimensional electromagnetic field calculation and particle push into a two-dimensional electrostatic field solve and particle push. This algorithm reduces the computational time by at least 2 to 3 orders of magnitude. Comparison with a fully explicit PIC model (OSIRIS) shows excellent agreement for problems of interest. QuickPIC simulations of the SLAC PWFA experiment have revealed important physics and achieved good agreement with experiment measurement. Theoretical analysis of the stability of acceleration can now be guided and verified by QuickPIC simulations.   

Session J3: Emerging Science of Solid State Lighting

Current State of the Art in High Brightness LEDs

LED's have been commercially available since the 1960's. For many years they were used primarily for indicator applications. The remarkable increase in materials technology and efficiency that has been achieved since the early 1990's for AlInGaP red and amber LEDs, and InGaN green and blue LEDs, has enabled the penetration of markets such as outdoor display, signaling, and automotive brake light and turn signal applications. White LEDs, which are either blue LEDs combined with a phosphor, or a combination of red, green, and blue LEDs, are being used in emerging applications such as cell phone flash, television backlights, projection, and automotive headlights. In addition, to efficiency improvements these applications have required the development of higher power packages and, in some of these applications which are etendue limited, higher luminance devices. High power devices are commercially available which are capable of 140 lumens output and have an efficacy of around 70 lm/W for white emission. New package and chip technologies have been demonstrated which have a luminance of 38 mega nits (Mcd/m$^{2})$, approximately 50{\%} more luminance than that of an automotive headlamp halogen bulb ($\sim $25 mega nits). The recent progress in materials technology, packaging, and chip technology makes it clear that LED's will become important for general illumination applications. The rate of LED penetration of this market will depend upon continued increases in performance and lower costs as well as better control of the white spectral emission. Efficiency, current density, and costs are closely linked because the cost in dollars/lumen is inversely proportional to how many lumens can be realized from each unit of device area for a given device type. Performance as high as 138 lm/W, and over 40{\%} wall plug efficiency, has been reported for low power research devices and over 90 lm/W for high power research devices. It is clear that high power commercial products with performance in excess of 100 lm/W will become available soon, which is substantially more efficient than incandescents ($\sim $15 lm/W) and compact fluorescents ($\sim $60 lm/W) and equivalent to high performance fluorescent lighting. Performance in the range of 150 lm/W or even higher is plausible in the longer term. This talk will include an overview of the status and trends in LED technology and applications, and the challenges which must be met in order for LED's to become widely utilized in general illumination.   

Charge Injection and Transport in Conjugated Polymers.

We will overview the state-of-the-art in our understanding of charge injection and transport in conjugated polymers. We start by discussing the identifying characteristics of this class of materials, especially in relation with their structure and morphology. We follow by reviewing the advantages and limitations of experimental techniques that are used to probe charge transport. We then embark on a discussion of the fundamentals of charge transport in organics. We follow a didactic approach, where we start from transport in crystalline semiconductors and gradually introduce corrections for space charge effects, for the influence of disorder on mobility, for high charge densities, and for electric field-dependent charge densities. We compare with experimental data from polyfluorenes. We then shift our attention to charge injection. We review some of the recent theories and compared their predictions to experimental data, again from polyfluorenes. We close by proposing directions for future work.   

Cross-Cutting Basic Research Needs for Solid State Lighting

The recent DOE workshop on basic research needs for solid-state lighting has identified a number of cross-cutting scientific research areas that have potential to impact solid-state lighting. Basic research in the following areas were identified as priorities: new functionality through heterogeneous nanostructures, innovative photon management, enhanced light-matter interactions, multiscale modeling for solid-state lighting, and precise nanoscale characterization. We will provide an overview of the challenges and opportunities in these areas and describe how advances here could impact solid-state lighting.   

Session J4: Polymer-based Composite Materials

Nanostructure Evolution in Polymer/Nano-object Hybrids

Significant advances have recently been made in the synthesis of nano-objects with well-defined functions. Various size and shape of nano-objects are now readily available. In order to find useful applications those nano-objects are often mixed with polymers. We investigated the effect of hard additives, i.e., \textit{interacting} magnetic nanoparticles (NPs), on the ordered morphology of block copolymers by varying NP concentration. In order to characterize the structural change of block copolymer associated with different NP loadings, small-angle X-ray scattering and transmission electron microscopy were employed. With the increase in NP concentration, domains of NP aggregates were observed. It is surprising to note that regular lattice-like aggregates with $\gamma $-Fe$_{2}$O$_{3}$ NPs induce an intriguing morphological transformation from the hexagonal cylinders to the body-centered cubic spheres via undulated cylinders of block copolymers, which does not show such morphological transition without NPs. These results are compared with the case where the interaction among NPs is relatively weak. In addition, we studied the effect of casting solvents and sample preparation conditions to confine such NPs in one of microphase separated domains. These results could add more flexibility in structural control and orientation of block templates in thin films opening up new applications.   

Curved Brushes: Ordering and Dynamics of Silica Polymer Nanocomposites

The structure and dynamics of polymer-tethered silica nanocomposites are examined here. Local dynamics of the hybrids suggest a significant increase in the glass transition temperature of the polymer chains, compared to the free polymer for the case of poly(butyl acrylate) systems. Mesoscale dynamics probed under quiescent conditions indicate a solid-like response for the nanocomposite, and this was seen to persist even upon dilution of the end-grafted chains with free PBA of approximately the same molecular weight. For the end-tethered hybrid, an ordered arrangement of the nanoparticles is observed using small angle x-ray scattering and transmission electron microscopy. Dilution results in a homogenous system of the hybrid with the free chains, and the resulting scaling of the correlation between hybrid domains suggest a fully penetrated brush system. In addition, linear viscoelastic characteristics of the blends were found to exhibit strong dependence on the hybrid volume fraction.   

The phase stability and properties of polymer -- nanoparticle blends

In our studies of nanoparticles blended with linear polymer melts two unusual phenomena were noted; the viscosity was reduced upon nanoparticle addition and the nanoparticles remained dispersed despite the interparticle gap being smaller than the polymer radius of gyration. The viscosity decrease is not easily explained and it appears as if it is related to introduction of free volume created by the vast surface area generated by the nanoparticles and elimination of entanglements via constraint release generated by the fast diffusing nanoparticles. Further study is required to fully understand this phenomenon. This leads to the key point, unless nanoparticles are well dispersed in polymer melts then one would not expect any unusual phenomena to exist. Furthermore, one would expect the equivalent of depletion flocculation to occur at moderate volume fraction since the interparticle gap becomes so small in nano-systems and this simply does not occur. We suggest the dispersion driving force is due to an enthalpy gain the nanoparticles experience. Consider the pure nanoparticle phase, the van der Waals forces effectively propagate over a distance of order $\delta $, however, the interstices between the nanoparticles could be larger than this resulting in a reduced cohesive energy. Thus, when a nanoparticle is dispersed in a polymer melt it gains molecular contacts or enthalpy. This is a true nanoscale phenomenon since if the nanoparticles are too small then the interstices are quite small and the driving force for dispersion is reduced while if they are too large then there are not enough of them per unit volume to cause a significant enthalpy gain for a given volume fraction. An optimum exists at a radius of approximately 3-5 nm. This phenomenon and others will be discussed in the seminar.   

Aggregation,Steric Stabilization,Bridging and Miscibility of Polymer Nanocomposites

Microscopic liquid state theory has been employed to study the potential of mean force (PMF), statistical structure, and phase separation of spherical nanoparticles in a dense polymer melt over a wide range of interfacial chemistry, chain length, and filler size and volume fraction conditions. As interfacial cohesion strength increases the nanoparticle organization evolves from contact depletion aggregation, to well dispersed behavior associated with a thermodynamically stable polymer coating, to polymer-mediated bridging of a variable degree of tightness. Near linear scaling of the PMF with the particle/monomer diameter ratio is found, and the spatial range of the interfacial attraction is important in determining nanoparticle organization. Spinodal demixing calculations predict an entropy-driven fluid-fluid phase separation for weak interfacial attractions, and an enthalpically driven network or complex formation type of phase separation in the strong cohesion regime. A miscibility window exists at intermediate interfacial attraction strengths which systematically narrows, and is ultimately destroyed, as particle size and/or direct filler-filler van der Waals attractions increase. The length-scale dependent real space statistical structure is quantified via calculations of the polymer and filler intermolecular pair correlation functions and partial scattering structure factors. At high filler volume fractions interference between the polymer organization near nanoparticle surfaces induces significant changes of filler packing. The presence of bound polymer layers in miscible nanocomposites results in microphase- separation-like features in the small angle collective polymer structure factor. Implications of the theoretical results for the design of thermodynamically and/or kinetically well-dispersed polymer nanocomposites, and the formation of nonequilibrium networks, will be discussed. The theory has also been generalized to treat the consequences of soft intermolecular repulsions, and nonspherical fillers including rod, disk and compact molecule-like shapes.   

Polymer brushes on nanoparticles: their positioning in and influence on block copolymer morphology.

Polymers brushes grafted to the nanoparticle surface enable the precise positioning of particles within a block copolymer matrix by determining the compatibility of nanoparticles within a polymeric matrix and modifying the interfacial properties between polymers and inorganic nanoparticle. Short thiol terminated polystyrene (PS-SH), poly(2-vinylpyridine) (P2VP-SH) and PS-$r$-P2VP with the molecular weight (M$_{n})$ of 3 kg/mol were used to control the location of Au nanoparticles over PS-b-P2VP diblock copolymer template. We will discuss further the approach of varying the areal chain density ($\Sigma )$ of PS-SH brushes on the PS coated particles, which utilizes the preferential wetting of one block of a copolymer (P2VP) on the Au substrate. Such favorable interaction provides the strong binding of Au particles to the PS/P2VP interface as $\Sigma $ of PS chains on the Au particle decreases. We find that at $\Sigma $ above a certain value, the nanoparticles are segregated to the center of the PS domains while below this value they are segregated to the interface. The transition $\Sigma $ for PS-SH chains (M$_{n}$ = 3.4 kg/mol) is 1.3 chains/nm$^{2}$ but unexpectedly scales as M$_{n}^{-0.55}$ as M$_{n}$ is varied from 1.5 to 13 kg/mol. In addition, we will discuss changes in block copolymer morphology that occur as the nanoparticle volume fraction (\textit{$\phi $}) is increased for nanoparticles that segregate to the domain center as well as those that segregate to the interface, the latter behaving as nanoparticle surfactants. Small \textit{$\phi $} of such surfactants added to lamellar diblock copolymers lead initially to a decrease in lamellar thickness, a consequence of decreasing interfacial tension, up to a critical value of \textit{$\phi $} beyond which the block copolymer adopts a bicontinuous morphology. I thank my collaborators G. H. Fredrickson, J. Bang, C. J. Hawker, and E. J. Kramer as well as funding by the MRL as UCSB from the NSF-MRSEC-Program Award DMR05-20418.   

Session J5: Topics from the Gordon Research Conference on Teaching Electricity and Magnetism

Student understanding of basic concepts in dc electric circuits

After instruction on dc circuits in a typical introductory physics course, students are often able to apply the formalism they have learned to analyze relatively complicated circuits, even those containing multiple batteries and multiple loops. However, research conducted over a period of many years has shown that despite facility with the equations, students often fail to understand some very basic concepts. Results from a recent investigation involving multiple batteries reveal the surprising extent of the gap between what is taught and what students learn. The findings have strong implications for instruction.   

The MIT TEAL Simulations and Visualizations in Electromagnetism

The Technology Enabled Active Learning (TEAL) Project at MIT has developed a broad range of 3D visualizations and simulations to foster student intuition about electromagnetic fields and phenomena (see http://web.mit.edu/8.02t/www/802TEAL3D/). In this talk we discuss the software approaches we use to create these simulations, including Macromedia Shockwave and Java 3D applets for interactive visualization, passive animations created with 3ds max, and the Dynamic Line Integral Convolution (DLIC) method for constructing time dependent representations of the electromagnetic field at close to the resolution of the computer display (Sundquist, 2003). The DLIC method, in particular, is far superior in delineating the spatial and temporal structure of fields as compared to e.g. field line displays or vector field grids. We also report on the use of these visualizations in instruction at the freshmen level. Our strong opinion is that for effective student learning, such visualizations must be embedded in a software framework for their interactive delivery. This ``guided inquiry'' framework is essential to influence and optimize what students take away from the visualizations. In our current research, we are delivering our visualizations using a commercial package, Addison Wesley's MasteringPhysics (MP), although any guided inquiry delivery system such as MP will be able to interact with our simulation software. We have released our Java 3D simulation software as open source with a liberal open source license (see http://jlearn.mit.edu/tealsim/ ), with support from the Davis Educational Foundation.   

A simple model for why an active learning approach works best: Experiences with a ``Jackson by Inquiry'' electromagnetism course

The development of an inquiry-based group learning studio lab[1] for the teaching of electromagnetism is described, which has the goal of facilitating the transition of students from passive listeners to active investigators or practicing physicists. We summarize the course design, implementation, and results, which show improved performance by students with weaker math and physics backgrounds or who are under-represented in physics. A number of assessment tools are considered and evaluated including comparison to standard lecture formats. A general strategy for presenting technically demanding material is given and a simple model is presented which relates the success of such structured inquiry approaches to recent research in neurophysiology, cognitive science and learning, and physics education. \begin{enumerate} \item B. R. Patton, ``Group Inquiry-Based Approach to Graduate Education in Physics: Can you do Jackson in a hands-on way?'', APS/AAPT Joint Meeting, Indianapolis, IN, 2-5 May 1996; B. R. Patton, Group Learning-Based Approach to the Graduate Electrodynamics Course: ``Jackson by Inquiry,'' APS Forum on Education Newsletter, Summer 1996. \end{enumerate}   

Optical Trapping and Manipulation in the Single- and Many-Body Limits

Analysis of optical dipole/scattering forces can be done at a variety of levels, some of which are appropriate to the undergraduate curriculum. The addition of simple holographic techniques has extended the basic capabilities of optical tweezing, making it a more viable tool for the assembly of micro-systems and organization of specimens into user-defined structures. In 2D, we have demonstrated an approach that allows optical forces alone to assemble microparticles over macroscopic areas. 3D structures pose greater challenges, but also significant opportunities. Our early efforts at filling a 3D lattice of optical traps led to an appreciation for the dynamics of injected microparticle streams, which yield a surprisingly successful method of sorting or re- routing within microfludic environments. We will discuss the status of efforts using optical trapping to create static many-body structures (both simple and complex), as well as recent results on dynamic interactions. At the same time, some of these techniques have clear pedagogical value, as will be emphasized.   

Session J6: Women in Academic Science: Balancing Career and Children

CSWP Panel Discussion: Women in Academic Science: Balancing Career and Family

Many people who are considering pursuing academic careers in science worry about how to balance career with family. One challenge is the two-body problem, where partners are searching for jobs that are reasonably close together. Another challenge, particularly for women, is children: many women worry about whether they can have children as well as successful careers, and if so, when might be the best time to have them. This panel discussion will bring together five women who span a range of stages in their faculty careers and who all have children. Several of them have spouses who are also academic scientists. They will discuss practical strategies that they have adopted to address the challenges of career and family, as well as their views on what departments and institutions can/should do to help.   

Session J7: Scientific Cooperation in the Middle East

Bridging the Rift -- Scientific Cooperation between Israel and Jordan

Bridging the Rift is a project designed to develop scientific collaborations between scholars from Jordan and Israel. Over the past three years scholars from five Jordanian universities and all the Israeli universities have participated in research together. The fields of emphasis are Microbiology and Ecology. Joint field trips, research planning meetings, and laboratory studies have already been carried out. A physical institution spanning the Israeli-Jordanian border is planned as the home of this long-term collaboration.   

SESAME, A Scientific Collaboration In The Middle East : Personal and Israeli Perspectives.

SESAME is an effort to nurture both high quality research and a novel scope of scientific collaboration in the Middle East. I will present a personal and an Israeli perspective on how the project came about, where it is now and where it may head for.   

Creating an enduring framework for scientific cooperation in the Middle East

There are few channels for Israelis and Arabs to communicate directly when tensions are high. Scientists, who always have channels open for scientific communication, have a special responsibility to remain in contact with their counterparts on the other side to provide an avenue for reasoned discourse. Jordanian engineer Dr. Hani Mulki, former foreign minister and now science advisor to the King of Jordan, once said that scientific cooperation should not be a byproduct of peace, but a driving force. Many of the senior Israeli, Palestinian, and Jordanian scientists know each other and know how to work together, but it can be difficult for them to meet or even to speak without the cover of an invitation from a foreign organization; younger scientists unknown to the foreign organizations have fewer opportunities. The activities sponsored by APS, NAS, AAAS, and others are playing an important role, but what also is required are national and regional scientific organizations that can independently convene meetings and provide an umbrella for collaborative research. The academies of sciences of Israel and Palestine and the Higher Council for Science and Technology of Jordan have been working together for nearly two decades on joint research, studies and conferences, but always under the sponsorship of the U.S. National Academies or other international organizations. They should be able to convene regional meetings and provide an umbrella for cooperative research that can be sustainable without a foreign presence. Since they are only a driving distance apart, there is much they can do together for little money. Strengthening these academies, especially the relatively new Palestine Academy for Science and Technology, should be a high priority. Foreign scientific organizations should include the academies of the region in their activities, as co-sponsors if possible, to enhance their stature and encourage a role as independent conveners and sponsors of cooperative research.   

Mobilizing the Global Scientific Enterprise to Foster Cooperation with the Middle East

International science cooperation - e.g. science for diplomacy - has long been an important tool in the US foreign policy approach. Cold War science exchanges and development of institutions such as IIASA in Vienna, Austria played a critical role in increasing contacts and building trust between the adversaries. American - Chinese science exchanges starting in the 1970's laid the groundwork for increased interactions between the two countries. While such a deep history of exchange with the Muslim world U.S. policy, scientific diplomacy should now be a central point of any broader diplomatic engagement with these countries. Although perceptions of the U.S. are at historical lows throughout the region, recent polling continues to show that U.S. science and scientists remain highly respected. In part as a response to this unpopularity of the U.S. throughout the broader Middle East, President Bush appointed Karen Hughes, one of his most trusted advisers, to serve as the Under Secretary of State for Public Diplomacy - a position that is charges with improving the U.S. image abroad. This presentation will demonstrate that Under Secretary Hughes has valuable resources within the scientific community that are willing, able and proactively engaging with the Muslim world. The paper will highlight some of the key areas of cooperation, and highlight areas and programs that AAAS is using to further engage elements of the Muslim scientific community.   

Session J8: Focus Session: Novel Superconductors IV: Intercalated graphites and related

Superconductivity in alkaline earth-intercalated graphites: CaC$_6$ and SrC$_6$

The recent discovery of superconductivity in alkaline earth-intercalated graphites CaC$_6$ ($T_c$ = 11.5 K) with substantially higher $T_c$'s than the previously known, has renewed the interest in the graphite intercalated compounds and stimulated a debate about the relevant pairing mechanisms. We have investigated the superconducting properties of high-quality CaC$_6$ samples, using specific heat ($C_P$) and magnetization measurements. For CaC$_6$, the exponential temperature dependence of the electronic $C_P$ and its linear dependence on the magnetic fields provide evidence for a fully-gapped, intermediate-coupled, and phonon-mediated superconductor without essential contributions from alternative paring mechanisms. However, the $C_P$ anomaly at $T_c$ is found to be much smaller than expected from theory, indicating a possible anisotropy in the superconducting gap. Consistently, the anisotropy of the upper critical field $H_{c2}^{\parallel}$/$H_{c2}^{\perp}$ is also larger than expected from the Fermi velocities, and shows significant temperature dependence below $T_c$. Recently, we also discovered the superconductivity in SrC$_6$ at $T_c$ = 1.65(6) K as well as the absence of superconductivity in BaC$_6$ down to 0.3 K. Similar to CaC$_6$, the $C_P$ anomaly of SrC$_6$ is somewhat lower than that theory predicted, but the discrepancy is much reduced. The anisotropy of $H_{c2}$ for SrC$_6$ is also found to be much smaller than that of CaC$_6$, indicating a reduced superconducting gap anisotropy. Finally, we will discuss the significantly lower $T_c$ of SrC$_6$ than CaC$_6$ as well as their positive pressure dependence in terms of the \textit{e-ph} coupling with the in-plane intercalant and the out-of-plane C phonon modes, based on \textit{ab-initio} calculations. Implications of the present findings on the superconducting mechanisms in alkaline-earth as well as alkali- intercalated graphites will also be given.   

Bulk evidence for single-gap s-wave superconductivity in the intercalated graphite superconductor C$_6$Yb

We report measurements of the in-plane electrical resistivity $\rho$ and thermal conductivity $\kappa$ of the intercalated graphite superconductor C$_6$Yb to temperatures as low as $T_c$/100. When a magnetic field is applied along the c-axis, the part of $\kappa$ associated with fermionic quasiparticles increases exponentially for $H_{c1} [Preview Abstract]

Superconductivity in Alkali-Earth intercalated graphite.

It has long been known that Graphite intercalated compounds (GICs) can display a superconducting behavior at low temperature. However, until the discovery of YbC$_6$ and CaC$_6$, the critical temperatures achieved were typically inferior to 5 Kelvin. Using density functional theory we study superconductivity in AC$_6$ with A=Mg,Ca,Sr,Ba. We find that at zero pressure Ca, Ba and Sr intercalated graphite are superconducting with critical temperatures (T$_c$) 11.5, 0.2 K and 3.0 K, respectively. We study the pressure dependence of T$_c$. We find that the SrC$_6$ and BaC$_6$ critical temperatures should be substantially enhanced by pressure. On the contrary, for CaC$_6$ we find that in the 0 to 5 GPa region, T$_c$ weakly increases with pressure. The increase is much smaller than what shown in several recent experiments. Thus we suggest that in CaC$_6$ stacking faults or a continous phase transformation,such as an increase in staging could occur at finite pressure. Finally we argue that, although MgC$_6$ is unstable, the synthesis of intercalated systems of the kind Mg$_x$Ca$_{1-x}$C$_y$ could lead to higher critical temperatures. \newline \newline [1] M. Calandra and F. Mauri, Phys. Rev. Lett. {\bf 95}, 237002 (2005) \newline [2] M. Calandra and F. Mauri Phys. Rev. B 74, 094507 (2006)   

Far-infrared signature of a superconducting gap in intercalated graphite CaC$_{6}$.

CaC$_{6}$ is exceptional in the series of intercalated graphite compounds because of its high superconducting transition temperature, $T_{c}$=11.5K. The superconducting gap, 2$\Delta $=25.6 $\pm $ 3.2cm$^{-1}$, measured by scanning tunneling spectroscopy (N. Bergeal et al., PRL \textbf{97}, 077003 (2006)), is consistent with the weak-coupling BCS type superconductivity. The superconducting gap can be directly probed also by far-infrared spectroscopy. We studied the reflectance $R$ of CaC$_{6}$ between 4 and 100cm$^{-1}$ from 3K to 15K. We see the signature of the superconducting gap in the reflectance ratio of superconducting state $R_{s}$ to the normal state $R_{n }$and can follow its temperature dependence. The appearance of the gap signature in $R_{s}/R_{n}$ tells us that CaC$_{6}$ is in the dirty limit. Different models, including an anisotropic gap and a multi-gap scenario, will be discussed to fit the optical data.   

A First-Principles Insight into the Superconductivity of Graphite Intercalation Compounds

Experimental evidences have estabilished that the recently discovered superconductivity in graphite-intercalation compounds (GICs) CaC$_6$ and YbC$_6$ is due to electron-phonon ($e-ph$) coupling. First-principles calculations predict for CaC$_6$ an intermediate $e-ph$ coupling ($\lambda \sim 0.83$), resulting from intercalant in-plane ($I_{xy}$) and carbon out-of-plane ($C_z$) vibrations. Whereas the softening of the $I_{xy}$ modes explains increase of T$_c$ with pressure [1], the presence of the $C_z$ peak is due to an interaction which is ``dormant'' in pure graphite. A simple analysis of the band structure of the GICs also permits to rule out the possibility of plasmon-meadiated superconductivity[1]. \newline \newline [1] J. S. Kim, L. Boeri, R. K. Kremer, and F. S. Razavi Phys. Rev B, in press and Phys. Rev. Lett. 96, 217002 (2006).   

Strong electron-phonon coupling in the rare-earth carbide superconductor La$_2$C$_3$

Superconductivity in rare earth carbides has attracted interested again after the recent discovery of the 18 K superconductor Y$_2$C$_3$. Here, we present the crystal structure as well as the superconducting properties of the rare-earth sesquicarbides La$_2$C$_3$ ($T_c$ $\approx$ 13.4 K) gained from low-temperature neutron powder diffraction, specific heat and electrical resistivity measurements. From a detailed analysis of specific heat as well as the comparison with the full potential electronic structure calculations, a quantitative estimate of the electron-phonon coupling strength and the logarithmic average phonon frequency is made. The electron-phonon coupling constant found to be $\lambda_{ph}$ $\sim$ 1.35, and the low energy phonon modes are responsible for the superconductivity. These results suggest that La$_2$C$_3$ is in the strong coupling regime and the relevant phonon modes are the La-related modes rather than the C-C stretching modes. The upper critical fields($H_{c2}$) show a clear enhancement with respect to the Werthamer-Helfand-Hohenberg prediction and amount to $H_{c2}(0)$ $\sim$ 20 T confirming the strong electron-phonon coupling.   

The potential for mean-field $d$-wave superconductivity in graphite

We investigate the possibility of inducing superconductivity in a graphite layer by electronic correlation effects. We use a phenomenological microscopic Hamiltonian[1] which includes nearest neighbor hopping and an interaction term which explicitly favors nearest neighbor spin-singlets through the well-known resonance valence bond (RVB) character of planar organic molecules. Treating the Hamiltonian in mean-field theory, allowing for bond-dependent variation of the RVB order parameter, we show that both $s$- and $d$-wave superconducting states are possible with the $d$-wave state having a significantly higher $T_c$ at finite doping. By using density functional theory we show that the doping induced from sulfur absorption on a graphite layer is enough to cause an electronically driven $d$-wave superconductivity at graphite-sulfur interfaces (see e.g. [2]). We will also briefly discuss applying our results in the case of the intercalated graphites as well as the validity of a mean-field approach. \newline [1] G. Baskaran PRB {\bf 65} 212505 (2002) \newline [2] S. Moehlecke {\it et al.} PRB {\bf 69} 134519 (2004)   

Possibility of superconductivity in high pressure phases of BC$_3$

Using an ab-initio pseudopotential-local-density-approximation, we study the possibility of a high-pressure transition of graphitic planar sp$^2$ bonded BC$_3$ into an sp$^3$ bonded covalent network. Energy barriers are examined for the predicted transition to the sp$^3$ phase and for the observed onset of phase separation in BC$_3$ under high pressure, high temperature conditions [V. L. Solozhenko \textit{et al.}, Appl. Phys. Lett. \textbf{85}, 1508 (2004)]. The sp$^3$ phase of BC$_3 $ is predicted to be metallic and superconducting and is similar to 25\% boron-doped diamond without boron clusters. Calculations of phonon frequencies and electron-phonon coupling allow for an estimate of the superconducting transition temperature.   

Probing electronic structure and electron-phonon interaction in borides using optical spectroscopy

We report optical properties of high-quality single crystals of boron type superconductor ZrB$_{12}$ (T$_{c}$=6 K) in the normal state from 20 to 300 K. The optical conductivity was measured from (6 meV-4 eV) by a combination of reflectivity and ellipsometry. The Drude plasma frequency and interband optical conductivity calculated by self-consistent full-potential LMTO method agree well with experiments. The $\alpha ^{2}$F($\omega )$ function extracted from optical spectra presents two peaks at 25 and 80 meV with partial coupling constants of 0.8 and 0.3 respectively. The low energy peak corresponds to the displacement mode of Zr inside B$_{24}$ cages, while the second one involves the rigid boron network. In addition to the usual narrowing of the Drude peak with cooling down, we observed an unexpected removal of about 10 {\%} of the Drude spectral weight which is partially transferred to the region of the lowest-energy interband transition ($\approx $ 1 eV). This effect may be caused by a delocalization of the metal ion from the centre of the B$_{24}$ cage. The discussion will refer to recent work on other boron rich compounds.   

Gapless Fermi Surfaces in anisotropic multiband superconductors in magnetic field.

We propose that a new state with a fully gapless Fermi surface appears in quasi-2D multiband superconductors in magnetic field applied parallel to the plane. It is characterized by a paramagnetic moment caused by a finite density of states on the open Fermi surface. We calculate thermodynamic and magnetic properties of the gapless state for both s-wave and d-wave cases, and discuss the details of the 1-st order metamagnetic phase transition that accompanies the appearance of the new phase in s-wave superconductors. We suggest possible experiments to detect this state both in the s-wave (2-H NbSe$_2$) and d-wave (CeCoIn$_5$) superconductors.   

Superconducting order parameter in NbSe$_{2}$ derived from the specific heat

To resolve the discrepancies on the superconducting order parameter of quasi-2D NbSe$_{2}$, the comprehensive specific heat measurements have been carried out.The thermodynamic consistence requires more than one energy scale of the order parameters The zero field data and the results of the mixed states respectively with $H$//$c$ and $H\bot c$ conclude: (1) $\Delta _{L}$=1.26 meV and $\Delta _{S}$=0.73 meV; (2) $N_{Se}$(0)/$ N$(0)=11{\%}$\sim $20{\%}; (3) $\Delta _{S}$ is 3-D and like on the Se derived Fermi surface. This present scenario largely removes the dispute over the order parameter existing in the previous literature. The alternative anisotropic$ s$-wave model is also discussed.   

Session J9: Superconductivity: Oxide Photoemission I and X-ray Scattering

A Proposed New Measurement of the Superconducting Gap in YBa$_{2}$Cu$_{3}$O$_{7}$

The superconducting energy gap of YBa$_{2}$Cu$_{3}$O$_{7}$ (YBCO) varies strongly with $\vec {k}$and from a sheet of the Fermi surface to another. The strong anisotropic superconducting gap in high Tc materials such as YBCO has led to conflicting d-wave and s-wave interpretations. We have utilized electronic wave functions from the ab-initio density functional calculation and the related electron-phonon interaction matrix elements for the calculation of the superconducting gap values of YBCO. For three pieces of the Fermi surfaces, the calculated superconducting gaps exhibit a strong anisotropy. In contrast, we have found that the superconducting gap on one sheet of the Fermi surface around S-point only shows a minor variation from about 18 meV to 25 meV. Especially, there is no node on this sheet of the Fermi surface. We propose a new test measurement of the superconducting gap of YBCO on this sheet of the Fermi surface around the S-point. This measurement is expected to shed light on the gap symmetry properties of high Tc superconductors. This work was funded in part by NSF (Award No. 0508245) and ONR (Grant No: N00014-05-1-0009).   

Dynamic Response Functions from Angle Resolved Photoemission Spectra

The linear response to an external probe as a function of energy and momentum is of great importance in elucidating the properties of complex materials. We introduce a formalism in the framework of diagrammatic k space approach with Random Phase Approximation (RPA), for calculating dynamic response functions using experimental single particle Green's function derived from ARPES spectra. Specifically we focus on using the single particle Green's function obtained from superconducting state of ARPES data in the High Tc cuprates to compute the dynamic spin susceptibility. We find good agreement between our results and the superconducting state neutron results, in particular the resonance at antiferromagnetic ordering wave vector, with its unusual `reverse magnon' dispersion. We anticipate, our formalism will also be useful in interpreting results from other spectroscopies such as optical and Raman responses.   

Self Energy Analysis of Photoemission spectral function from parent cuprates, Ca$_{2}$CuO$_{2}$Cl$_{2}$

Self energy $\Sigma $ (\textbf{k},$\omega )$ is the fundamental function that describes the effects of many-body interactions on an electron in a solid. But it is very difficult to extract the self energy from experimental data, especially for non-metallic materials. In this paper we developed a new and general method with which one can extract the self energy from angle-resolved photoemission spectroscopy (ARPES) data in the full \textbf{k}-$\omega $ space. We demonstrate the validity of this method by applying it to the ARPES data from Ca$_{2}$CuO$_{2}$Cl$_{2}$ (CCOC). We find the values for the imaginary part of the self energy is compatible with the value obtained by measuring the peak width.   

The hierarchy of multiple many-body interaction scales in high-temperature superconductors

Many-body interaction is key to novel properties of quantum matter. As an extreme example, the complexity due to charge, spin, and lattice interactions in high-T$_{c}$ superconductors makes it difficult to identify the essential microscopic ingredients for the basic model. Energy scales where these interactions are manifest usually provide important insights into the nature of the interactions. The energy-momentum dispersion relationship measured by angle-resolved photoemission spectroscopy (ARPES) provides an excellent tool for identifying these scales. To date, the focus of the discussion has been on the low energy anomaly near 0.03-0.09eV. Here we present improved experimental data from four families of high-T$_{c}$ superconductors that reveal a hierarchy of many-body interaction scales focused on: the low energy anomaly (``kink'') of 0.03-0.09eV, a high energy anomaly of 0.3-0.5eV, and an anomalous enhancement of the width of the LDA-based CuO$_{2}$ band extending to energies of approximately 2 eV.   

Energy scales revealed by ARPES study on four layered cuprate superconductor Ba$_{2}$Ca$_{3}$Cu$_{4}$O$_{8}$(O$_{\delta }$F$_{1-\delta })_{2}$

Recently discovered four layered cuprate superconductor Ba$_{2}$Ca$_{3}$Cu$_{4}$O$_{8}$(O$_{\delta }$F$_{1-\delta })_{2}$ posseses various properties that differ from the conventional cuprate High T$_{c}$ superconductors, making itself exceptional to the well known high T$_{c}$ superconductivity phase diagram. The understanding of this discrepancy will provide important implications for high T$_{c}$ theory. We study this material by Angular Resolved Photoemission Spectroscopy (ARPES) and find that the electronic band structure of this four layered system exhibits clear difference from the previously studied hole- or electron- doped cuprate superconductors such as Bi2212 or NCCO. The multiple energy scales associated with its electronic structures not only show the complexity of this system, but also provide hints to understand its uniqueness.   

The evolution of electronic structure of Bi$_2$Sr$_{2-x}$Bi$_{x}$CuO$_{6+\delta}$ revealed by ARPES

Bi$_{2}$Sr$_{2-x}$Ln$_{x}$CuO$_{6+\delta}$ (Ln is a trivalent element) is a good candidate to investigate the effects of charge doping and potential disorder to the properties of the high-Tc cuprates. High-quality single crystals of Bi$_2$Sr$_{2-x}$Bi$_x$CuO$_{6+\delta}$ (Bi-Bi2201) have been synthesized over a wide substitution range ($0 [Preview Abstract]

High-energy kink in high-temperature superconductors

Photoemission studies show the presence of a high energy anomaly in the observed band dispersion for two families of cuprate superconductors, Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{4+\delta }$and La$_{2-x}$Ba$_{x}$CuO$_{4}$. The anomaly, which occurs at a binding energy of approximately 340 meV, is found to be doping and momentum independent. The magnitude of the effect is momentum dependent. Scattering from short range or nearest neighbour spin excitations is found to supply an adequate description of the observed phenomena.   

Systematic ARPES study of n-doped cuprate superconductors

In contrast to the hole-doped high-temperature superconductors, for which the Cu2+ long-range antiferromagnetism (AF) is suppressed at low doping, the AF order is more robust and extends to higher doping in the case of the electron-doped superconductors RE2-xCexCuO4 (RE = Pr, Nd, Sm) and (Pr,La)2-xCexCuO4. Even though this long-range ordering is suppressed at optimal doping, neutron measurements and Hubbard model calculations suggest the persistence of short-range fluctuations. In order to investigate the impact of these fluctuations on the electronic structure of the electron-doped superconductors, we have performed systematic angular resolved photoemission spectroscopy measurements of optimally doped Pr2-xCexCuO4 and (Pr,La)2-xCexCuO4 samples. We present and discuss our recent results.   

{\it s}-wave-like excitation in the superconducting state of electron-doped cuprates with {\it d}-wave pairing

It has been quite controversial whether the superconducting (SC) pairing symmetry is {\it s}- or {\it d}-wave in electron-doped cuprates such as Nd$_{2-x}$Ce$_x$CuO$_4$ (NCCO) and Pr$_{2-x}$Ce$_x$CuO$_4$ (PCCO). In view that many experimental measurements to study the SC pairing symmetry only give direct information about the quasiparticle excitation gap and not the SC order parameter, we explore a physical mechanism to show the {\it s}-wave-like quasiparticle excitation under the {\it d}-wave SC pairing in electron-doped cuprates and intend to reconcile the contradictory experimental results. Our idea is based on the intrinsic Fermi surface (FS) evolution with doping as revealed by ARPES measurements on NCCO. It was found that at low doping only a small FS pocket appears around $(\pi,0)$ and with increasing doping a new FS pocket around $(\pi/2,\pi/2)$ will emerge. We argue that the FS pocket around $(\pi/2,\pi/2)$ has not yet formed until doping reaches about the optimal value $x=0.15$. Therefore in the underdoped regime, even if the SC order parameter is $d$-wave which vanishes along the diagonal line, the quasiparticle excitation gap is still finite and looks $s$-wave-like due to the absence of the FS across that line. This makes it possible, with $d$-wave SC pairing, to understand those experiments which evidenced the $s$-wave quasiparticle excitation.   

Spectral function analysis of an e-doped Hi-Tc superconductor near optimal doping, revisited

By comparison with the p-doped high transition temperature superconductors, their e-doped counterparts might give further insight into the unusual underlying physics. High resolution angle-resolved photoemission spectroscopy (ARPES) data of an e-doped cuprate near optimal doping is presented to further enrich previous comprehensive work on the subject \footnote{N.P. Armitage et al., PRB 68, 064517 (2003); H. Matsui et al., PRLetters 94, 047005 (2005)}. Spectral function analysis is also used to discuss band renormalizations. Other findings will be discussed as well.   

Intermediate energy structure of cuprates using Resonant Inelastic X-ray Scattering

We present a comprehensive study of the charge-transfer excitations in the 1-8 eV range using the burgeoning technique of resonant inelastic X-ray scattering (RIXS). Surprisingly, we find that the charge-transfer gap, distinct at around 2.25 eV in Mott insulating La$_{2}$CuO$_{4}$, is also discernible in the high-T$_{c}$ superconductor HgBa$_{2}$CuO$_{4+\delta }$. In addition, we are able to identify many distinct, weakly dispersive features above the charge-transfer gap of La$_{2}$CuO$_{4}$ [1-3] and the model high-Tc superconductor HgBa$_{2}$CuO$_{4+\delta }$ [1]. Detailed extension of this work in La$_{2}$CuO$_{4}$ reveals previously unresolved systematics in the vicinity of the charge-transfer gap, and a distinct dependence on scattering geometry of both the charge-transfer gap and the high-energy excitations. We interpret this scattering-geometry dependence as arising from the intrinsic symmetry selectivity of the RIXS/Raman process, and suggest that similar experiments can give definitive identification of excitation symmetry. [1] L. Lu et al., Phys. Rev. Lett. 95, 217003 (2005). [2] L. Lu et al. (to appear Phys. Rev. B 74; cond-mat/0607311) [3] J. N. Hancock et al. (in preparation).   

RIXS Spectra for Ladder Cuprate $Sr_{14}Cu_{24}O_{41}$

The ladder compound $Sr_{14}Cu_{24}O_{41}$ is of interest both as a quasi-one-dimensional analog of superconducting cupartes and as a superconductor in its own right when $Sr$ is substituted by $Ca$. In order to model recent resonant inelastic x-ray scattering (RIXS) spectra of this compound, we studied the simpler $SrCu_{2}O_{3}$ system in which the crystal structure contains very similar ladder planes. We approximated the LDA dispersion of $SrCu_{2}O_{3}$ by two different tight-binding models - either a copper only model with two bands or a copper plus planar oxygen model with seven bands. Due to the glide symmetry of the structure, the period of dispersion along the ladder is $4\pi$. Strong correlation effects were treated by assuming an anti-ferromagnetic ground state for both models. The resulting dispersion of the filled band at half filling matches the experimental ARPES spectra, and the RIXS spectra are in good agreement with experimental results. Work supported in part by the USDOE   

Inelastic X-ray scattering study of the bond stretching phonon mode in Bi$_2$Sr$_{2x}$Cu$_2$O$_{6+\delta}$

The phonon dispersions of the single layer high temperature superconductor Bi$_2$Sr$_{2-x}$Cu$_2$O$_{6+\delta}$ along the [$\xi$ 0 0] direction have been determined by inelastic x-ray scattering. The two highest longitudinal phonon branches, associated with the Cu-O bond stretching and out-of-plane oxygen vibration, are clearly resolved for the first time. The comparison with La$_{2}$Sr$_x$Cu$_2$O$_{0}$ will be discussed.   

Structures and properties of Ni$^{1+/2+}$ nickelates with infinite NiO$_{2}$ layers

Layered mixed valence Ni$^{1+/2+}$ nickelates possess similar crystal and electronic structures to Cu$^{2+/3+}$ high temperature superconducting cuprates. Only a few Ni$^{1+/2+}$ nickelates have been identified and they properties have not been reported so far. We present a first systematic study of Ln$_{n+1}$Ni$_{n}$O$_{2n+2}$, Ln = La or Nd, which structures could be described as an intergrowth of {\{}LnO$_{2}${\}} fluorite and infinite layer {\{}LaNiO$_{2}${\}}$_{n}$ blocks. The crystal structures of the new Ln$_{3}$Ni$_{2}$O$_{6,}$ Ln$_{4}$Ni$_{3}$O$_{8,}$ Ln$_{5}$Ni$_{4}$O$_{10}$ phases have been confirmed by X-ray and neutron powder diffraction. X-ray absorption spectroscopy data proves the 1+/2+ oxidation state and planar coordination of Ni atoms. Magnetic susceptibility data of Ln$_{n+1}$Ni$_{n}$O$_{2n+2}$ will be discussed.   

Session J10: Superconductivity: Vortex Imaging

Low Temperature STM Study of Vortex Motion on Fe doped NbSe2

We investigated the vortex motion around magnetic Fe impurities on type II superconductor NbSe2 by a home built low temperature STM. Using Scanning Tunneling Spectroscopy Maps we recorded the movie of the motion at 4 K with a very slow decaying rate of the magnetic field ($\sim $ 5 nT/s). The map images were taken with a 400 nm by 400 nm field of view and in a 0.75 T magnetic field to start with. Each frame of the movie has $\sim $ 109 vortices and takes $\sim $ 8 min to acquire. Scanning tunneling spectroscopy data show that the superconductivity is destroyed at the impurity sites, which indicates that they serve as attractive pinning centers for the vortex lattice. The behavior of the overall motion of the vortex lattice can be explained by the Larkin-Ovchinnikov collective pinning theory. The average speed of the motion is $\sim $ 5 pm/s. Our STS movie data display the pinning and depinning events of a single vortex around the pinning center. A flux creep model will be exploited to understand the effect of the pinning centers on the vortex motion.   

Local vortex--defect interaction in moving vortex lattices observed by STM

When applying a magnetic field to a type II superconductor, part of the magnetic flux penetrates the sample forming a current vortex. At high enough fields and low enough defect concentration the vortices form a 2D triangular lattice. We observed the vortex lattice on NbSe$_2$ single crystals using STM ($B=250-750$ mT, $T=4.2$ K). Due to a slow decay of the magnetic field of our superconducting magnet ($\sim -5$ nT/s) the vortices collectively move at an average speed of about 5 pm/s. The motion was observed by tracking the center of a vortex across consecutive images of the vortex lattice. The motion shows distinct acceleration/deceleration cycles we associate with collective pinning events on nearby defect sites. A more subtle observation was the deviation of the vortex positions from their `expected' location within the lattice of up to 3 nm. A similar effect was found in 2D simulations of a moving vortex lattice near defect sites. Since it takes an additional force to move a vortex out of position, we can identify subsurface defects and analyze the defect--vortex interaction. Results of the analysis will be presented.   

Lorentz Imaging of Superconducting Flux Vortices with a Commercial Transmission Electron Microscope

Magnetic flux penetrates type II superconductors along normal channels called flux vortices, each containing a single quantum of flux. It is beneficial to image these vortices and study their response to external stimuli as they determine the performance of many superconducting devices. Tonomura's research group have demonstrated that vortices can be imaged by transmission electron microscopy because of the deflection the electrons suffer as they pass through the magnetic flux within the vortices (Harada K. \textit{et al.}, Nature 360, 51, 1992). This technique offers spatial resolution superior to other techniques, real-time imaging and is sensitive to magnetic flux throughout the material, not simply surface fields. To our knowledge, Tonomura's is the only group to have successfully employed this technique and their experiments required custom-built high voltage microscopes. Here we demonstrate that flux vortices can be imaged with a commercially available electron microscope, opening the field for other researchers. We show images of flux vortices in Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta }$ and analyze their arrangements as a function of applied magnetic field and temperature.   

Single Vortex Resolution Imaging of the Flux Front in a YBa$_{2}$Cu$_{3}$O$_{7-\delta}$ Single Crystal

We have imaged the vortex state in an optimally doped, detwinned YBa$_{2}$Cu$_{3}$O$_{7-\delta}$ single crystal, using magnetic force microscopy (MFM). The structure of the flux front, the boundary between vortices and anti-vortices, is studied with single vortex resolution, as it evolves with applied magnetic field. We find the front to be corrugated on a scale of several microns. On a smaller scale, the front is composed of alternating ``fingers'' of vortices and anti-vortices. We also observe stable vortex-antivortex pairs.   

Magnetic Induction Profile in Superconductor/Ferromagnet Bilayers

Strong suppression of flux density peaks at the edge of a superconducting film was observed by magneto-optical imaging the magnetic induction profiles of an YBCO superconducting film on a magnetic substrate in perpendicular magnetic fields, The observed induction profile is in a striking contrast to the case of superconducting films on a non-magnetic substrate that display sharp flux density peaks at the edges of the films. The influence of magnetic substrate on the flux distribution in superconductor films may be modeled by considering the formation of a virtual infinite stack of superconducting films due to the magnetic mirror effect. We also found that the flux patterns in the magnetic substrate were strongly influenced by the flux distribution in the superconductor. These observations, results of computer simulations, and their implications to the transport and magnetization properties of superconducting films will be discussed.   

Dragging Individual Vortices to Probe the Dimensionality of Pinning in YBa$_{2}$Cu$_{3}$O$_{7-\delta}$

We have used a magnetic force microscope (MFM) to image and to manipulate individual vortices in optimally doped YBa$_{2}$Cu$_{3}$O$_{7-\delta}$ samples: a 200nm film and a detwinned single crystal. In the film, if the force exerted by the MFM tip is strong enough to overcome the pinning potential, a pinned vortex jumps as a whole to a new pinning site. We find a wide spread of depinning forces, attesting to the importance of point pinners as opposed to pinning along one-dimensional defects. The behavior in the single crystal is very different. Even when a vortex is pinned the shape of its image is distorted, perhaps indicating meandering of the vortex line to take advantage of pinning centers as it traverses the crystal. When we drag a vortex, it tilts significantly before depinning, as signified by pronounced stretching of its image. This effect is highly anisotropic and depends on the pulling direction.   

Manipulation of the magnetic flux in superconductor by the ferromagnetic domains in SC/FM hybrid

We studied magneto-optically the magnetic flux entry and exit in SC/FM hybrid of a ferromagnetic permalloy film sputtered on the superconducting NbSe2 single crystal. The FM film had growth induced perpendicular anisotropy and the labyrinth equilibrium domain structure. However, we could align the domain walls in a desired direction by application of a strong enough in-plane field. Thus formed stripe domains introduce a pronounced directionality for the vortex motion in the underlying superconductor. The effect persists up to the fields of the stripe domain collapse and does not depend on the temperature at which the domain walls were polarized. It does not change at heating the sample and cooling back below Tc. We discuss the effect in terms of the domain wall pinning of vortices in conditions when the domain size is larger than the coherence length and propose a scheme for manipulating the transport properties of superconductors by the ferromagnetic domains.   

Magnetic Force Microscopy of Superconducting Vortices in an Ordered Array of Artificial Pinning Centers

Ordered arrays of artificial pinning centers were fabricated in Nb thin films using anodic aluminum oxide (AAO) as a template. These artificial pinning arrays have a triangular lattice parameter of 105 nm and antidot diameters of about 50 nm. The nanohole arrays show only a small decrease in the superconducting transition temperature, Tc$\simeq$7.1 K, from comparable unprocessed Nb thin films. Enhancement of the magnetization at the first, second and third matching fields (matching field = 2170 Oe) were observed in the magnetization half-loops of these arrays at 5 K. Magnetic Force Microscopy (MFM) was used to image the nanohole arrays above and below Tc . These images clearly show the nanohole lattice. Individual vortices have been imaged at low fields and their movement within the lattice is being explored. Further, domain rings have been imaged at low and high fields at temperatures between 5 and 5.5 K. Their properties are currently being investigated.   

Vortex dynamics in mesoscopic weak-pinning superconducting channels with a Corbino geometry.

We report transport measurements of vortex flow dynamics in mesoscopic weak-pinning channels of a-NbGe with strong-pinning NbN channel edges. The channels are arranged in circular patterns on a Corbino disk geometry, thus eliminating the influence of edge barriers to vortex entry on the dynamics. The number of vortices which can be detected at particular flow velocities is limited by the method for measuring the flux flow voltage and the channel configuration. We discuss potential applications of this system for guiding vortices around nanofabricated structures free from edge barriers.   

Structured pinning potentials for guiding vortex motion in superconductors.

Nanofabricated pinning structures can be used to guide vortices in superconductors through various potential energy landscapes. We report transport measurements of vortex flow dynamics in structured weak-pinning channels of a-NbGe with strong-pinning NbN channel edges. By arranging the channels in circular patterns on a Corbino disk geometry, we eliminate the influence of edge barriers to vortex entry on the dynamics. Patterning channel edges with different shapes allows us to explore the influence of the confinement potential on the vortex dynamics. We discuss one such pattern with channel edges in an asymmetric sawtooth configuration for investigations of vortex ratchet dynamics.   

Dynamic flux-quantum phases in weak-pinning V$_{3}$Si and Re$_{3}$W

The dynamics of transport-driven flux quanta in the \textit{absence }of pinning is a fundamental phenomenon little understood and studied by few. This is mainly because of a rarity of highly homogeneous type II samples with few pinning defects, combined with the technical challenge of passing high levels of transport current through such samples. These issues are addressed by the use of ultrasonically soldered leads and pulsed currents, in addition to the availability of relatively defect-free samples of the low-temperature superconductors, V$_{3}$Si and Re$_{3}$W. This enabled the study the critical-current ``peak effect'' in V$_{3}$Si [\textit{PRB }\textbf{67}, 104516], which also included the observation of metastable phases in connection with the peak effect, still to be reported in greater detail. Another observation is that of dissipative flux flow phases, in both V$_{3}$Si and Re$_{3}$W, along with evidence of an \textit{approach} towards the highly-ordered Bardeen-Stephen phase of free flux flow. The field dependence of free flux flow resistivity is also of interest in probing vortex \textit{core size} effects [\textit{PRB }\textbf{71}, 134505]. All of these are to be discussed in detail. \textit{Research at ORNL supported by DOE Office of Electricity Delivery and Energy Efficiency and DOE Office of Science, Basic Energy Sciences.}   

Vortex-antivortex molecules in superconducting films with magnetic dot arrays

Following earlier works [Milosevic and Peeters, PRB (2003), PRL (2004)], we studied the vortex-antivortex stabilization in a superconducting film under a square array of magnetic dots of variable size. The theoretical side of the investigation was done within the Ginzburg-Landau theory, and main findings comprise: (i) multi-shell vortex-antivortex structures, (ii) the profound interaction between neighboring vortex-antivortex molecules through exchange of ``valence'' antivortices, and (iii) dual interaction of stabilized vortex-antivortex pairs and magnetic dots with excess flux-lines of the applied homogeneous magnetic field. On experimental side, the results are corroborated by scanning Hall probe measurements, performed on a 80nm thick Pb film, on top of a square array (period 5$\mu $m) of magnetic dots of four sizes - R=0.522, 0.738, 0.808, and 0.902$\mu $m, etched out of a [2nm Pt] [0.6nm Co/1.0nm Pt]$_{10}$ multilayer film with perpendicular magnetization. A 20nm thick Ge layer was evaporated on top of the dots to avoid the proximity effect. In measurements performed at T=5K, direct SHPM images showed the structure of antivortices between the magnetic dots, whereas the successive difference images revealed the positioning of additional vortices in applied homogeneous magnetic field.   

Vortex-antivortex phenomena in superconductors with antidot arrays

We investigated in detail the vortex configurations in superconducting films with regular antidot-arrays within the non-linear Ginzburg-Landau theory, where demagnetization effects and overlapping vortex cores are fully taken into account (contrary to the London approach). In addition to the well-known matching phenomena, we predict: (i) the \textit{nucleation of giant-vortex states} at interstitial sites; (ii) the \textit{combination of giant- and multi-vortices} at rational matching fields; and (iii) for particular interstitial vorticity, the symmetry imposed creation of \textit{vortex-antivortex configurations}. As a consequence of (iii), we predict \textit{resistance maxima }at particular matching fields, opposite to the expected minima due to commensurability effects. Using the same principle, we stabilized vortex-antivortex molecules in finite submicron superconducting polygons by strategically placed nanoholes. Compared to earlier predictions, we \textit{enhanced} the stamina of the antivortex with respect to temperature, applied fields and geometrical defects in the sample. Further, increased vortex-antivortex spacing and pronounced amplitudes of the local magnetic field in our system make these fascinating structures \textit{observable} by e.g. Scanning Tunneling or Hall probe microscopy.   

Novel Commensurability Effects and Enhanced Pinning at Nonmatching Fields for Vortices Interacting with Diluted Periodic Pinning Arrays

Using numerical simulations, we demonstrate that periodic pinning arrays that have been diluted by removing some fraction of the pinning sites at random have pronounced commensurability effects at the {\it same} field strength as undiluted pinning arrays. The commensuration can occur at fields significantly higher than the field corresponding to one-to-one matching between the diluted pinning array and the vortices, and the effect persists for periodic arrays with up to 90 percent dilution. We show that samples with diluted periodic pinning arrays produce a considerable enhancement of the critical current for fields above the first matching field compared to samples with purely random pinning arrangements. These results suggest that diluted periodic pinning arrays may be a promising geometry to increase the critical current in superconductors over a wide magnetic field range.   

Influence of Au layer on the morphology and superconductivity of the ultra-thin Pb film using Low-temperature STM/S

Thin film superconductivity is a subject of great scientific importance. Recently by using epitaxial thin Pb films, two papers reported the observation of quantum oscillations of thickness-dependent superconductivity. By using ex situ transport measurements on Pb thin films grown on Si(111) substrate and subsequently covered with 2 ML of Au, Guo et al. reported Tc oscillation between 22 and 28 MLs and a rapid decrease of Tc below 20 ML. On the other hand, Eom et al., by using in situ tunneling spectroscopy to measure the superconducting gap directly, reported persistent quantum oscillations of superconductivity from 18 ML down to 5 ML without any sign of quenching. One explanation of such apparent inconsistency is the existence of the Au capping layer used in the ex-situ transport measurements. Here we explore the role of Au capping layer on superconductivity of Pb thin film directly using STM/S. We show that the Au capping layer induces significant roughening of the Pb thin films. Moreover, we found that the deposition of Au first induces the formation of AuPb alloy followed by Au overlayer. Direct measurement of superconducting gaps on the film at different stages of Au deposition are also performed. The details of how Au overlayer impact the superconductivity of thin Pb films will be presented.   

Session J11: Correlated Organic Conductors

Mixed spin-charge solitons and thermodynamics of (TMTTF)$_2$X

The (TMTTF)$_2$X salts are quasi-one-dimensional materials that undergo two phase transitions as the temperature is lowered from 300 K under ambient pressure. The high temperature transition at T$_{CO}$ $\sim$ 100 K is to a charge-ordered (CO) state. The low temperature transition is often to a spin-Peierls (SP) state that appears at T$_{SP}$ $\sim$ 10 K, and that competes with the CO state. We have investigated the thermodynamics of these systems within an extended Hubbard Hamiltonian that includes (a) on-site and nearest neighbor Coulomb interactions, and (b) bond- and site-coupled quantum phonons. From calculations of charge, bond and spin-susceptibilities we are able to explain the transition from the CO to the SP state. The CO state corresponds to the charge occupancy scheme ...1010... (where `1' and `0' denote charge-rich and charge-poor sites respectively), while the SP state has charge occupancy ...1100.... The transition from the CO to the SP phase as temperature is lowered is driven by spin effects: At high temperatures, high-spin states dominate the free energy, and favor the ...1010... CO configuration. At low temperatures, spin singlet states dominate the free energy and instead favor a singlet SP state with the ...1100... charge pattern. In the temperature region T$_{SP} < T < T_{CO}$ there occur mixed spin-charge solitons that are domain walls between the ...1010... and ...1100... charge patterns.   

Finite-Temperature Phase Transitions in Quasi-One-Dimensional Molecular Conductors

Phase transitions to symmetry-broken states in quarter-filled quasi-one-dimensional molecular conductors, such as DCNQI$_2X$, TMTTF$_2X$, and EDO-TTF$_2X$, are studied theoretically. We consider extended Hubbard chains with on-site, intra-chain, and inter-chain Coulomb interactions, coupled to the lattice degree of freedom by Peierls-type and Holstein-type electron-lattice interactions. We apply the numerical quantum transfer-matrix method to an effective one-dimensional model, treating the inter-chain term within mean-field approximation. Finite-temperature properties are investigated for the charge ordering, the dimer-type Mott transition (bond dimerization), and the spin-Peierls transition (bond tetramerization), by computing the temperature dependences of the order parameters together with those of the charge and spin susceptibilities. A coexistent state of charge order and bond dimerization exhibiting dielectricity is predicted in a certain parameter range, even when intrinsic dimerization is absent. Reference: H. Seo, Y. Motome and T. Kato, preprint(cond-mat/0611499) to be published in J. Phys. Soc. Jpn. {\bf 76} (2007) No. 1.   

Soliton Wall Superlattice Phase in Organic Conductor (Per)$_{2}$Pt(mnt)$_{2}$ in a Magnetic Field

We suggest a model [1] to explain the appearance of a high resistance high magnetic field charge-density-wave (CDW) phase, discovered in quasi-one-dimensional (Q1D) organic conductor (Per)$_{2}$Pt(mnt)$_{2}$. In particular, we show that the Pauli spin-splitting effects improve the nesting properties of a realistic Q1D electron spectrum, and, therefore, a high resistance Peierls CDW phase is stabilized in high magnetic fields. In intermediate and very high magnetic fields, a periodic soliton wall superlattice (SWS) phase is found to be a ground state. We suggest to study the predicted phase transitions between the Peierls and SWS CDW phases to discover a unique SWS state. [1] A.G. Lebed and Si Wu, Physical Review Letters, submitted (2006).   

Nature of superconducting state in the new phase of (TMTSF)$_{2}$PF$_{6}$ under pressure

The unusual phase has been recently observed in the organic material (TMTSF)$_{2}$PF$_{6}$, where superconductivity coexists with spin-density wave in the pressure interval p$_{c1}<$p$<$p$_{c}$ below the first order transition into superconducting or normal metal phase. Assuming that the coexistence takes place on the microscopic scale, we consider the properties of the intermediate phase. We show that the new superconducting state inside spin-density wave phase just above p$_{c1}$ must bear a triplet pairing.   

Angular magnetoresistance oscillations in quasi-one-dimensional organic conductors in the presence of a crystal superstructure

Crystal superstructures, produced by anion ordering in the quasi-one-dimensional organic conductors $\rm(TMTSF)_2ReO_4$ and $\rm(TMTSF)_2ClO_4$, modify electron spectra in these materials and generate effective tunneling amplitudes between distant chains. These amplitudes cause multiple peaks in the interlayer conductivity for the magnetic field orientations along the rational crystallographic directions (the Lebed magic angles). The different wave vectors of anion ordering in $\rm(TMTSF)_2ReO_4$ and $\rm(TMTSF)_2ClO_4$ result in the odd and even Lebed angles, as observed experimentally.\\ Reference: cond-mat/0608317.   

Angular and Temperature Dependent $^{77}$Se NMR in the Metallic and Field-Induced Spin Density Wave State in (TMTSF)$_{2}$ClO$_{4}$

We present an exploratory investigation of the NMR pulse-power and magnetic field direction dependence of the $^{77}$Se NMR line shapes and relaxation rates in the metallic and field-induced spin density wave (FISDW) state of the quasi-one-dimensional organic conductor (TMTSF)$_{2}$ClO$_{4}$. By reducing the integrated NMR pulse power (via width and/or pulse height), the limitations of the enhancement factor below the FISDW transition are overcome, and the $^{77}$Se spin-lattice relaxation rate 1/T$_{1}$ can be measured in both the metallic and FISDW states vs. temperature and field direction. Our results on the temperature dependence of 1/T$_{1 }$in the vicinity of the FISDW transition, and also a description of the temperature and field direction dependence of the NMR spectra, will be presented.   

Superconductivity in (TMTSF)$_2$ClO$_4$ probed by $^{77}$Se NMR

Superconductivity in the Bechgaard salts (TMTSF)$_2$X, with X=PF$_6$, ClO$_4$, survives well beyond the paramagnetic limit set by the transition temperature $T_c\approx$1K. As a result, it has been hypothesized that the spin pairing is triplet. We report on measurements of the $^{77}$Se Knight shift and spin-lattice relaxation rate $T_1^{-1}$, conducted {\it in situ} with interlayer resistivity, deep within the superconducting state of (TMTSF)$_2$ClO$_4$. At fields $H_0\approx$10kOe aligned along the $\mathbf{a}-$ and $\mathbf{b'}-$axes, the Knight shift reveals a decrease in spin susceptibility $\chi_s$ that is likely consistent with singlet pairing. The field dependence of $T_1^{-1}$ at temperatures $T\ll T_c$ exhibits a very sharply-defined increase at a field $H_s\approx$15kOe. For $H_0>H_s$, $T_1^{-1}$ is close to the normal state value, even though $H_{c2}\gg H_s$ and $R_{zz}=0$ to within experimental uncertainty. We discuss the implications for interpreting the results as evidence for a crossover, or a phase transition within the superconducting state.   

Angular Dependent Magnetoresistance Measurements of (Per)$_{2}$Au(mnt)$_{2}$ Under Pressure

The quasi-one-dimensional organic conductor (Per)$_{2}$Au(mnt)$_{2}$ has a charge density wave (CDW) ground state when cooled below a transition temperature of T$_{CDW} \quad \sim $ 12 K under ambient pressure. The CDW state is largely suppressed by applying a pressure of $\sim $ 6 kbar, as shown by a dramatic increase in low temperature conductivity where the behavior remains slightly activated, providing evidence of a mixed CDW-metal state. Oscillations under pressure are observed in the magnetoresistance (MR) which agree well with band structure estimates of the Fermi surface and are explained by Stark quantum interference. The angular dependence of the MR oscillations has been studied using the continuous rotation of a pressure cell in constant magnetic fields, aligned with the crystallographic planes of the sample. The results will be discussed within the context of known MR angular effects (i.e. Lebed or Danner-Kang-Chaikin oscillations) as well as the inhomogenous CDW-metal state which may exist in this pressure range.   

Phase diagram of pressure-induced superconductor $\beta $-(BDA-TTP)$_{2}$\textit{MX}$_{4}$ ($M$=Fe, Ga and $X$=Cl, Br) with localized magnetic moments

We have investigated transport and magnetization properties of $\beta $-(BDA-TTP)$_{2}$\textit{MX}$_{4}$ ($M$=Fe, Ga and $X$=Cl, Br) as a function of pressure, temperature and magnetic field. The title material undergoes metal-insulator transitions above 100 K at ambient pressure. The insulating phase is suppressed with pressure and superconductivity eventually appears above $P_{c}$= 4.5 kbar ($X$=Cl) and 13 kbar ($X$=Br). The general temperature-pressure (\textit{TP}) phase diagram is similar each other, while higher pressure is required for $X$=Br compounds to suppress the insulating state and induce the superconductivity. Pressure dependent DC magnetization studies on $\beta $-(BDA-TTP)$_{2}$FeCl$_{4}$ compound revealed that the AFM ordering persist well above $P_{c}$. In spite of similarity of phase diagram between $M$=Fe and $M$=Ga compounds, magnetoresistance results show distinct behaviors, which indicates the magnetic interaction with the conduction electrons are still effective. The comparison between $X$=Cl and $X$=Br compounds suggests the anion-size effect rather than the existence of localized magnetic moments plays more important role in determining the ground state.   

High field ESR study of the \textit{pi}-$d$ interaction effect in beta-(BDA-TTP)$_{2}$MCl$_{4 }$(M=Fe, Ga)

Novel magnetic organic conductors with \textit{pi}-$d$ interaction have commanded attention since the discovery of field induced superconductivity. One of them, beta-(BDA-TTP)$_{2}$FeCl$_{4}$, has alternating donor molecules and quasi 2$D$ electrical properties. Previous studies of electrical and magnetic properties show an M-I transition at 120K and an AF transition at $T_{N}$=8.5K, suggesting an exchange interaction between the conduction electrons and the Fe$^{3+} \quad d$-electrons. The properties of beta-(BDA-TTP)$_{2}$GaCl$_{4}$ are similar with exception of the absence of the AF transition, which is apparently due to the absence of \textit{pi}-$d$ exchange interaction. We report angular/temperature dependent 240GHz quasi optical ESR measurements on both compounds to probe the magnetic properties. The Ga compound signals follow the donor molecule structure, and show no magnetic order at any temperature. The Fe compound signals are quite different from the Ga compound, and exhibit AF behavior below $T_{N}$. The difference of Fe and Ga compounds will be discussed in terms of the interaction between localized and itinerant magnetic moments.   

Metastable domains at the pressure induced neutral-ionic transition of TTF-CA

Tetrathiafulvalene-Chloranil (TTF-CA) is the prototypical organic charge transfer (CT) salt whose neutral-ionic and dimerization (Peierls) transitions have been studied on cooling or under pressure. Volume changes switch the ground state from a band insulator with a fractional CT from TTF to CA of $\rho \sim$ 0.3 in a regular stack to a Mott insulator with $\rho >$ 0.5 in a dimerized stack. TTF-CA spectra indicate electron-vibration coupling to both lattice (e-ph) and molecular (e-mv) modes that lead to competing instabilities. Near the metallic point of the rigid system, a one-dimensional adiabatic Hubbard model with linear e-ph and e-mv coupling leads to metastable domains with different $\rho$, $\rho'$ that are thermally accessible at 300 K over a limited bistability range. The transition of TTF-CA single crystals at 1 GPa indicates a pressure range with two resolved $\rho$, $\rho'$. The model also describes the first order transition at 81 K at ambient pressure and generates anharmonic potential energy surfaces. These quantum transitions are driven by volume changes without contributions from electronic excited states.   

Mapping the temperature-dependent quasiparticle scattering rate over the Fermi surface of an organic superconductor

The interlayer magnetoresistance $\rho_{zz}$ of the organic metal $\kappa$-(BEDT-TTF)$_2$Cu(NCS)$_2$ is studied in fields of up to 45~T and at temperatures $T$ from 0.5~K to 30~K. The peak in $\rho_{zz}$ seen in in-plane fields, a definitive signature of interlayer coherence, remains to $T$s exceeding the Anderson criterion for incoherent transport by a factor $\sim 30$. Angle-dependent magnetoresistance oscillations are modeled using an approach based on field-induced quasiparticle paths on a 3D Fermi surface, to yield the $T$ dependence of the scattering rate $\tau^{-1}$. The results suggest that $\tau^{-1}$ does not vary strongly over the Fermi surface, and that it has a $T^2$ dependence due to electron-electron scattering. These findings are contrasted with recent experiments on cuprates, and their implications for models of organic superconductivity (e.g. FLEX) are discussed.   

Coexisting fluctuations of charge ordering in quasi-2D organic conductors, $\theta$-(ET)$_2X$

Charge ordering is a crystallization of electrons driven by strong electronic correlations, and is one of the central issues in organic conductors. In particular, the charge ordering in 1/4-filled quasi-2D materials, $\theta$-(ET)$_2X$, attracts much attention since the anisotropic triangular lattice structure enables us to study the stability of charge order under the geometrical frustration and quantum fluctuation. A systematic phase diagram was obtained for the anion $X$ which controls the frustration, and surprisingly, an unusual coexistence of charge fluctuations with different wave numbers was observed in the quantum critical regime where the transition temperature goes to zero. Some exotic phenomena such as the strongly non-linear I-V characteristics and large magneto-resistence are observed in this regime, possibly induced by the anomalous properties of the charge degree of freedom. Here we theoretically study how the charge order and its fluctuations develop in the frustrated systems by applying the random-phase approximation to the extended Hubbard model with electron-phonon couplings. We successfully reproduce the coexisting charge fluctuations as well as the phase diagram. The coexistence originates from the competition between the kinetic energy and the Coulomb repulsion in the intermediate correlation regime, and is characteristic of the charge degree of freedom.   

Anisotropy of the competing superconducting and magnetic states in quasi-2D organic conductor $\kappa $-(BEDT-TTF)$_{2}$Cu[N(CN)$_{2}$]Br: An elastic investigation

Ultrasonic measurements performed on the quasi-2D organic conductor $\kappa $-(BEDT-TTF)$_{2}$Cu[N(CN)$_{2}$]Br reveal a phase separation between superconductivity and magnetism in the vicinity of the Mott transition line. We report here longitudinal (L) and transverse (T) ultrasonic velocity measurements propagating perpendicularly to the highly conducting planes; a magnetic field up to 18 Tesla could be applied along the same direction to differentiate the superconducting phase from the magnetic one. The huge velocity dip observed between 30 and 40 K and associated to a compressibility increase driven by the electronic degrees of freedom is not observed for T-waves polarized along [001]; this implies that only magnetic fluctuations associated to 1D sheets of the Fermi surface can couple to the ultrasonic waves. Around T$_{c}$= 12 K, both the temperature profile and the amplitude of the elastic anomalies are highly dependent on the wave polarization. A magnetic field investigation of these anomalies not only establishes the anisotropic character of the superconducting anomaly, but it reveals also the onset of a magnetic transition below 15 K over the same temperature range as the superconducting one. These anomalies likely favor a multi-component superconducting order parameter.   

Session J12: Focus Session: Spin Control and Dynamics in Quantum Dots

Single spins in diamond: polarization, readout, and coherent control

The Nitrogen-Vacancy (N-V) color center in diamond is well suited for studying electronic and nuclear spin phenomena, since its spin state can be both initialized and read out optically. Moreover, N-V center spins may allow for quantum information processing, as measurements have shown long room- temperature electron spin coherence times well into the microsecond regime. Here, we report on recent experimental progress towards coherent control and coupling of single spins in diamond. Using magneto-photoluminescence imaging and electron spin resonance (ESR) measurements at room temperature, we have investigated single N-V center spins that are coupled to electron spins of nearby nitrogen (N) defects. These N spins are optically inactive (`dark'), but can be detected via the N-V center, as the N-V and the N spins are coupled via the magnetic dipolar interaction. Some of the N-V centers are strongly coupled to only one single N spin, allowing the controlled polarization and readout of this single `dark' N spin \footnote{R. Hanson, F. M. Mendoza, R. J. Epstein and D. D. Awschalom, Phys. Rev. Lett. {\bf 97}, 087601 (2006)}. From time-resolved pump-probe measurements we find the relaxation time of the single N electron spin to be 75 microseconds at room temperature. More recently, we have demonstrated the coherent control of the N-V center spin using optical detection of pulsed ESR and spin echo techniques \footnote{R. Hanson, O. Gywat and D.D. Awschalom, Phys. Rev. B {\bf 74}, 161203(R) (2006)}. Using these tools at different static magnetic fields, we have found that the main source of decoherence for the N-V center spins in our sample is the dipolar coupling to the surrounding bath of N spins. These results pave the way towards room-temperature coherent control of coupled spin states in diamond.   

Valley Kondo Effect in Silicon Quantum Dots

Recent progress in the fabrication of quantum dots using silicon opens the prospect of observing the Kondo effect associated with the valley degree of freedom. We compute the dot density of states using an Anderson model with infinite Coulomb interaction U. The density of states is obtained as a function of temperature and applied magnetic field in the Kondo regime using an equation-of-motion approach to obtain the Green's functions of the electrons. We predict the appearance of a very complex peak structure near the Fermi energy, much richer than the one or two peaks of the usual spin Kondo effect. We also show that the valley index is typically not conserved when electrons tunnel into a silicon dot. Analysis of the conductance should enable experimenters to understand the interplay of Zeeman splitting and valley splitting, as well as the dependence of tunneling on the valley degree of freedom.   

Tuning Hole g-Factors in Self-Assembled InAs/GaAs Quantum Dots with an Electric Field

The g-factors of holes in quantum dots (QDs) determine the energy splittings of the spin states in a magnetic field, influencing spin precession, spin lifetimes, and photoluminescence polarization. Modulation of the g-factor by an electric field may permit spin manipulation for quantum information processing. Hole g-factors in quantum wells have a large anisotropy between the in-plane (g= 0) and growth (g=2.3) directions[1]. Calculations of hole g-factors for InAs/GaAs[2], CdTe[3], and Ge/Si QDs[4] have also indicated size dependency. Using 8-band k.p theory, we calculated electric field dependent hole g-factors on a variety of InAs/GaAs QDs. We find a large anisotropy: g=0.75 and 2.5 for B along (1-10) and (001) respectively for an elliptical dot with Eg=1.136, and g=0.059 and 2.8 for a round dot with Eg=1.133. A 100kV/cm field along (001) changes the (1-10) g-factor from 0.75 to 1.1 in the elliptical dot (0.059 to 0.058 for the round dot), and the (001) g-factor changes from 2.5 to 2.3 (2.8 to 2.9). [1] Sapega et al., PRB 45, 4320 (1992). [2] C. Pryor and M. E. Flatte, PRL 96, 026804 (2006). [3] Prado et al., PRB 69, 201310(R) (2004). [4] Nenashev et al., PRB 67, 205301 (2003).   

Non-destructive Kerr rotation measurements of a single spin in a quantum dot

A single electron spin in a quantum dot forms a natural two state system for use in quantum information processing. The ability to measure this spin without destroying the system is an important step towards observing various quantum measurement-related phenomena. In contrast to previous experiments, we have performed non-destructive Kerr rotation measurements on a single electron spin confined in a charge-tunable semiconductor quantum dot\footnote{J. Berezovsky \textit{et al., Science Express}, 9 November 2006, (10.1126/science.1133862).}. This measurement technique provides a means to directly probe the spin off-resonance, thus minimally disturbing the system. Energy-resolved Kerr rotation spectra demonstrate that we are probing a single electron, and also yield information about the optically-pumped spin polarization as a function of quantum dot charging. These results point the way towards quantum non-demolition measurements and optically-mediated entanglement of spins in the solid state.   

Kerr rotation studies of single electron spin dynamics in a quantum dot

Kerr rotation measurements are used to directly and non-destructively probe the dynamics of a single electron spin in a charge-tunable quantum dot \footnote{ J. Berezovsky, M. H. Mikkelsen, O. Gywat, N. G. Stoltz, L. A. Coldren, and D. D. Awschalom,{\em Science Express}, 9 November 2006, (10.1126/science.1133862)}. The dot is formed by interface fluctuations of a GaAs quantum well and embedded in a vertical optical cavity. Using Hanle techniques, we perform single electron Kerr rotation measurements at $T=10\mathrm{K}$ in order to monitor the depolarization of an optically pumped electron spin within an applied transverse magnetic field. This reveals information about the time averaged transverse spin lifetime, $T_2^*$. At gate voltages for which the charging rate of the dot is relatively low, the results yield a $T_2^*$ in agreement with values expected from the hyperfine interaction in these materials. In contrast, at larger charging rates, we find that $T_2^*$ is strongly reduced, indicating the importance of additional decoherence mechanisms in that regime.   

Driven coherent oscillations of a single electron spin in a quantum dot

The ability to control the quantum state of a single electron spin in a quantum dot is at the heart of recent developments towards a scalable spin-based quantum computer. In combination with the recently demonstrated controlled exchange gate between two neighbouring spins [1], driven coherent single spin rotations would permit universal quantum operations. In this talk, I will discuss the experimental realization of single electron spin rotations in a gate-defined GaAs double quantum dot. We coherently control the quantum state of the electron spin by applying short bursts of an on-chip generated oscillating magnetic field [2]. This allows us to observe up to eight Rabi oscillations of the electron spin in a microsecond burst. Via Ramsey-type pulse sequences we measure an apparent time-averaged coherence time which is limited by the hyperfine interaction with the nuclear spins. We erase these nuclear spin effects to a large extend via spin-echo pulse sequences and recover the intrinsic coherence time. [1] J.R. Petta et al., Science 309, 2180--2184 (2005). [2] F.H.L. Koppens et al., Nature 442, 766-771 (2006).   

Spin Coherence Modulated Trion Transitions and Probabilistic Initialization in Charged Semiconductor Quantum Dots

The presence of symmetry breaking in a three-level $\Lambda $ system consisting of two spin ground states and a charged exciton (trion) state leads to new features, where the population excited to the trion state is modulated by the spin coherence. This phenomenon is due to the unique semiconductor environment of the quantum dot (QD) system, which allows for two simultaneously orthogonal spinor axes. In addition, the polarization dependent excitations due to the double spinor axes of the system can be utilized to create a net spin from a completely mixed spin state, which is impossible to achieve through unitary operation of the spin system. This result provides an important application to the practical implementation of ultrafast spin based quantum computation in the semiconductor QD system in terms of qubit initialization.   

Radio frequency charge sensing and nuclear polarization of a two-electron double quantum dot

We report charge-sensing measurements of a two-electron double quantum dot using an electrometer based on a radio-frequency quantum point contact (rf-QPC). In analogy with the radio frequency single electron transistor (rf-SET) the rf-QPC makes use of a LC impedance transformer and radio frequency reflectometry to achieve high charge sensitivity over a bandwidth exceeding 20MHz. In addition, we use controlled nanosecond pulsing of the double-dot potential to drive singlet to triplet transitions that develop a partial polarization of the nuclear spins. Using the rf-QPC, the subsequent diffusion and dynamics of nuclear polarization is examined on fast timescales.   

Fast spin state preparation in a single charged quantum dot

Electron spins trapped inside of semiconductor dots (QD) are promising candidates for quantum bits (qubits). Quantum computation requires both the initialization of qubits with a high fidelity and a continuous supply of fresh ancillary qubits. The latter is the key for quantum error correction (QEC). Here, we demonstrate fast spin state preparation (laser cooling of an electron spin) in a single charged InAs self-assembled QD by applying magnetic field in the Voigt geometry. The preparation efficiency of the spin state is 98.9{\%} at a magnetic field of 0.88T, which corresponds to the cooling of spin from 5 K (experimental temperature) to 0.06 K. To reach the same efficiency by thermal equilibration, a 69 T static magnetic field should be applied. The spin cooling rate of $3\cdot 8\times 10^9s^{-1}$ is achieved. This is three orders of magnitude faster than the spin decoherence rate, which is on the order of$1\times 10^6s^{-1}$.   

Exchange-controlled single-spin rotations in quantum dots

We show theoretically that arbitrary coherent rotations can be performed quickly (with a gating time $\sim 1\,\mathrm{ns}$) and with high fidelity on the spin of a single electron confined to a quantum dot using exchange. These rotations can be performed using experimentally proven techniques for rapid exchange control, without the need for spin-orbit interaction or ac electromagnetic fields. We expect that implementations of this scheme would achieve gate error rates on the order of $\eta \sim 10^{-3}$, within reach of several known error-correction schemes.   

Mixing of trion spin states in single and coupled dots from electron-hole and electron-electron exchange

Polarized light spectroscopy is of interest for initializing and reading the electron (e) spin state in quantum dot (QD) systems for quantum information applications. An additional electron-hole (e-h) pair is created in the QD giving rise to a transient trion [1]. The mechanisms behind the spin-nonconserving (asymmetric) e-e exchange and of the e-h exchange are important for understanding the spin dynamics of the trion. Here, first we show the importance of the long-range e-h exchange for the flip-flop mechanism in the lowest triplet of a single QD, particularly for highly-symmetric QDs. This adds to the strong e-e asymmetric exchange in a cylindrical QD [2]. Second, we consider a double-dot system, and we describe the combined effect of e-h and e-e asymmetric exchange in the lowest (delocalized) triplet by comparison to the first excited (localized) triplet [1] M. E. Ware et al., PRL 95(17), 177403 (2005) [2] S. C. Badescu and T. L. Reinecke, cond-mat/0610405   

Electric Dipole Spin Resonance for Heavy Holes in Quantum Dots

We propose and analyze a new method for manipulation of a heavy hole spin in a quantum dot. Due to spin-orbit coupling between states with different orbital momenta and opposite spin orientations, an applied rf electric field induces transitions between spin-up and spin-down states. This scheme can be used for detection of heavy-hole spin resonance signals, for the control of the spin dynamics in two-dimensional systems, and for determining important parameters of heavy-holes such as the effective $g$-factor, mass, spin-orbit coupling constants, spin relaxation and decoherence times.   

Quantum Hall Ferrimagnetism in lateral quantum dot molecules

We demonstrate the existence of ferrimagnetic and ferromagnetic phases in a spin phase diagram of coupled lateral quantum dot molecules in the quantum Hall regime. The spin phase diagram is determined from Hartree-Fock Configuration Interaction method as a function of electron numbers N, and magnetic field B. The quantum Hall ferrimagnetic phase corresponds to spatially imbalanced spin droplets resulting from strong inter-dot coupling of identical dots. The quantum Hall ferromagnetic phases correspond to ferromagnetic coupling of spin polarization at filling factors between nu=2 and nu=1 [1]. [1] Ramin M. Abolfath, and Pawel Hawrylak, Phys. Rev. Lett 97, 186802 (2006).   

Session J13: Focus Session: Multiferroics III

Interatomic spin-orbit coupling: mechanism for spin-spiral-caused ferroelectricity

There are two general classes of mechanisms that have been proposed for spin-spiral caused ferroelectricity, one based on ionic displacements as primary cause, the other on charge distortion without ionic displacements. Here we discuss the latter$^{1,2}$. The mechanism proposed here is illustrated by a model where a pair of ions a and b each have low-lying s- and excited p- states with a prescribed spin state $\chi_a$ for the a-site states, similarly for the b-site, and there are 2 electrons; interatomic spin orbit coupling resides in inter-ion hopping due to s-p matrix elements of the spin-orbit coupling operator $\propto \nabla V \times \mathbf{p}\cdot\mathbf{s}$, $V, \mathbf{p}, \mathbf{s}$ are 1- electron potential, momentum, spin, respectively. Assuming the symmetry of a nearest-neighbor pair of cubic- spinel B-sites (there's no center of inversion (coi)) we find an electric dipole moment in the direction $\mathbf{r}_ {ab}\times(\mathbf{S}_a\times\mathbf{S}_b)$, as was found when there is a coi$^{1,2}$. For the spins in a chain parallel to the spiral wave vector in CoCr$_2$O$_{4}$, direction [110]$^3$, this results in ferroelectricity, as observed$^{4}$; the spins in a [1,-1,0]-directed chain, give an antiferroelectric component. Extension to a pair of Cr$^{3+}$ ions will be discussed.\\ 1. H. Katsura et al,Phys. Rev. Lett.\textbf{95},057205 (2005)\\ 2. T. A. Kaplan and S. D. Mahanti, cond-mat/0608227, 10 Aug 2006 \\ 3. N. Menyuk et al, J. Phys. (Paris) \textbf{25}, 528 (1964)\\ 4. Y. Yamasaki et al, Phys. Rev. Lett. \textbf{96}, 207204 (2006)   

Magnetoelectric properties of cobalt oxides with low dimensional structures

Since the discovery of novel ferroelectric transition due to spiral spin structures in TbMnO$_3$, materials with spin frustration or nontrivial spin structures have attracted renewed interest as a promising candidate for new magnetoelectrics. In this context, we have focused on compounds with low dimensional structures because they often possess geometrical frustration and resultantly exhibit nontrivial spin structures. In this work, we have investigated the magnetic and dielectric properties of cobalt oxides with low dimensional structures. The subject compound, BaCo$_2$Si$_2$O$_7$ single crystal, is a derivative from Ba$_2$CuGe$_2$O$_7$ in which the spiral spin structure is reported below 3.26K\@. We have substituted Co$^{2+} $ for Cu$^{2+}$ to increase the transition temperature. The crystallographic symmetry of the obtained crystal at RT was confirmed to be $C2/c$ which does not break the inversion symmetry. The Weiss temperatures estimated in paramagnetic region are $-20$K ($H$$\| $$c$) and $-74$K ($H$$\bot $$c$), indicating the large magnetic anisotropy. The weak ferromagnetic magnetization rises up at 21K, where the dielectric constant perpendicular to the $c$ axis ($\varepsilon_{\bot}^c$) decreases concomitantly. In addition, we have observed the magnetocapacitance effect below 21K: $\Delta \varepsilon_{\bot} ^c (\mu_0H_{\bot}^c$=8T$)/ \varepsilon_{\bot}^c (0)$ reaches 0.2\% at 5.5K\@. This result suggests that there exists a coupling between magnetism and dielectricity. Results of Ba$_2$CoSi$_2$O$_7$ will also be presented.   

Ferroelectricity in a quantum chain magnet.

Multiferroics with enhanced cross-coupling effects exhibit magnetic orders with broken centrosymmetry. It turns out that the lattice relaxation through exchange striction associated with the magnetic orders with non-centrocymetry is the origin of magnetism-induced ferroelectricity. Among the exchange strictions, the Dzyaloshinskii-Moriya type interaction becomes active when ferroelectricty is induced by spiral magnetic orders. Herein, we report our suprising discovery that a quantum chain magnet exhibits ferroelectricity when the spiral magnetic order sets in.   

Direct transition from a disordered to a multiferroic phase

We report the first direct transition from a paramagnetic and paraelectric phase to an incommensurate multiferroic in a triangular lattice antiferromagnet. Ferroelectricity is only observed when the magnetic structure has chirality and breaks inversion symmetry. A magnetic field extinguishes the electric polarization through a change in the magnetic symmetry. A Landau expansion of symmetry-allowed terms in the free energy demonstrates that the chiral magnetic order of the triangular lattice antiferromagnet gives rise to a pseudoelectric field, whose temperature dependence agrees with that of the observed electric polarization. The observed multiferroic behavior provides a theoretically tractable example of ferroelectricity from competing spin interactions.   

Mapping Structural Phase Separation in Eu$_{0.5}$Y$_{0.5}$MnO$_{3}$ using 3D X-ray Microdiffraction

Phase coexistence in multicomponent manganite systems is known to occur over a wide range of length scales and strongly influences the magnetic and electronic properties. We have used 3D synchrotron x-ray Laue microdiffraction to investigate domain formation and local lattice structure in bulk Eu$_{0.5}$Y$_{0.5}$MnO$_{3}$ single-crystals. X-ray microdiffraction yields 3D spatially-resolved maps of the crystal structure, orientation and lattice parameter, while microfluorescence yields depth-integrated composition maps. The x-ray measurements reveal alternating lamella of orthorhombic Eu-rich and hexagonal Y-rich phases with a self-organized periodicity of $\sim $15 microns. Both phases maintain a well-defined long-range ($\sim $mm) average crystal orientation with respect to the growth direction and to each other. However, small local variations in both orientation (i.e. mosaic) and lattice parameter (strain and composition) are observed, and the possible origins and implications of these inhomogeneities will be discussed.   

Resonant magnetic scattering of multiferroic HoMnO$_{3}$

The multiferroic material HoMnO$_{3 }$displays electrical polarization below the Curie temperature $T_{C}$ = 875 K and antiferromagnetic Mn$^{3+}$ ordering at N\'{e}el temperature, $T_{N}$~= 75 K. The ferroelectric phase possesses hexagonal P6$_{3}$cm symmetry with polarization $P_{c}$~=~5.6~$\mu $C~cm$^{-2}$ along the hexagonal $c$ axis. In order to shed further light on the magnetic order in this compound, element specific X-ray resonant magnetic scattering was performed at the Ho $L_{III}$ absorption edge. Resonance enhancement was observed in both quadrupole and dipole channels below 40 K. Measurement of ($0 0 l)$ and $(h 0 l) $( $l$ odd) reflections have resolved contradictions present in the literature regarding the magnetic order of Ho$^{3+}$ moments. From 40 K down to 6 K, Ho$^{3+ }$ moments order according to magnetic space group P6$_{3}^{\mbox{'}}$cm$^{\mbox{'}}$ with different values for the ordered moment on the Wyckoff sites 2a and 4b. According to this space group, Ho moments are ferromagnetically correlated in the \textbf{a-b} plane and antiferromagnetically correlated in the \textbf{c}-direction. The coincidence of the transition temperature (40 K) for Ho$^{3+ }$moment ordering, spin rotation for Mn$^{3+}$ moments, and sharp anomaly in dielectric constant indicates that the interplay between ferroelectricity and magnetism is strongly related to the Ho magnetic order.   

Crystal Field Excitations in Multiferroic HoMnO$_3$

Antiferromagnetic and ferroelectric order coexist in hexagonal HoMnO$_3$, and strong coupling between these two order parameters has been previously observed. Neutron scattering measurements of the low-energy excitations in HoMnO$_3$ reveal a complex spectra of Ho$^{3+}$ crystal-field excitations which depend upon both temperature and applied magnetic field. These crystal-field changes are correlated with changes in the magnetic symmetry of the Mn$^{3+}$ magnetic sublattice. Measurements of the magnon excitations near these crystal field levels indicates strong coupling between the Mn$^{3+}$ moments and the Ho$^{3+}$ crystal field levels. This coupling may play a critical role in explaining the interaction of ferroelectricity and antiferromagnetism in HoMnO$_3$.   

Ferroelectricity driven by Y $d^0$-ness with re-hybridization in YMnO$_3$

Recently multiferroicity, in which magnetism and ferroelectricity co-exist, takes much attention due to its exotic magnetoelecric (ME) phenomena. Hexagonal $R$MnO$_3$ ($R$ = Ho, Er, Tm, Yb, Lu, Y, Sc) exhibits multiferroicity with high ferroelectric and low antiferromagnetic transition temperature ($T_E >$ 600K, $T_M\sim$ 90K). The hexagonal structure ($P6_3cm$) brings out soft mode phonons required for ferroelectricity, but its driving force has been puzzled. We investigated electronic structure of hexagonal multiferroic YMnO$_3$ using the polarization dependent x-ray absorption spectroscopy (PXAS) at O $K$- and Mn $L_{2,3}$-edges. PXAS exhibits strong polarization dependence at both edges, reflecting anisotropic Mn 3$d$ orbital occupation. Moreover, the O $K$-edge spectra show that Y $4d$ states are strongly hybridized with O $2p$ ones, resulting in large anomalies in Born effective charges on off-centering Y- and O-ions. These results manifest that the Y $d^0$-ness with re-hybridization is the driving force for the ferroelectricity, and suggest a new approach to understand the multiferroicity in the hexagonal manganites.   

Spin-lattice coupling in RMnO3 (R=rare earth and Y) perovskites

The magnetic order in RMnO3(R=rare earth and Y) perovskites is quite sensitive to the R3+ ionic radius. Type A magnetic order has been observed for R=Gd?La. For R=Dy and Tb, no classic magnetic order was observed down to the lowest temperature. The rest members of the family show a type E magnetic order. As far as we know, the lattice response to the magnetic order has not been systematically studied. Here we will discuss the lattice response to the magnetic order studied by synchrotron x-ray powder diffraction and thermal conductivity.   

Epitaxially stabilized hexagonal TbMnO$_{3}$ thin films

We observed that hexagonal TbMnO$_{3}$ thin films showed multiferroic properties with enhanced ferroelectricity. The hexagonal TbMnO$_{3}$ thin film shows 1.6 \textit{$\mu $}C/cm$^{2}$ as the remnant polarization value, which is 20 times larger than that of its orthorhombic bulk phase. In addition, the ferroelectric Curie temperature is shifted to 60 K compared to the low temperature value (27 K) of its bulk orthorhombic phase. Interestingly, the hexagonal TbMnO$_{3}$ film displayed the emergence of a new antiferroelectric-like phase just above 65 K, which is the first of its kind in the family of multiferroic hexagonal manganites. A clear anomaly in dielectric constant near the antiferromagnetic Neel temperature ($T_{N}\sim $70 K) shows the possible coupling between the spin and charge degrees of freedom. This is indeed confirmed by the observed second-order magnetoelectric effect below $T_{N}$.   

Spin structures of magnetic phases in magnetic ferroelectrics, RMn2O5(R = Y and Tb)

We report polarized and unpolarized neutron diffraction data obtained from single crystals of magnetic ferroelectrics, RMn2O5(R=Y and Tb). Each system undergoes, upon cooling, more than one magnetic and ferroelectric phase transitions. By using the group representation theory and fitting the data, we have determined the spin structures of the different phases to elucidate the microscopic mechanism of the static spin-charge coupling in the multiferroics. Our results show that the magnetic and ferroelectric phases of the two systems have spin structures that can only be constructed by a linear combination of the basis functions of two two-dimensional representations of the magnetic space group. Implication of the spin structures to the electric polarization of the systems and to theoretical models will also be discussed.   

Magnetic excitations in magnetic ferroelectrics, RMn$_2$O$_5$ (R = Y and Tb)

We report inelastic neutron scattering data obtained from powder and single crystal samples of magnetic ferroelectrics, RMn$_2$O$_5$ (R=Y and Tb). In these systems, magnetic moments are lying on the crystalline ab-plane, and the spontaneous electric polarization occurs along the b-axis. Our data shows that there are several different magnetic excitation modes of Mn spin waves upto 15 meV. The lowest energy excitation is the sliding mode of the moments in the ab-plane, called phason. We have performed linear spin wave calculations to reproduce our data including dispersions as well as polarizations of the spin wave excitations. Relation of the spin waves to electric polarization will be discussed.   

Resonant and Non-resonant x-ray Scattering Studies on Multiferroic TbMn$_{2}$O$_{5}$

Comprehensive x-ray scattering studies on single crystal TbMn$_{2}$O$_{5}$, including resonant scattering Mn $L$ edge, Tb $L$ edge and $M$ edges, have been carried out. Non-resonant x-ray scattering data provide the first crystallographic evidence of symmetry lowering. The x-ray intensities were observed at a forbidden Bragg position in ferroelectric phase. Their temperature dependence as well as q-dependence of CDW peaks are consistent with exchange striction mechanism for multiferroicity in the sample. Intensities of incommensurate CDW peaks show anomalous temperature dependences, which are attributed to existence of magnetic ordering having different temperature dependences. Resonant scattering data confirmed that magnetic moments of Mn$^{3+}$, Mn$^{4+}$ and Tb have different temperature dependences below T$_{N} \quad \sim $ 41K.   

Elucidaion on the effects of hydrostatic pressure on multiferroic, HoMn2O5

HoMn$_{2}$O$_{5}$ has been the subject of intense study as a mutltiferroic material in which both ferroelectricy and magnetic ordering coexist. Earlier work has shown that the ferrolelectric polarization and the dielectric constant are strongly effected by the application of a magnetic field. At low temperatures, as the system's magnetic structure lowers from a commensurate to an incommensurate magnetic phase, ferrolectricity is suppressed and the system becomes paralelectric. It has recently been shown that the applications of hydrostatic pressures of 6 Kbar can stabilize the ferroelectric phase of HoMn$_{2}$O$_{5}$ at low temperatures. During this talk, we discuss the results of neutron diffraction experiments performed on the BT1 powder diffractometer at the NCNR that correlate this preservation of ferroelectricty with changes in the magnetic structure.   

Pressure Induced Stabilization of the Ferroelectricity and Commensurate Magnetic structure in $R$Mn$_{2}$O$_{5}$ ($R$=Tb,Dy,Ho)

Measurements of ferroelectric (FE) polarization were done on $R$Mn$_{2}$O$_{5}$ ($R$=Tb,Dy,Ho) with applied isotropic pressures up to 17kbar. At low temperatures, the high polarization commensurate magnetic state destabilizes into a low polarization incommensurate magnetic state. This is shown by a sharp drop in the ferroelectric polarization with an associated increase in the dielectric constant. This feature was found to be pressure dependent and is suddenly quenched upon passing an $R$-dependent critical pressure, implying the stabilization of the high ferroelectric polarization state which is coincident with the commensurate magnetic structure. The direct correlation between the commensurability of the magnetic structure and the high polarization state is further revealed by high pressure neutron powder diffraction measurements on HoMn$_{2}$O$_{5 }$showing the pressure induced phase transition from the incommensurate to the commensurate magnetic state.   

Session J14: Focus Session: Domain Wall Motion and Spin Dynamics

Effects of domain wall width on current- and field-driven wall motion

Magnetic domain wall motion can be driven by a magnetic field or by a spin-polarized electric current traversing the wall. The velocity of field-driven wall motion [1] depends on the details of the domain wall structure and varies in direct proportion to the wall width. By contrast, a current is predicted to augment the velocity of a domain wall by an amount that is \textit{independent} of its structure. Using high-bandwidth scanning Kerr polarimetry, we have studied field- [1] and current-driven [2] wall dynamics in Permalloy nanowires whose widths span a broad range. Wall width is a function of the wire cross-sectional geometry, and the field-driven wall mobility varies in proportion to the calculated wall width. However, the mobility of current-driven motion also depends on wire geometry and is strongly correlated with the field-driven wall mobility. The results will be discussed in relation to available spin-torque models. [1] G. S. D. Beach, et al., Nature Mater. 4, 741 (2005) [2] G. S. D. Beach, et al., Phys. Rev. Lett. 97, 057203 (2006).   

Current induced resonance of vortex domain wall in permalloy nanowires

We explore the current induced resonant excitation of magnetic domain walls in permalloy nanowires which have vortex structures. Domain walls (DWs) are injected and pinned at artificial pinning sites or notches patterned by electron beam lithography in nanowires 300 nm wide and 20 nm thick. Ac current excitation of vortex DW has been measured using RF rectification method where ac-current is applied to nanowire by using bias-tee and at the same time dc-voltage difference across the nanowire has been measured with voltmeter. The ac current results in a translational motion of the DW vortex core which accompanies the resistance change of sample due to the anisotropic magnetoresistance effect at the DW. This resistance change gives rise to a small but measurable dc voltage along the nanowire and allows us to detect its resonance. The resonance frequency is very sensitive to external magnetic field applied along the wire. We interpret the resonance to be due to a magnetic field induced motion of the DW within the pinning potential arising from the notch. Using the DW energy profile calculated from micromagnetic simulations and a 1d analytical model we obtain good agreement with the experimentally observed field dependence of the resonant frequency.   

Thermally-Assisted Current-Driven Domain Wall Motion

Starting from the stochastic Landau-Lifschitz-Gilbert equation, we derive Langevin equations that describe the nonzero-temperature dynamics of a rigid domain wall. We derive an expression for the average drift velocity of the domain wall $\langle \dot{r}_{\rm dw} \rangle$ as a function of the applied current, and find qualitative agreement with recent magnetic semiconductor experiments. Our model implies that at any nonzero temperature $\langle \dot{r}_{\rm dw} \rangle$ initially varies linearly with current, even in the absence of non-adiabatic spin torques.   

Oscillatory dependence of current driven domain wall motion on current pulse length

The motion of domain walls (DW) in magnetic nanowires driven by spin torque from spin-polarized current is of considerable interest. Most previous work has considered the effect of dc or $\sim $microsecond long current pulses. Here, we show that the dynamics of DWs driven by nanosecond-long current pulses is unexpectedly complex. In particular, we show that the current driven motion of a DW, confined to a pinning site in a permalloy nanowire, exhibits an oscillatory dependence on the current pulse length with a period of just a few nanoseconds [1]. This behavior can be understood within a surprisingly straightforward one dimensional analytical model of the DW's motion. When a current pulse is applied, the DW's position oscillates within the pinning potential out of phase with the DW's out-of-plane magnetization, where the latter acts like the DW's momentum. Thus, the current driven motion of the DW is akin to a harmonic oscillator, whose frequency is determined by the ``mass'' of the DW and where the restoring force is related to the slope of the pinning potential. Remarkably, when the current pulse is turned off during phases of the DW motion when it has enough momentum, the amplitude of the oscillations can be amplified such that the DW exits the pinning potential well \textit{after} the pulse is turned off. This oscillatory depinning occurs for currents smaller than the dc threshold current, and, moreover, the DW moves against the electron flow, opposite to the propagation direction above the dc threshold. These effects can be further amplified by using trains of current pulses whose lengths and separations are matched to the DW's oscillation period. In this way, we have demonstrated a five fold reduction in the threshold current required to move a DW out of a pinning site, making this effect potentially important for technological applications. [1] L. Thomas, M. Hayashi, X. Jiang, R. Moriya, C. Rettner and S.S.P. Parkin, Nature 443, 197 (2006).   

Spin-Transfer-Torque-Driven Domain-Wall Dynamics in Permalloy Nanowires

Pulse-current-driven domain-wall dynamics in Permalloy nanowires are studied using high-temporal-resolution magneto-optical techniques. The time-resolved measurements elucidate mechanisms responsible for stochastic variation in pulse-current-stimulated wall displacements, and resolve factor-of-10 disagreements between prior experimental $^{[1,2]}$ and theoretical determinations $^{[3]}$ of domain-wall velocity and spin-flip efficiency in magnetic nanowire structures. Current pulses with different width and amplitude are used to probe the domain-wall motion. By reducing the pulse width, higher current densities can be achieved, leading to more complex domain structures (probed by MFM) in the final state. [1] A. Yamaguchi et al. PRL 92, 077205-1, 2004 [2] M. Klaui et al. PRL 95, 026601-1, 2005 [3] Z. Li and S. Zhang, PRL 92, 207203-1, 2004   

Coherent precession of propagating domain walls in permalloy nanowires

We report on domain wall (DW) dynamics in permalloy nanowires. We demonstrate the precessional nature of the DW propagation above the Walker breakdown field. Time resolved resistance measurements were performed on 200 nm wide 10 nm thick permalloy nanowires. Oscillations in resistance are observed when the DW propagates along the nanowire. The frequency of this oscillation varies linearly with magnetic field, according to the Larmor precession frequency. By contrast, current passing through the nanowire has relatively little effect on the oscillation frequency even though it is large enough to influence the DW velocity. To explore the origin of this resistance oscillation, dc resistance measurements were performed on permalloy nanowires with a pinning center located along the nanowire. The state of the DW pinned at the pinning center can be inferred from the nanowire's resistance. By using a combination of current and magnetic field, the time at which the DW arrives at the pining center can be tuned, allowing us to show that the chirality of the domain walls reverses periodically as the wall propagates along the nanowire.   

Damping in Ferromagnets: Landau-Lifshitz versus Gilbert

We first note a number of qualitative and quantitative arguments favoring Landau-Lifshitz over Gilbert damping in ferromagnets. We then explicitly demonstrate a classical Fokker-Planck-like derivation of the macroscopic damping rate in terms of thermodynamic fluctuations (fluctuation-dissipation). Because out-of-equilibrium fluctuations are driven toward equilibrium by their excess thermodynamic energy, the damping term is proportional to the transverse component of the effective field, thus yielding Landau-Lifshitz damping with an explicit expression for the damping coefficient. This damping is unaffected to lowest order in systems with spin transfer torque (STT). Recent experiments on current-driven domain wall motion have a natural interpretation in terms of the so-called adiabatic STT.   

Interactions between spin polarized current and local spin systems: A Quantum Mechanical Approach

In this talk, we present the first fully quantum mechanical calculation of the spin flip interaction between spin polarized current and local spin systems. Dynamics of local spins will be illustrated as many electrons pass through the chain. The talk will focus on the description of the approach, density matrix formalism used in the calculations and the behavior of the system for different configurations. The interplay between electron-spin and spin-spin interaction, effect of domain walls, limiting cases for interaction strengths, spin degree of freedom, and comparison to LLG model will be presented.   

Giant voltage response to magnetic field of model granular magnetic films and spin mixing effects

Magneto-thermogalvanic voltage (MTGV) is the magnetic field dependence of the AC voltage measured across a sample subjected to a DC current and to an oscillation of its temperature. Large field sensitivity was found when the measurement was applied to a thin film made by the technique of cluster-assembled materials. [1] Clusters of Co with an average size of 15 atoms per cluster in a copper matrix gave this result. Similar large changes were observed using silver as a matrix. The size of this effect compared to GMR, its temperature and field dependence, in these and other nanostructures, demonstrate that a different process than that responsible for GMR is the determining mechanism. Argument in favour of an asymmetry in the spin mixing process is given. \newline \newline [1] S. Serrano-Guisan et al., \underline {Nature Materials} \textbf{5,} 730 (2006)   

Dynamics of spin flipping

Interactions between a spin polarized current and a ferromagnetic spin chain will lead to the eventual flipping of the spins. We study the dynamics of spin flipping due to Kondo- like interactions between an electron and a spin chain. Interactions within the chain are taken to be of Heisenberg type. The full time dependent quantum mechanical problem is solved within a density matrix formulation. We present the time evolution of the electron wave packet and of the spin expectation values as the electron passes through the chain. The electron transmission probability is calculated as a function of electron momentum and interaction coupling strength. We observe excitations induced by spin transfer and resonant transmission regimes. Deviations from quasi-classical treatments of magnetic moments are discussed.   

Non-Equilibrium Spin Dynamics in the Subpicosecond Regime

Femto-second laser pulses are becoming an important tool that allows us to explore non-equilibrium spin dynamics at short time (high frequency) scales [1]. It has therefore become apparent [2] that more rigorous treatments are needed to correctly address spin relaxation at these energies. I will show how functional-methods of calculations of correlation energies in electron gas [3] can be successfully adapted to the problem of relaxation in magnetic systems [4]. The study of short time response entails a careful treatment of initial conditions. Our formalism naturally takes care of this and avoids the assumption that the system has been in equilibrium in the infinite past, an assumption common in Boltzmann-type treatments. As an example, we discuss possible non-equilibrium effects due to ultrasonic attenuation on spin relaxation when the magnon sub-system is initially near the Curie point. \newline [1] A. V. Kimmel et al, Nature 435, 655 (2005); L. Guidoni et al., Phys. Rev. Lett. 89, 017401 (2002). \newline [2] A. Rebei and J. Hohlfeld, Phys. Rev. 97, 117601 (2006); A. Rebei and M. Simionato, Phys. Rev. B 71, 174415 (2005). \newline [3] A. Rebei and W. N. G. Hitchon, Int. J. Mod. Phys. B 17, 973 (2003). \newline [4] A. Rebei, W.N.G. Hitchon, and G. J. Parker, Phys. Rev. B 72, 064408 (2005).   

Thermal hysteresis in transport properties of chromium films due to Spin Density Wave (SDW) quantization and Domain-wall scattering

Magnetotransport measurements were made on four Cr films of thicknesses 3500$^{o}$A, 560$^{o}$A, 430$^{o}$A and 175$^{o}$A sputtered on MgO substrates. We observe thermal hysteresis in the resistivity and Hall coefficient. Two types of hysteresis are observed, one in a temperature range of tens of Kelvin above 200 K and the other from the Neel temperature down to about 50 K. The first type is seen in two of the films, 175$^{o}$A and 560$^{o}$A. By looking at the Hall conductance in the vicinity of this hysteresis, we show that it arises directly from the SDW. The second type of hysteresis is absent in the thinnest 175$^{o}$A film, but increases in magnitude with film thickness, and resistivity is always higher during cooling than warming. We conclude that the first type of hysteresis is caused due to discrete changes in the number of incommensurate SDW nodes due to confinement in the thickness dimension, and the second type is caused due to changes in the domain wall configuration with temperature, leading to a reduction in anti-ferromagnetic (AFM) SDW domain wall scattering.   

Direct measurement of antiferromagnetic domain fluctuations

We present coherent x-ray speckle measurements of slow nanoscale dynamics of domain walls separating microscopic regions with different orientations of the spin- (charge-) density waves in Cr bulk samples. Between 150K and 30K domain wall fluctuations slow down as the sample temperature is lowered, consistent with classical thermal activation model. Below 30K, however, the characteristic domain fluctuation timescale remains constant, possibly due to the cross-over between thermally activated and quantum tunneling mechanisms of domain wall relaxation.   

Session J15: Focus Session: Frustrated 1D Magnetism

Quantum helimagnetism of the frustrated spin 1/2 chain LiCuVO4

Inelastic neutron scattering, susceptibilit y, and high-field magnetization identify LiCuVO4 as a nearest-neighbour ferromagnetic, next-nearest-neigh bour rustrated, quasi-onedimensional helimagnet,whic h is largely influenced by quantum fluctuations. Complementary band structure calculations provide a microscopic model with the correct sign and magnitude of the major exchange integrals. \newline \newline In collaboration with M. Enderle, C. Mukherjee, B. Fak, R. K. Kremer, J.-M. Broto, H. Rosner, S.-L. Drechsler, J. Richter, J. Malek, A. Prokofiev, W. Assmus, H. Rakoto, M. Rheinstadter, \'Ecole Polytechnique F\'ed\'erale de Lausanne.   

Properties of frustrated one dimensional S=1/2 Heisenberg antiferromagnets in zero and applied fields

The Phase diagram for a S = 1/2 $J_{1}$-$J_{2}$ exchange model on a one dimensional chain offers phases with no classical analogue. We will present a summary of experimental findings on quasi-one dimensional S = 1/2 (Cu$^{2+}$) chain systems with competing interactions along the chain, which lie in the frustrated helix part of this phase diagram. Compounds in this region of the phase diagram have similar magnetic properties, including a common magnetic structure with an incommensurate helix which is polarized along the chain. The application of a magnetic field applied in the helix plane offers a rich phase diagram with yet unknown phases. This will be discussed primarily for the quasi one dimensional compound LiCuVO$_{4}$.   

Specific-heat and magnetocaloric-effect study of the S=1/2 frustrated-chain antiferromagnet Cu$_{3}$(CO$_{3})_{2}$(OH)$_{2}$

Azurite, Cu$_{3}$(CO$_{3})_{2}$(OH)$_{2}$, is a natural mineral in which the S=1/2 spins of Cu$^{2+}$ form frustrated chains. The magnetization of this material exhibits a plateau at 1/3 of the saturation value, at magnetic fields between 16 T and 26 T applied in the chain direction, and the specific heat at zero field shows a broad peak at 4 K followed by a sharp peak at 1.8 K [1]. We have measured specific heat up to 18 T and found that the broad peak gets smaller with increasing field and disappears above 10 T. The sharp peak begins to separate into a peak and a shoulder at 5 T, and the temperature difference between these anomalies becomes larger up to 13 T. These observations are well supported by magnetocaloric-effect data taken at the same time. In the plateau region, an exponential temperature dependence is observed in the specific heat at low temperatures, indicating an energy gap for low-lying excited states. Detailed specific-heat and magnetocaloric-effect data and the magnetic phase diagram up to 18 T will be presented. [1] H. Kikuchi \textit{et al,} Phys. Rev. Lett. \textbf{94}, 227201 (2005)   

Commensurate Fluctuations in the Pseudogap and Incommensurate Spin-Peierls Phases of TiOCl

We have performed x-ray scattering measurements on the unconventional spin-Peierls system TiOCl and the closely related doped compound Ti$_{(1-x)}$Sc$_{x}$OCl (x = 0.01, 0.03). In pure TiOCl these measurements reveal the presence of commensurate dimerization peaks within both the incommensurate spin-Peierls phase (T$_{c1}<$ T $<$ T$_{c2})$ and the so-called pseudogap phase (T$_{c2}<$ T $<$ T*). This commensurate scattering is slightly shifted in Q-space relative to the commensurate long-range ordered state below T$_{c1}$, and has a fairly narrow width in Q, suggesting correlation lengths greater than 100 angstroms. Below T* $\sim $ 130K, the integrated intensity of the scattering over the commensurate and incommensurate positions grows continuously as a function of decreasing temperature. In addition, measurements performed on the doped compound show that the substitution of non-magnetic Sc$^{3+}$ ions (S = 0) onto Ti$^{3+}$ (S = 1/2) sites appears to suppress commensurate fluctuations and prevent the development of a commensurate long-range ordered state.   

High-pressure structural investigation of the spin-Peierls compound TiOCl.

TiOCl is a S=1/2 layered Mott insulator which displays a spin-Peierls state at low-temperatures involving the dimerization of the Ti ions. Although carrier localization is expected to be weak, it has proven difficult to dope charges in the system through partial chemical substitution. To explore if TiOCl could instead host a bandwidth-driven insulator-to-metal transition under external pressure, we investigated the crystal structure at pressures up to 17 GPa by means of powder and single crystal synchrotron x-ray diffraction in diamond anvil cells.   

NMR and $\mu $SR studies of Sc (S=0) doping effects in a spin-Peierls system TiOCl

TiOCl is a model one-dimensional S=1/2 chain system with an incommensurate phase below T$_{c}\sim $95K and a spin-Peierls ground state below T$_{c}$'$\sim $65K. We report a detailed NMR and $\mu $SR investigation of the local lattice and spin environment in undoped and Sc (S=0) doped Ti$_{1-x}$Sc$_{x}$OCl (x = 0, 0.01, and 0.03). Based on $^{35}$Cl NMR lineshape measurements, we show that all Ti$_{1-x}$Sc$_{x}$OCl samples exhibit signatures of short-range Peierls distortion starting from as high as T*$\sim $130K. Moreover, the NMR line splits into two peaks below T$_{c}\sim $95K, corresponding to two types of local lattice geometries. Only undoped TiOCl develops sharp double peaks below T$_{c}$'=65K. $\mu $SR measurements reveal no evidence for a magnetic long range order down to 2K, even for x=0.03. We will compare our results with x-ray scattering measurements (J.P. Clancy et al., this meeting) and doped CuGeO$_{3}$.   

Hole-doping of Ca$_{1-x}$Na$_x$V$_2$O$_4$ ($x = 0 - 0.5$) With Zig-Zag Vanadium Chains

CaV$_2$O$_4$ crystallizes in an orthorhombic $Pnam$ structure with $S=1$ zig-zag V chains along the $c$-axis. In this low-dimensional, insulating system the triangular arrangement of V atoms with $J_1\approx J_2$ leads to competing frustrating antiferromagnetic (AF) interactions. Our recent studies on powders and single crystals of CaV$_2$O$_4$ show long-range AF ordering at a N$\acute{\rm e}$el temperature $T_{\rm N} \sim 75 - 78$~K (with a monoclinic distortion at $\sim 145-150$~K) and signatures of partial spin-freezing below 20~K. We have tried doping CaV$_2$O$_4$ into the metallic state by substitution at the Ca site to drive V into fractional valence states. We have succeeded in replacing Ca upto 50\% by Na at 1200~$^{\circ}$C. Powder XRD patterns of our Na-substituted samples are nearly single-phase CaV$_2$O$_4$-type, while the $c$-axis lattice parameter decreases sharply - Thus Na indeed substitutes for Ca instead of occupying interstitial positions. The room temperature resistance of Na-doped sintered pellets decreases significantly. High field ($H$ = 1 T) dc magnetization measurements show a steep fall in $T_{\rm N}$ while low field ($H$ = 100 Oe) data suggest onset of spin-glass like behavior as the Na content increases. We shall present our results and discuss the evolution from a partially disordered AF insulator to a spin-glass.   

$^{51}$V NMR Study of the Magnetic Structure of the Frustrated Zig-Zag Spin-1 Chain Compound CaV$_2$O$_4$

$^{51}$V NMR measurements have been performed on a single crystal of orthorhombic (at room temperature) CaV$_2$O$_4$ in zero applied magnetic field and with a small perturbing field up to $H = 2$~T, at temperatures well below the N\'eel temperature $T_N = 78$~K\@. The $c$-axis is parallel to the chains. At $H = 0$, a broad $^{51}$V NMR spectrum with a peak at 237~MHz was observed. The effective local hyperfine field $H_{{\rm eff}} = 21.2$~T corresponding to the peak frequency 237~MHz is in good agreement with expectation for the V$^{3+}$ $S = 1$ spin state. In $\vec{H}\perp c$, the spectrum splits into two parts that are equally separated from the peak position at zero field. The separation of the parts depends strongly both on the magnitude and direction of $\vec{H}$ with respect to the crystal axes. Our NMR results are consistent with a collinear antiferromagnetic spin structure with the spin direction along the \textit{b}-axis, which together with the magnetization data suggest that the antiferromagnetic long-range order arises from an order-from-disorder mechanism. We also present the temperature and orientation dependence of the spin-lattice relaxation rate $1/T_1$.   

Bilinear-Biquadratic Spin 1 Heisenberg Zig-Zag Chain

Recent theoretical studies raised the possibility of a realization of spin nematic states in the S=1 triangular lattice compound NiGa2S4. We study the bilinear-biquadratic spin 1 Heisenberg chain in a zig-zag geometry by means of Density Matrix Renormalization Group (DMRG) and Exact Diagonalization (ED). We present the phase diagram focusing on antiferromagnetic interactions. Adjacent to the known Haldane / double Haldane and the extended critical phase with dominant spin nematic correlations we find a trimerized phase with a non-vanishing energy gap. We discuss results for different order parameters, energy gaps, correlation functions and the central charge, and make connection to field theoretical predictions for the phase diagram.   

Static Holes in Geometrically Frustrated Bow Tie Ladder

Doping of the geometrically frustrated bow-tie spin ladder \footnote{C. Waldtmann, A. Kreutzmann, U. Schollwock, K. Maisinger, and H.-U. Everts, Phys. Rev. B {\bf 62}, 9472 (2000).} with static holes is investigated by a complementary approach using exact diagonalization and hard-core quantum dimers. Results for the thermodynamics in the undoped case, the singlet density of states, the hole-binding energy, and the spin correlations will be presented. We find that the static holes polarize their vicinity by a localization of singlets in order to reduce the frustration. As a consequence the singlet polarization cloud induces short range repulsive forces between the holes with oscillatory longer range behavior. For those systems we have studied, most results for the quantum dimer approach are found to be qualitatively if not quantitatively in agreement with exact diagonalization. The ground state of the undoped system is non-degenerate with translationally invariant nearest-neighbor spin correlations up to a few unit cells, which is consistent with a spin liquid state or a valence bond crystal with very large unit cell.   

Magnetic and transport properties of a one dimensional frustrated t-J model for vanadate nanotubes

We propose a one-dimensional model consisting of a chain with a t-J Hamiltonian coupled to a Heisenberg chain in a frustrated geometry to describe the appearance of the ferromagnetic phase which has been experimentally observed in vanadate nanotubes. This model contains a mechanism of frustration suppressed by doping suggested by L. Krusin-Elbaum, {\it et al.} [Nature {\bf 431}, 672 (2004)]. We study, using numerical techniques in small clusters, the relation between magnetic order and transport properties in the proposed model, and we perform a detailed comparison of the properties of this model with those of the ferromagnetic Kondo lattice model (FKLM). For this comparison, a number of results for the latter model, obtained using the same numerical techniques, will be provided to complement those results already available in the literature. We conclude that it that does not appear to be a true ferromagnetic order in the proposed model, but rather an incommensurate ferrimagnetic one, and contrary to what happens in the FKLM, electronic transport is somewhat suppressed by this ferrimagnetic order.   

Quantized Berry phase for itinerating singlets in one-dimensional t-J mod

The quantized Berry phase as a local order parameter of gapped quantum liquids is proposed for characterization of a topological or quantum order in various models including strongly correlated electron systems[1]. We apply the scheme to calculate the quantized Berry phase in the $t-J$ model, where the Berry phase is quantized as trivial or non-trivial value, i.e., 0 or $\pi$, due to some anti-unitary symmetry. One-dimensional $t-J$ model with a few electrons gives a realization of itinerating singlets when the exchange energy $J$ is large. Although the charge excitation is gapless, the spin gap is finite. Then we can calculate the Berry phase by treating low energy states as a degenerated multiplet. To use a local spin singlet as a order parameter, we define the Berry phase by a local spin twist. It is found that the Berry phase is quantized actually and becomes uniform and nontrivial when the number of electrons $N=4n+2$, with $n$ being an integer [2]. \newline [1] Y. Hatsugai, cond-mat/0603230 to appear in J. Phys. Soc. Jpn.\newline [2] I. M. and Y.Hatsugai. unpublished   

Field-Induced New Phases in the S=1/2 Three-Leg Spin Tube

The S=1/2 three-leg frustrated antiferromagnetic spin tube is investigated using the numerical diagonalization, density matrix renormalization group, and effective field theory techniques. Varing one of the three rung couplings, a quantum phase transition between the spin gap and gapless phases was predicted in our previous work[1]. The present phenomenological renormalization approach gives a phase diagram of the spin gap. In addition we study the magnetization process of the regular triangle spin tube. Our present effective field theory between the lower critical field where the spin gap vanishes, and the upper critical field where the magnetization is saturated, predicts two field-induced new phases appear; the one has the vector chiral order and the other has an inhomogeneous distribution of the magnetization. These two phases coexist with the Tomonaga-Luttinger liquid phase. The prediction is confirmed by the numerical diagonalization and finite-size scaling analyses.\newline [1] T. Sakai, M. Matsumoto, K. Okunishi, K. Okamoto and M. Sato: Physica E 29 (2005) 633.   

Session J16: Focus Session: Molecular Magnets

Interactions Between Thin Metallic Films and Mn$_{12}$-Acetate.

Single-molecule magnets are a novel class of materials which have been extensively studied in recent years. One such molecule is Mn$_{12}$-Acetate, [Mn$_{12}$O$_{12}$(CH$_{3}$COO)$_{16}$(H$_{2}$O)$_{4}$]$\cdot $2CH$_{3}$COOH$\cdot $4H$_{2}$O. Its high-spin ground state (S=10) at low temperatures leads to many interesting phenomena. Here we explore the effect these molecules have on the electronic transport properties of a normal-conducting, metallic thin film in the temperature range from 0.2K to 1K and magnetic fields up to 3T. The magnetoresistance of Au films is measured in agreement with published results. Measurement of Au films with a few monolayers of Mn$_{12}$-Ac on the surface show a different behavior.   

Magnetism of Rubidium Cobalt Hexacyanoferrate Nanoparticles.

Although photoinduced magnetism in nanoparticles of Prussian blue analogs has been reported, these samples are superparamagnetic. We have generated and characterized nanoparticles of Rb$_{j}$Co$_{k}$[Fe(CN)$_{6}$]$_{l}\cdot $nH$_{2}$O, which exhibit photoinduced magnetism and, for the largest particles, long-range ferrimagnetism with finite coercive fields. The synthesis involves the variation of the concentration of the poly(vinylpyrrolidone), PVP, the encapsulating polymer, which controls the resulting particle sizes. From HR-TEM, the particle size distributions have been obtained for four batches of samples, with mean diameters ranging from nominally 3~nm to 13~nm. Upon irradiation with white light at 5~K, all samples exhibit photoinduced magnetism. Magnetization studies indicate that the smallest particles are superparamagnetic, while the largest ones are ferrimagnetic with long-range ordering temperatures ($T_{c}$ $\sim $ 17~K) and coercive fields ($H_{c}$ $\sim $ 250~G) varying with particle size in a manner consistent with the predictions of finite-size scaling.   

Fabrication of nano-gapped single-electron transistors for transport studies of individual single-molecule magnets

Three terminal single-electron transistor devices utilizing Al/Al2O3 gate electrodes were developed for the study of electron transport through individual single-molecule magnets (SMMs). These devices were patterned via multiple layers of optical and electron beam lithography. Al gate electrodes were allowed to oxidize in the ambient atmosphere overnight, creating a robust Al2O3 insulating layer. The single-electron transistor devices were then treated with O2 plasma and Mn12-(3-thiophenecarboxylate) SMMs were self-assembled on the surface. These molecules are Mn12-acetate derivatives, which have been functionalized with thiophene groups and are known to attach to Au surfaces. Self-assembly of the molecules was verified using scanning probe microscopy and XPS measurements. Nano-gapped electrodes were produced at low temperature by electromigration of the 90 nm wide Au wire, reliably yielding 1-3 nm gaps in which the SMM can be situated. We show that the nano-gap spacing can be fine tuned by adding resistance in series with the nanowire. Electron transport measurements were then performed to reveal gate dependent low level (less than 40 meV) excitations in the conductance of a single Mn12 SMM.   

Will spin-relaxation times in molecular magnets permit quantum information processing?

Certain computational tasks can be efficiently implemented using quantum logic, in which the information-carrying elements are permitted to exist in quantum superpositions. To achieve this in practice, a physical system that is suitable for embodying quantum bits (qubits) must be identified. Some proposed scenarios employ electron spins in the solid state, for example phosphorous donors in silicon, quantum dots, heterostructures and endohedral fullerenes, motivated by the long electron-spin relaxation times exhibited by these systems. An alternative electron-spin based proposal exploits the large number of quantum states and the non-degenerate transitions available in high spin molecular magnets. Although these advantages have stimulated vigorous research in molecular magnets, the key question of whether the intrinsic spin relaxation times are long enough has hitherto remained unaddressed. Using X-band pulsed electron spin resonance, we measure the intrinsic spin-lattice ($T_1$) and phase coherence ($T_2$) relaxation times in molecular nanomagnets for the first time. In Cr$_7M$ heterometallic wheels, with $M$ = Ni and Mn, phase coherence relaxation is dominated by the coupling of the electron spin to protons within the molecule. In deuterated samples $T_2$ reaches 3~$\mu$s at low temperatures, which is several orders of magnitude longer than the duration of spin manipulations, satisfying a prerequisite for the deployment of molecular nanomagnets in quantum information applications.   

Transverse magnetization and transient oscillations in the quantum tu nneling of molecular magnets

We calculate the response of a molecular magnet subject to a time-varying magnetic field and perturbatively coupled to a heat bath. The effective model of a triangle of Heisenberg spins and weak anisotropies is as has been used to model the molecular magnets $\{V_{15}\}$ and $\{Cu_3\}$. Oscillations parallel and transverse to the field direction correspond to transient effects in quantum tunneling. We propose that observations of these oscillations, particularly those transverse to the field, may be an effective way to probe the details of level repulsion and coupling to the environment.   

Energy relaxation between low lying tunnel split spin-states of the single molecule magnet Ni$_4$

We have developed integrated magnetic sensors to study quantum tunneling of magnetization (QTM) in single molecule magnet (SMMs) single crystals. These sensors incorporate a microstrip resonator (30~GHz) and a micro-Hall effect magnetometer. They have been used to investigate the relaxation rates between the 2 lowest lying tunnel split spin-states of the SMM Ni$_4$ ($S=4$). EPR spectroscopy at 30~GHz and 0.4~K and concurrent magnetization measurements of several Ni$_4$ single crystals are presented. EPR enables measurement of the energy splitting between the 2 lowest lying superposition states as a function of the longitudinal and transverse fields. The energy relaxation rate is determined in two ways. First, in cw microwave experiments the change in spin-population together with the microwave absorption directly gives the relaxation time from energy conservation in steady-state. Second, direct time-resolved measurements of the magnetization with pulsed microwave radiation have been performed. The relaxation time is found to vary by several orders of magnitude in different crystals, from a few seconds down to smaller than 100~$\mu$s. We discuss this and the form of the relaxation found for different crystals and pulse conditions.   

Electrodes for Molecular Spin-Valves

Realization of spin devices based on the spin-state of magnetic molecules remains a difficult challenge due to the lack of a reliable molecular electrode fabrication process. We have successfully fabricated magnetic Molecular Junctions (MJ's) by having paramagnetic molecular clusters molecules span across the surface of a metal-insulator-metal tunnel junctions (MJT) [Ta/Co/NiFe/Al$_{2}$O$_{3}(\sim $2nm)/NiFe] at the exposed cross-junction pattern edge. Interestingly the current from $\sim $1$\mu $A to $\sim $1nA (RT, 100mV bias) a short time after molecular attachment presumably due to magnetic ordering. Low temperature in-plane magnetization (77 K, 0.4T) further increased magnetic ordering and decreased the junction current to $\sim $1pA level. Magnetic force microscopy (MFM) spatially showed strong antiferromagnetic coupling between the top and bottom magnetic electrodes. SQIUD- magnetometer study on an array of MJT dots (4um diameter) showed reduction in magnetization after molecular attachment consistent with antiferromagnetic coupling and the dramatic changes in magneto-junction current (uA to pA).   

Ferromagnetism in Organic Iron Phthalocyanine Thin Films

Organic iron phthalocyanine (FePc) thin films were deposited with the planar molecule either parallel or perpendicular to the substrate. Hysteretic ferromagnetic loops are observed below 5 K, lower than the previously found 15 K temperature for short range ordering in bulk powder samples [1]. An induced molecular magnetic anisotropy is found based on ac-susceptibility measurements with the magnetic field parallel and perpendicular to the substrate. The molecular plane spacing, as determined from X-ray diffraction, is correlated with the magnetic susceptibility. This indicates that the molecular spacing, controllable by appropriate substrate and growth temperature selection, is an important parameter for the magnetic properties of FePc. Work supported by AFOSR-MURI. [1] M. Evangelisti, J. Bartolome, L. J. de Jongh, and G. Filoti, Phys. Rev. B \textbf{66}, 144410 (2002).   

High frequency (240 GHz) ferrimagnetic resonance (FMR) of room temperature organic based magnetic semiconductor V[TCNE]$_{x}$ (x$\sim $2) films

V[TCNE]$_{x}$ (x$\sim $2) is an organic based ferrimagnetic semiconducting material ($\rho _{300K}\sim $10$^{2}\Omega $.cm and activation energy, E$_{a} \quad \approx $ 0.5 eV) with an ordering temperature well above room temperature. Magnetoresistance (MR) behavior of this material has been explained on the basis of spin polarization of charge carriers in the $\pi $* electronic subbands of [TCNE]$^{-}$ forming a `half-semiconductor'.[1,2] X-band ($\sim $9 GHz) ferrimagnetic resonance (FMR) studies on V[TCNE]$_{x}$ (x$\sim $2) have been reported earlier.[3] Temperature and angular dependence of FMR spectra of V[TCNE]$_{x}$ (x$\sim $2) films, obtained using $\sim $240 GHz radiation, indicate the coexistence of long-range magnetic ordering and glassy behaviors. These results will be discussed in terms of competing interactions between V$^{2+}$ and [TCNE]$^{-}$ spins based on the local structural order. 1.V.N. Prigodin et. al., Adv. Mater. \textbf{14}, 1230 (2002). 2.N.P. Raju et. al., J. Appl. Phys., \textbf{93}, 6799 (2003). 3.R. Plachy et. al., Phys. Rev. B \textbf{70}, 064411 (2004).   

Exotic quantum magnetization process observed in the \{Cu$_3$\} triangular spin ring

We present a comprehensive set of pulsed field magnetization, ESR, and NMR measurements on the triangle spin ring system [Cu$_{3}$(H$_{2}$O)$_{3}(\alpha $-XW$_{9}$O$_{33})_{2}$]$^{12- }$(X=As, Sb). We observed half step magnetization and hysteresis loops for X=As in a fast sweeping magnetic field of $\sim $10$^{4}$T/s at 0.4 K. These features become less pronounced for X=Sb. A comparative ESR study of both compounds reveals that Dzyaloshinskii-Moriya (DM) interactions are weaker in X=Sb than X=As because of the size difference between the diamagnetic heteroatom X. This leads to a reduction of an anti-level crossing gap in X=Sb compared to X=As. This is consistent with the NMR results which show an appreciable peak of the spin-lattice relaxation rate 1/T$_{1}$ at anti-level crossing fields of 2 and 4.4 T only for X=Sb. Our work suggests that the dependence of half step magnetization on X in a nanocluster system arises from a delicate balance between the adiabatic magnetization and the relaxation rate, relying on DM interactions.   

Magnetic ordering and switching of iron porphyrin molecules on a substrate

We have studied the structural ordering and the magnetic coupling of in-situ sublimated Fe-based porphyrin molecules on epitaxially grown Ni and Co films on Cu(100) by means of X-ray absorption spectroscopy and X-ray magnetic circular dichroism at third generation synchrotron radiation facilities, in an experimental study which is combined by density functional theory (DFT). We demonstrate the necessary sensitivity to probe the magnetic properties even for sub-monolayer porphyrin coverages. We show that due to 90 degree super-exchange interaction between Fe atoms in the molecules and atoms in the substrate (Co or Ni) the paramagnetic molecules can be made to order ferro magnetically and even have their magnetisation direction switched by a magnetisation reversal of the substrate. Theory is demonstrated to reproduce the experimental observations.   

Spatial and Morphology Controlled Magnetic Patterns on Organic Monolayers

We report on the effect of polarity of self-assembled monolayers on magnetic properties and morphology of a deposited magnetic material. Sputtering of Permalloy (Ni79Fe21) on a patterned self-assembled monolayer (SAM) structure results in formation of ferromagnetic film on polar regions and superparamagnetic clusters on non-polar regions of the SAM. The existence of two distinct morphologies of the deposited magnetic material can be attributed to the difference in wettability of the SAM surface. These results demonstrate new opportunities for integration of controlled regions with different magnetic behavior without using conventional lithography.   

Magnetic ordering of new molecule-based magnet: [Fe(TCNE)(NCMe)$_{2}$][FeCl$_{4}$].

The magnetic properties of [Fe(TCNE)(NCMe)$_{2}$][FeCl$_{4}$] (TCNE = tetracyanoethylene), a molecule-based magnet synthesized via reaction of FeCl$_{2}$(NCMe)$_{2}$ with TCNE in CH$_{2}$Cl$_{2}$. $M(T)$ is discussed. Both \textit{$\chi $'}($T)$ and \textit{$\chi $}''($T)$, ac susceptibilities are almost frequency independent, and exhibit a sharp peak at $\sim $90 K in accord with $T_{c}$. The zero field cooled (ZFC) and field cooled (FC) magnetic susceptibilities, \textit{$\chi $}$_{ZFC}(T)_{ }$and \textit{$\chi $}$_{FC}(T)$, at 5 Oe rise sharply below 95 K indicative of a magnetic transition. \textit{$\chi $}$_{ZFC}(T)$ reaches maximum at 88 K followed by a rapid decrease suggesting an antiferromagnetic(AFM) ground state attributed to AFM coupling between ferrimagnetically ordered [Fe[TCNE](NCMe)$_{2}$]$^{+}$ layers. In contrast, \textit{$\chi $}$_{FC}(T)$ rises upon further cooling suggesting a strong irreversibility and indicating the presence of a remanant magnetization below 90 K, which increases upon cooling. [Fe(TCNE)(NCMe)$_{2}$][FeCl$_{4}$] is the initial member of a new class of magnets. It is the first metal-TCNE magnet with direct bonding between metal ion and [TCNE]$^{- }$whose structure has been determined, and it possesses a novel planar $\mu _{4}$-[TCNE]$^{-}$ spin coupling unit.   

Session J17: Elastomers & Gels

Depth Dependence of Shear Properties in Articular Cartilage

Articular cartilage is a highly complex and heterogeneous material in its structure, composition and mechanical behavior. Understanding these spatial variations is a critical step in designing replacement tissue and developing methods to diagnose and treat tissue affected by damage or disease. Existing techniques in particle image velocimetry (PIV) have been used to map the shear properties of complex materials; however, these methods have yet to be applied to understanding shear behavior in cartilage. In this talk, we will show that confocal microscopy in conjunction with PIV techniques can be used to determine the depth dependence of the shear properties of articular cartilage. We will show that the shear modulus of this tissue varies by over an order of magnitude over its depth, with the least stiff region located about 200 microns from the surface. Furthermore, our data indicate that the shear strain profile of articular cartilage is sensitive to both the degree of compression and the total applied shear strain. In particular, we find that cartilage strain stiffens most dramatically in a region 200-500 microns below the surface. Finally, we will describe a physical model that accounts for this behavior by taking into account the local buckling of collagen fibers just below the cartilage surface and present second harmonic generation (SHG) imaging data addressing the collagen orientation before and after shear.   

Determining Local Mechanical Properties of Soft Materials with Cavitation Rheology

To guide the development of tissue scaffolds and the characterization of naturally heterogeneous biological tissues, we have developed a method to determine the local modulus at an arbitrary point within a soft material. The method involves growing a cavity at the tip of a syringe needle and monitoring the pressure of the cavity at the onset of a mechanical instability. This critical pressure is directly related to the local modulus of the material. The results focus on the network development of poly(lactide)-poly(ethylene oxide)-poly(lactide) triblock copolymer and poly(vinyl alcohol) hydrogels. These materials serve as model materials for tissue scaffolds and soft biological tissues. This new method not only provides an easy, efficient, and economical method to guide the design and characterization of soft materials, but it also provides quantitative data of the local mechanical properties in naturally heterogeneous materials.   

Soft Segment Orientation Effects on the Morphology

A series of polyurethane elastomers were designed with varying poly(ethylene oxide) (PEO) lengths. Segmented polyurethanes containing higher soft segment molecular weight (4600 g/mol) demonstrated a lamellar morphology, a result of the highly crystalline hard and soft domains. Polyurethanes containing lower molecular weight PEO (1000 g/mol) showed less microphase segregation at similar hard segment contents, though shifting to a copolymer (PEO-PPO-PEO, 1900 g/mol) soft segment recovered domain segregation. High molecular weight PEO-containing polyurethanes showed a tendency to neck upon deformation, which likely resulted from the largely crystalline soft domains. Low molecular weight PEO-containing polyurethanes (1000 g/mol and 1900 g/mol) did not show a tendency to neck during deformation due to the lesser extent of microphase segregation and/or domain crystallinity. We have also investigated the effect of chemistry on morphology, thermal, and mechanical properties by varying chain extender, macrodiol, and isocyanate. Through these experiments, the goal was to determine the molecular mechanisms most responsible for mechanical property enhancement. Molecular architecture was shown to play a more prominent role in dynamic mechanical properties than hard segment percentage; amorphous polyurethane samples displayed drastic stiffness changes with strain rate.   

Structure and mechanical properties of hydrophobically modified hydrogels

Chemically crosslinked hydrogels based on polyacrylic acid chains modified with short hydrophobic C12 side groups have been synthesized in water at a polymer concentration varying between 4 and 10 {\%} in weight. In the absence of hydrophobic groups, the hydrogels behave as soft elastic solids with an elastic modulus in the few kPa range. With the introduction of even a few percent of C12 hydrophobic side chains, the dissipative component of the shear modulus increases by two orders of magnitude, while the elastic component remains unchanged. This causes a large increase in the hysteresis at large strains and an increase in the fracture toughness of the gel. We demonstrate that this change in properties is due to the formation of labile nanoclusters of the hydrophobic groups in the aqueous phase.   

Molecular Origins for the Superior Toughness of Double-Network Hydrogels

Double network hydrogels (DN-gels) are the toughest of crosslinked polymer networks which contain water at more than 90{\%} water by volume. The order-of-magnitude increase in the fracture toughness of a highly swollen but brittle polyelectrolyte network obtained from the addition of a linear polymer is non-intuitive and intriguing. Here, we present insights into the change in the total and the individual molecular structures of DN-gels obtained from recent neutron scattering measurements. The structure of individual components within the DN-gels was obtained by using a deuterium-labeled monomer in conjunction with contrast-matching methods. A working hypothesis for the toughening mechanism has been proposed based on the scattering data and other supporting measurements$.$   

Neutron scattering from polyelectrolyte solutions in the presence of a hydrophilic polymer

The structure of highly charged polyelectrolytes in water has been extensively studied. Existing models adequately describe the small-angle scattering from polyelectrolytes using characteristic physical parameters such as the Debye screening length. However, the influence of a hydrophilic polymer on the structure of a polyelectrolyte in solution such as the case of Double-Network Hydrogels has not been widely studied. Here, we present our recent neutron scattering results from such a multicomponent system. This talk discusses neutron scattering from a monovalent polyelectrolyte in water and in an aqueous solution of neutral polymer capable of hydrogen bonding. The thermodynamic parameters that govern the mutual interactions between the charged polyelectrolyte, neutral polymer, and water are used as parameters to successfully describe the observed neutron scattering results.   

Neutron scattering from double-network hydrogels subjected to uniaxial extension

Double-network hydrogels (DN-gels) are water swollen polymer networks with load bearing abilities similar to that of an articular cartilage. Our neutron scattering measurements offered important insights into the molecular origins for the superior toughness of DN-gels. Here, we discuss recent neutron scattering results from DN-gels subjected to uniaxial extension. These results are further supported by flow birefringence measurements on the solution equivalents of DN-gels subjected to uniaxial extension. The deformation behavior of DN-gels is contrasted with that of complex hydrogels containing nanosized inorganic fillers or figure-of-eight crosslinkers.   

The Conformational Elasticity Theory and Its Applications

A theory that physically describes rubber elasticity including large scale behavior, internal energy contribution and different elastic behaviors of chemically different polymers, so called ``conformational elasticity theory,'' has been recently developed with predicted stress as a function of axial ratio of ellipsoid model. Using short chain rotational isometric state (RIS) model we simulated 2-dimensional chain conformation distribution map (one axes is the end-to-end distance of a polymer chain and the other is the conformational energy). It is very important to propose a microscopic mechanism of the distribution evolution during the polymer deformation. The total tension, the internal energy force and the entropy force can be obtained in any elongation step, and very close to the experimental data. This theory includes interaction energy and distinguishes different chemical structures, thus providing the opportunity to make some efforts in analysis of the physical junction and the entanglement occurring to the system during elongation.   

Modeling mechanochemical transduction in chemo-responsive gels.

Using the recently developed gel lattice spring model, we study mechanochemical transduction in chemo-responsive gels undergoing the Belousov-Zhabotinsky reaction. More specifically, we examine how to harness an applied mechanical force to trigger the propagation of traveling chemical waves, which then lead to oscillations within gels that were initially non-oscillating. In our two dimensional simulations, we introduce the presence of an applied force by uniformly decreasing the thickness of the sample from its initial value. We isolate the system parameters for which the decreasing of the thickness of the sample cases causes the transition from the non-oscillatory to the oscillatory state. In addition, we illustrate that even if the system is not driven into the oscillatory state, the nature of the pressure-induced transient oscillations are of interest since the waves can indicate the strength of the mechanical impact. We define how this type of the mehanochemical transduction depends on the reaction parameters and on the gel's cross-link density. Our studies clarify the sensitivity of the chemo-responsive gel to mechanical deformation and indicate the extent to which the gels can be harnessed as sensors of the mechanical impact.   

Thermoreversible Ion Gels by Block Copolymer Self-assembly in Ionic Liquids

Ion gels, formed by swelling a polymer network with ionic liquids, have been shown to be promising candidates towards highly conductive solid-state electrolytes. Through the self-assembly of triblock copolymers in room-temperature ionic liquids, transparent ion gels could be obtained. Due to the low copolymer concentration, the ionic conductivity of the resulting ion gels is only modestly affected by the triblock copolymer network. By further selecting thermo-responsive end blocks in the triblock copolymers, thermoreversible ion gels were developed. The gelation behavior, ionic conductivity, rheological properties, and microstructure of the ion gels were investigated in detail. The results presented here suggest that triblock copolymer gelation is a promising way to develop highly conductive ion gels.   

Diblock copolymers containing compositionally-uniform poly(HEMA-co-DMAEMA)

Hydroxyethyl methacrylate (HEMA) and dimethylaminoethyl methacrylate (DMAEMA) have been investigated as precursors for pH-responsive hydrogels. DMAEMA contains tertiary amine functionality that is reversibly protonated below its pKa. The swelling characteristics of poly(HEMA-co-DMAEMA) hydrogels are dependent on the distribution of DMAEMA, which in turn depends on the monomer composition and the monomer reactivity ratios. We find that the reactivity ratios are highly solvent dependent. Gradient copolymers are favored in most solvents at all monomer compositions. In dimethylsulfoxide, however, the reactivity ratios are near unity; compositionally-uniform poly(HEMA-co-DMAEMA) copolymers can therefore be synthesized at any composition. We have synthesized diblock copolymers containing poly(HEMA-co-DMAEMA) by a combination of atom transfer radical polymerization and click chemistry. The resulting diblock copolymers have controlled molecular weights, molecular weight distributions, and comonomer distributions, and they form well-defined periodic nanoscale structures consistent with their molecular characteristics.   

Small Angle Neutron Scattering Studies of the Counterion Effects on the Molecular Conformation and Structure of Charged G4 PAMAM Dendrimers in Aqueous Solutions

The structural properties of generation 4 (G4) poly(amidoamine) starburst dendrimers (PAMAM) with an ethylenediamine (EDA) central core in D$_{2}$O solutions have been studied by small angle neutron scattering (SANS). Upon the addition of DCl, SANS patterns show pronounced inter-particle correlation peaks due to the strong repulsion introduced by the protonation of the amino groups of the dendrimers. By solving the Ornstein-Zernike integral equation (OZ) with hypernetted chain closure (HNC), the dendrimer-dendrimer structure factor S(Q) is determined and used to fit the experimental data. Quantitative information such as the effective charge per dendrimer and its conformational change at different pH values is obtained. The results show clear evidence that significant counterion association occurs, strongly mediating the inter-dendrimer interaction. The influence of interplay between counterions and molecular protonation of dendrimers imposes a strong effect on the dendrimer conformation and effective interaction.   

Creasing of soft surfaces under compression

The surface of a soft material may form sharp creases when placed under compression. Though this mechanical instability was first recognized over 40 years ago, very little is known about the structures that result. In particular, we focus on the creasing of surface-attached hydrogels, materials that provide an excellent means of controlling properties such as biocompatibility, adhesion, and tribology. Large compressive stresses are generated within such gels upon swelling, leading to formation of creases that can dramatically alter surface properties. This instability may be exploited to enable the reversible formation of topographic patterns to actively control surface properties. We will present measurements of the critical compression at which surface creases form and how the soft surfaces deform as they fold, and will describe the preparation of surfaces that reversibly fold and flatten in response to external stimuli.   

Schallamach Wave Periodicity in Soft Elastomer Friction

From the dynamics of biomaterial interfaces to the interpretation of nanoscale characterization of polymer interfaces, the friction of soft polymer layers is critical to a wide range of advanced materials. A dominant mechanism in the friction of soft material interfaces is the onset and propagation of Schallamach waves. Schallamach waves are ``tunnels'' of air that provide relative displacement between the slider and the substrate rather than the instantaneous interfacial failure involved with stick-slip. We present a fundamental relationship between the periodicity of Schallamach waves ($\lambda )$ and the ratio of interfacial adhesion (G$_{c})$ over complex elastic modulus (E*). This deconvolution of bulk and interfacial contributions to the friction of soft materials leads to interesting predictions that will impact material design for a wide range of applications.   

Complex diffusion in biopolymer networks with added molecular crowding

Intra- and extracellular diffusion can depend sensitively on the environmental details. In general the diffusion is hindered and can be subdiffusive, varying heterogeneously due to molecular crowding and interactions with an immobile polymer network. We study the combined effects of polymer-hindrance and molecular crowding using particle tracking and correlation analyses applied to microspheres diffusing in Type I collagen with added polyethylene glycol.   

Session J19: Frontiers in Electronic Structure Theory I

Post-Hartree-Fock Correlation Models

We have developed post-Hartree-Fock correlation models for all of dynamical, nondynamical, and dispersion correlations, based on real-space modelling of the correlation hole. Many of the outstanding problems in the density-functional theory of atomic, molecular, and condensed matter systems arise from local exchange approximations. Our post-Hartree-Fock approach circumvents these. The latest developments will be reported,   

Resolutions of the Coulomb Operator

The ``Resolution of the Identity Operator'' \begin{equation} \hat{I} \equiv | \chi_n \rangle \langle \chi_n | \end{equation} is a mathematical device that can be used to decouple the bra and ket in an overlap matrix element \begin{equation} \label{eq:RI} \langle f | g \rangle = \langle f | \chi_n \rangle \langle \chi_n | g \rangle \end{equation} through the introduction of an infinite complete expansion basis $\{\chi_n\}$. In practical implementations, where the basis set is finite and incomplete, (2) yields systematic approximations to difficult overlap integrals and is widely used in quantum physics and chemistry. We will present an analogous ``Resolution of the Coulomb Operator'' \begin{equation} r_{12}^{-1} \equiv | \phi_n \rangle \langle \phi_n | \end{equation} which allows one to expand Coulomb matrix elements \begin{equation} \label{eq:RC} \langle f | r_{12}^{-1} | g \rangle = \langle f | \phi_n \rangle \langle \phi_n | g \rangle \end{equation} and we will discuss the potential utility of (4) in the efficient treatment of the matrix elements that arise in quantum chemistry and elsewhere.   

Spin Polarization Resolved Energetics of a Quasi One Dimensional Electron Gas

This work extends that of Casula et. al.$^1$ by using Quantum Monte Carlo to calculate the exact energy of a quasi one dimensional electron gas at nonzero polarizations. We present a parameterization of the correlation energy suitable for LSDA density functional calculations$^2$. The energy of the momentum resolved spin and charge excitations is also calculated via the intermediate scattering function$^3$. At low densities, correlation opens a gap for charge excitations near $2 k_f$ for each spin species. The modes with periodicity close to the mean interparticle spacing are softened due to the formation of a quasi Wigner crystal. These effects disappear as the density increases and correlation becomes less important. The calculated excitation spectrum agrees with the long wavelength behavior predicted by Luttinger liquid theory. \begin{itemize} \item[{[1]}] M. Casula, S. Sorella and G. Senatore, cond-mat/0607130 (2006) \item[{[2]}] Abedinpour, Polini, Xianlong and Tosi, private communication. \item[{[3]}] S. Yamamoto, Physical Review Letters, {\bf 75}, 3349 (1995) \end{itemize}   

Electronic Counting Rules for the Stability of Metal-Silicon Clusters

Theoretical investigations of the ground state geometries, electronic structure, spin magnetic moment and the stability of the metal encapsulated neutral, cationic, and anionic MSi$_{16}$ ( M= Sc, Ti, V) clusters have been carried out within a gradient corrected density functional formalism. ScSi$_{16}^{-}$, TiSi$_{16}$, and VSi$_{16}^{+}$ are found to be particularly stable in agreement with recent experiments. It is shown that the enhanced stability can be reconciled within a model where each Si atom coordinated to the metal contributes one electron to the valence pool. We propose the use of the bond critical points (BCP) from the topological analysis of the electronic density, in order to identify the Si sites that are bonded to the metal atom. Clusters where the total number of valence electrons obtained by summing one electron from each Si site coordinated to metal atom and the valence electrons of the metal attain 20 are found to be particularly stable. Combined with the earlier reported stability at 18 electrons, it is proposed that such valence pools might be looked upon as a nearly free electron gas inside a silicon cage.   

Compact Representations of Kohn-Sham Invariant Subspaces

We present an algorithm for the computation of reduced numerical representations of the solutions of the Kohn-Sham equations. The method allows for {\em a priori} control of the error caused by the reduction process. When applied to Kohn-Sham wavefunctions expanded on a plane-wave basis, this approach leads to a substantial reduction of the size of the datasets used to restart first-principles simulations, with controlled loss of accuracy. Examples of applications to jellium, liquid water and carbon nanotubes will be presented. A comparison with representations in terms of maximally localized Wannier functions will also be discussed.   

Density Functional Theory in Transition Metal Chemistry: A Self-Consistent Hubbard U approach

Transition metals ions are reactive centers for a broad variety of biological and inorganic chemical reactions. Despite this central importance, density functional theory calculations based on local density or generalized gradient approximations (GGA) often fail qualitatively and quantitatively to describe multiplet splittings, relaxed structures, and reaction barriers for these systems. We have recently proposed$^{1}$ augmenting the GGA functional with a Hubbard U which is obtained from a self-consistent linear response procedure. This fully ab initio GGA+U approach provides excellent agreement with accurate, correlated-electron quantum chemistry calculations for paradigmatic cases that include the ground state of the iron dimer and addition-elimination reactions on bare FeO$^+$. We also show how a GGA+U approach may be applied to large-scale biological systems by preserving the favorable scaling of traditional density functional approaches with improved accuracy. 1) H. J. Kulik, M. Cococcioni, D. Scherlis and N. Marzari, PRL (2006).   

Linear Scaling First-Principles DFT Calculations with Grid-based Adaptive Orbitals

As an alternative to the Plane Waves approach for accurate and unbiased Density Functional Theory (DFT) simulations, we have developed a real-space approach which completely avoids use of Fourier transforms. An effective O(N) complexity is achieved by representing the electronic structure as a set of localized nonorthogonal orbitals. The efficiency of the approach has been demonstrated recently for molecular dynamics simulations in the microcanonical ensemble [J.-L. Fattebert and F. Gygi, Phys. Rev. B 73, 115124 (2006)]. Adapting the position of the localization regions on the fly is a key feature to enable accurate MD simulations. In this talk, we will report recent developments in adapting the size of localization regions to improve efficiency and address very general electronic structure problems.   

How localized is ``local?'' Efficiency vs. accuracy of $O(N)$ domain decomposition in local orbital based all-electron electronic structure theory

Numeric atom-centered local orbitals (NAO) are efficient basis sets for all-electron electronic structure theory. The locality of NAO's can be exploited to render (in principle) all operations of the self-consistency cycle $O(N)$. This is straightforward for 3D integrals using domain decomposition into spatially close subsets of integration points, enabling critical computational savings that are effective from $\sim$tens of atoms (no significant overhead for smaller systems) and make large systems (100s of atoms) computationally feasible. Using a new all-electron NAO-based code,$^1$ we investigate the quantitative impact of exploiting this locality on two distinct classes of systems: Large light-element molecules [Alanine-based polypeptide chains (Ala)$_n$], and compact transition metal clusters. Strict NAO locality is achieved by imposing a cutoff potential with an onset radius $r_c$, and exploited by appropriately shaped integration domains (subsets of integration points). Conventional tight $r_c\le$~3{\AA} have no measurable accuracy impact in (Ala)$_n$, but introduce inaccuracies of 20-30 meV/atom in Cu$_n$. The domain shape impacts the computational effort by only 10-20~\% for reasonable $r_c$. \newline $^1$ V. Blum, R. Gehrke, P. Havu, V. Havu, M. Scheffler, \emph{The FHI Ab Initio Molecular Simulations (aims) Project}, Fritz-Haber-Institut, Berlin (2006).   

Bonding in elemental boron: a view from electronic structure calculations using maximally localized Wannier functions

Boron exhibits the most complex structure of all elemental solids, with more than 300 atoms per unit cell arranged in interconnecting icosahedra, and some crystallographic positions occupied with a probability of less than one. The precise determination of the ground state geometry of boron---the so-called $\beta $-boron structure--has been elusive and its electronic and bonding properties have been difficult to rationalize. Using lattice model Monte Carlo optimization techniques and \textit{ab-initio} simulations, we have shown that a defective, quasi-ordered $\beta $ solid is the most stable structure \textit{at zero} as well as finite T. In the absence of partially occupied sites (POS), the perfect $\beta $-boron crystal is unstable; the presence of POS lower its internal energy below that of an ordered $\alpha $-phase,\textit{ not mere an entropic effect}. We present a picture of the intricate and unique bonding in boron based on maximally localized Wannier (MLWF) functions, which indicates that the presence of POS provides a subtle, yet essential spatial balance between electron deficient and fully saturated bonds. This work was performed under the auspices of the U.S. Dept. of Energy at the University of California/ LLNL under contract no. W-7405-Eng-48.   

Structural distortions in AlF$_3$ derived using density functional methods

The crystal structure of AlF$_3$ at high temperatures has a simple cubic lattice. Below $\sim 730$K the structure transforms to a rhombohedral ($\alpha$-phase) structure with R$\overline{3}$c symmetry, due to an unstable $R_5^-$ phonon. Density-functional based methods, from the least accurate rigid-ion model to highly-accurate all-electron Kohn-Sham models, yield the triply degenerate $R_5^-$ phonon that becomes unstable with decreasing volume at some critical volume $V_c$. Significant variations for $V_c$ and the equilibrium volume $V_0$ among the models lead to large uncertainties for the energy differences between the cubic and rhombohedral structures, indicating that present density functional models are not reliable for accurate quantitative results in this case.   

Quantum Monte Carlo studies of transition metal atoms and molecules

We study electron correlation in selected transition metal atoms and molecules from the 3d series by variational and fixed-node diffusion Monte Carlo methods. We test several types of orbitals such as RHF, UHF, B3LYP and atomic natural orbitals in building the Slater determinants. We explore also several types of wave functions based on single determinant, GVB, limited CI expansions with both unoptimized and reoptimized weights. The aim of this study is to estimate the accuracy of various wave functions with regard to fixed-node biases and to provide benchmarks for high accuracy calculations with these types of atoms.   

Session J20: Insulators: Radiation Detection, Defects, Thermal Properties

Substitutional NaCl hydration in ice

Na$^{+}$ and Cl$^{-}$ can replace water molecules in ice Ih, with minimal lattice strain and without disrupting the crystal's H-bond network. \textit{Ab initio} calculations show that substitutional solvation is optimally endothermic by only 0.50 eV per NaCl formula unit. Consistent with Na$^{+}$ and Cl$^{-}$ ionic radii of 1.0 and 1.8 {\AA}, Na$^{+}$ ions in the optimal structure lie 2.43 {\AA} from their four equidistant, nearest O-atom neighbors, and the Cl$^{-}$ ions 3.02 {\AA} from theirs. Solvation of \textit{interstitial} ions is less favorable by at least 1.5 eV. These results are cautionary for molecular dynamics simulations of ionic solvation in ice; a correction for the presence of water interstitials is needed if the number of molecules in the simulation cell is chosen too large, and another for the generation of free Bjerrum defects, if anion and cation solvation are treated separately.   

Investigation of silicon complexes in Si-doped calcium phosphate bioceramics

Silicon doped calcium phosphate materials have drawn great interest as bioceramics for bone repair due to their enhanced bioactivity. However, the low level of doping in these materials, generally $\sim $1 wt.{\%}, makes it difficult to determine the effects the silicon has on the structure of these materials. In this study, silicon substituted hydroxyapatite (Si-HA), silicon stabilized alpha tricalcium phosphate (Si-TCP), and a multi-phase mixture consisting of approximately 75{\%} Si-TCP with the remainder being mainly Si-HA have been synthesized using isotopically enriched silica containing $^{29}$Si. $^{29}$Si magic-angle spinning nuclear magnetic resonance spectroscopy (MAS-NMR) has been used to examine the silicon complexes within these materials resulting from the substitution of SiO$_{4}^{4-}$ for PO$_{4}^{3- }$and the required charge compensation mechanism needed to achieve this. Previous ab initio studies on these materials have investigated charge compensation mechanisms to suggest possible silicon complexes and these serve as a basis for interpreting the NMR results.   

Dose-Rate Dependence of Ionizing Radiation Damage in Silicon Transistors

The primary effects of ionizing radiation on silicon transistors is caused by the effects of electrons and holes created in the oxide portions of these devices. The holes can become trapped in the oxide, and they also create traps at the semiconductor-silicon interface. In the best- accepted explanation, the holes release hydrogen from source sites, often near the interface, and this hydrogen causes the interface traps by reacting with hydrogen-passivated Si dangling bonds at the interface. In this presentation, the transient electrical effects in a silicon device will be computed as a function of radiation duration, total dose, oxide defects, silicon doping and other physical variables. These calculations reveal mechanisms, such as bimolecular defect reactions, that make the damage dependent on the radiation dose rate if the total radiation dose exceeds a threshold dose.   

Information-Driven Exploration of Crystal Chemistries for Radiation Detection Materials

The ability to suggest promising materials and a priori eliminate unfruitful inquiry is the key to new crystal chemistry searches. Variable spaces tend to be large and poorly defined, and property measurements (and computations) of candidate materials are not abundant. For simple binary systems, the presence of structural polymorphs and higher order compositions for A(m)B(n); m,n=1-3 would combinatorially generate over 300,000 candidates, greatly complicating our ability to explore candidate spaces. We have used knowledge extraction methods to evolve structural signatures to direct searches using performance-based criteria. The exploratory data methods used both supervised (support-vector machines) and unsupervised (disorder-reduction and principal-component) classification methods for structural signature development. The development of new candidates for radiation detection materials will be used as the case example for this talk.   

Electronic structure of lead pyrophosphate

Lead Pyrophosphate Pb$_{2}$P$_{2}$O$_{7}$ is of interest for potential radiation detection applications and use in long term waste storage. It forms in triclinic P${\bar{1}}$ crystals and can also be grown as glasses. We performed electronic structure calculations using the crystal structure which determined by Mullica et. al (J. Solid State Chem (1986)) using x-ray diffraction and found large forces on atoms suggesting that the refined atomic positions were not fully correct. Here we report first principles structure relaxation and a revised crystal structure for this compound. We analyze the resulting structure using pair distribution functions and discuss the implications for the electronic properties. This work was supported by DOE NA22 and the Office of Naval Research.   

Full potential LAPW calculations of positron lifetimes in materials

We report positron lifetime calculations for a large number of semiconductors and insulators, including materials of interest for radiation detection. These include CdTe, ZnTe, lanthanide trihalides, orthophosphates, ZnO. Trends in lifetimes with structural features are discussed.   

Simulated electrolyte-metal interfaces -- Li$_3$PO$_4$ and Li

There has recently been a lot of interest in solid electrolyte materials such as LiPON developed at Oak Ridge National Laboratory\footnote{J. B. Bates, N. J. Dudney, and co-workers, {\em{Solid State Ionics}}, {\bf{53-56}}, 647-654 (1992).} for use in Li-ion batteries and other technologies. We report on the results of our model calculations on idealized interfaces between Li$_3$PO$_4$ and Li metal, studying the structural stability and the ion mobility, using first-principles density functional techniques with the {\em{PWscf}} and {\em{pwpaw}} codes.\footnote{http://www.pwscf.org and http://pwpaw.wfu.edu.} Starting with a supercell constructed from Li$_3$PO$_4$ in its crystalline $\gamma$-phase structure and several layers of Li metal, we used optimization and molecular dynamics techniques to find several meta-stable configurations. The qualitative features of the results are consistent with experimental evidence that the electrolyte is quite stable with respect to Li metal.\footnote{N. J. Dudney in Gholam-Abbas Nazri and Gianfranco Pistoia, Eds., {\em{ Lithium Batteries: Science and Technology}}, Chapt. 20, pp. 623$-$642, Kluwer Academic Publishers, 2004. ISBN 1-4020-7628-2.} In addition to stability analyses, we plan to study Li-ion diffusion across the interface.   

Li-ion diffusion mechanisms in crystalline Li$_3$PO$_4$ electrolytes

Using first principles electronic structure methods and ``nudged elastic band'' optimization techniques,\footnote{Using the Quantum ESPRESSO package http://www.pwscf.org/.} we examine ideal Li-ion diffusion in crystalline Li$_3$PO$_4$ electrolytes, considering both vacancy and interstitial mechanisms. The simulations are performed in supercells containing 16 Li$_3$PO$_4$ units. We determine the activation barriers for several plausible diffusion paths, considering the effects of the exchange-correlation functional forms, of the crystalline form in the $\beta$- and $\gamma$- structures, and also the effects of substitutional N. Using the generalized gradient approximation, results for $\gamma$-Li$_3$PO$_4$ show diffusion barriers of 0.6-0.7~eV for the vacancy mechanism with a small dependence on the crystallographic direction. For the interstitial mechanism, the diffusion barriers are 0.8~eV and 1.3~eV along the ${\bf{b}}$- and ${\bf{c}}$-axes, respectively. The larger activation barriers of the interstitial mechanism are closer to experimental measurements on polycrystalline and single crystal samples\footnote{J. Solid St. Chem. {\bf{115}}, 313 (1995), Cryst. Rep. {\bf{46}}, 864 (2001) } which find $E_a \approx 1.1-1.3$~eV.   

Negative-thermal-expansion ZrW$_{2}$O$_{8}$. Elasticity and pressure

The elasticity of the negative thermal expansion (NTE) compound ZrW$_{2}$O$_{8}$ is rather strange: the solid softens as its volume decreases on warming. Does ZrW$_{2}$O$_{8}$ also soften when pressure alone is applied? Using pulse-echo ultrasound in a large-volume moissanite anvil cell, we find an unusual decrease in bulk modulus with pressure at 300K. Our results are inconsistent with conventional lattice dynamics, but a framework-solid-based non-linear model with many degrees of freedom predicts elastic softening as increases in either temperature or pressure reduce volume. The pressure-induced phase transition from $\alpha $-ZrW$_{2}$O$_{8}$ (cubic) to $\gamma $-ZrW$_{2}$O$_{8}$ (orthorhombic) is found to take place at $\approx $ 0.5 GPa, result confirmed by Raman spectroscopy.   

Specific heat of tri-glycine sulfate in electric field

Measurements of the specific heat, polarization and dielectric constant are reported in electric fields up to 3 kV/cm in order to investigate domain energetics in the hydrogen-bonded ferroelectric tri-glycine sulfate. Although the shape of the specific-heat anomaly at $T_C = 322.5\, K$ is thermally broadened in unpoled crystals, this shape changes into the characteristic $\lambda$-shape expected for a continuous transition with the application of a 220$\,$V/cm electric field. The $\lambda$ transition below $T_C$ depends on $T-T_C$ in a range $T-T_c\leq~10~K$ with a critical exponent, $\beta$ = - 0.39. Similarly we find that below $T_C$ the experimental dielectric constant obeys an inverse-power law of the form, $\varepsilon(T) = a/(T-T_C)^\beta$, with the constant $a = 1244\, \mathrm{K}$ and the exponent $\beta=0.4$.   

Effect of temperature on the thermodynamic properties of Na$_2$Ti$_3$O$_7$

The equilibrium structure of the compound Na$_2$Ti$_3$O$_7$ has been obtained via the minimization of the total energy within Local Density Approximation (LDA) based on Density Functional Theory (DFT), the calculated equilibrium volume are in agreement with available experimental values. In the meantime, the thermodynamic properties are investigated applying nonempirical Debye-like model combining with the first principle theory in the quasi-harmonic approximation. The evaluated equilibrium volume using this model agrees with the values from \textit{ab intio} and from experiment. Our results demonstrate that this method can provide reliable predictions for the temperature dependence of these quantities such as the equation of state, the bulk modulus, the heat capacity, and the thermal expansion in detail. And our calculated thermodynamic properties are all in agreement with available experimental data.   

Session J21: Medical Physics: Approaches to Cancer Treatment

The use of Monte Carlo methods in heavy charged particle radiation therapy.

This presentation will demonstrate the importance of Monte Carlo (MC) simulations in proton therapy. MC applications will be shown which aim at 1. Modeling of the beam delivery system. MC can be used for quality assurance verification in order to understand the sensitivity of beam characteristics and how these influence the dose delivered. 2. Patient treatment dose verification. The capability of reading CT information has to be implemented into the MC code. Simulating the ionization chamber reading in the treatment head allows the dose to be specified for treatment plan verification. 3. 4D dose calculation. The patient geometry may be time dependent due to respiratory or cardiac motion. To consider this, patient specific 4D CT data can be used in combination with MC simulations. 4. Simulating positron emission. Positron emitters are produced via nuclear interactions along the beam path penetration and can be detected after treatment. Comparison between measured and MC simulated PET images can provide feedback on the intended dose in the patient. 5. Studies on radiation induced cancer risk. MC calculations based on computational anthropomorphic phantoms allow the estimation of organ dose and particle energy distributions everywhere in the patient.   

Simulating the migration of multiple cancer cells in the bloodstream

We model the migration of cancer cells that have broken away from a tumor and are circulating in the bloodstream. Using the immersed boundary method and culling from literature the material properties of cancer cells, we solve for the deformation of the cells represented as ``immersed boundaries'' being advected by the shear flow of the bloodstream. We solve for the magnitude of the deformation as a function of the flow magnitude as well as the adhesive properties between the cancer cells and the endothelial cells of the bloodstream. We also simulate the migration characteristics as a function of the migrating cell density. From these, we calculate rough approximations of the metastatic rate and efficiency.   

Multicatheter Device for Brachytherapy Treatment

Low dose rate brachytherapy treatment for prostate cancer encompasses the delivery of capsules containing radioactive material into the prostate's cancerous tissue via injection through needles. High dose rate brachytherapy treatment for prostate cancer follows the same concept with the difference that the radioactive source has a higher activity and it is placed temporarily into the patient. For this reason, the source is driven by an afterloading device that moves the source into the catheters and back into a shielded container. From both HDR and LDR brachytherapy, two issues remain unaddressed: homogeneity and localization. Sources not being homogeneous result in a delivered dose that does not correspond to the treatment plan. In the case of HDR, the afterloader not always places the source where it should within the catheter. This results in undertreatment of the cancerous tissue as well as damage to healthy tissue. To address both issues we have placed scintillating fiber into brachytherapy needles. If placed geometrically around the radioactive seeds we are able to check for homogeneity in the sources. At the same time, by analyzing the detected signals we are trying to determine the exact physical position of the seeds within the catheter. Using a radioactive source, we have taken measurements to calibrate the device and measurements under water to simulate living tissue environment. Results are discussed.   

Calibration Of An Active Mammosite Using A Low Activity Sr-90 Radioactive Source

The latest involvement of the Brachytherapy research group of the medical physics program at Hampton University is in the development of a scintillating fiber based detector for the breast cancer specific Mammosite (balloon device) from Cytyc Inc. Recent data were acquired at a local hospital to evaluate the possibility of measuring the dose distribution during breast Brachytherapy cancer treatments with this device. Since sub-millimeter accuracy in position is required, precision of the device relies on the accurate calibration of the scintillating fiber element. As part of a collaboration work, data were acquired for that purpose at Hampton University and subsequently analyzed at Morgan State University. An 8 mm diameter strontium-90 radioactive field source with a low activity of 25 $\mu $Ci was used along with a dedicated LabView data acquisition system. We will discuss the data collected and address some of the features of this novel system.   

Brachytherapy with an improved MammoSite Radiation Therapy System

Accelerated partial breast irradiation treatment utilizing the MammoSite Radiation Therapy System (MRTS) is becoming increasingly popular. Clinical studies show excellent results for disease control and localization, as well as for cosmesis. Several Phase I, II, and III clinical trials have found significant association between skin spacing and cosmetic results after treatment with MRTS. As a result, patients with skin spacing less then 7 mm are not recommended to undergo this treatment. We have developed a practical innovation to the MammoSite brachytherapy methodology that is directed to overcome the skin spacing problem. The idea is to partially shield the radiation dose to the skin where the skin spacing is less then 7 mm, thereby protecting the skin from radiation damage. Our innovation to the MRTS will allow better cosmetic outcome in breast conserving therapy (BCT), and will furthermore allow more women to take advantage of BCT. Reduction in skin radiation exposure is particularly important for patients also undergoing adjuvant chemotherapy. We will present the method and preliminary laboratory and Monte Carlo simulation results.   

Recent advances in radiation cancer therapy

This paper presents the recent advances in radiation therapy techniques for the treatment of cancer. Significant improvement has been made in imaging techniques such as CT, MRI, MRS, PET, ultrasound, etc. that have brought marked advances in tumor target and critical structure delineation for treatment planning and patient setup and target localization for accurate dose delivery in radiation therapy of cancer. Recent developments of novel treatment modalities including intensity-modulated x-ray therapy (IMXT), energy- and intensity modulated electron therapy (MERT) and intensity modulated proton therapy (IMPT) together with the use of advanced image guidance have enabled precise dose delivery for dose escalation and hypofractionation studies that may result in better local control and quality of life. Particle acceleration using laser-induced plasmas has great potential for new cost-effective radiation sources that may have a great impact on the management of cancer using radiation therapy.   

An Active MammoSite{\copyright} for Breast Cancer Treatment

Breast brachytherapy using the MammoSite{\copyright} balloon catheter is one of the latest developments in breast cancer treatment and is the most performed method of brachytherapy. A high activity $^{192}$Ir radioactive source is pushed inside the shaft of the device until it reaches the center of the balloon. The latest involvement of the Brachytherapy research group of the medical physics program at Hampton University is in the development of a scintillating fiber based detector for the breast cancer specific MammoSite{\copyright} balloon catheter from Cytyc, Inc. During the summer 2006, data were acquired at a local hospital (Bon Secours DePaul Medical Center) to evaluate the possibility of measuring the source location and dose distribution during breast brachytherapy cancer treatments with this device. Two 0.5 mm$^{2}$ and 1.0 mm$^{2}$ scintillating fibers were used for these experiments. We used two modified MammoSite{\copyright} devices, each housing an extra tubing within which the fibers were inserted. The results from these runs confirm the possibility of an active MammoSite{\copyright} to monitor the location of the source as well its dose distribution during patient treatment. We will describe the experimental setup and discuss the data.   

Optical Interferometric Response of Living Tissue to Cytoskeletal Anticancer Drugs

Living tissue illuminated by short-coherence light can be optically sectioned in three dimensions using coherent detection such as interferometry. We have developed full-field coherence-gated imaging of tissue using digital holography. Two-dimensional image sections from a fixed depth are recorded as interference fringes with a CCD camera located at the optical Fourier plane. Fast Fourier transform of the digital hologram yields the depth-selected image. When the tissue is living, highly dynamic speckle is observed as fluctuating pixel intensities. The temporal autocorrelation functions are directly related to the degree of motility at depth. We have applied the cytoskeletal drugs nocodazole and colchicine to osteogenic sarcoma multicellular spheroids and observed the response holographically. Colchicine is an anticancer drug that inhibits microtubule polymerization and hence prevents spindle formation during mitosis. Nocodazole, on the other hand, depolymerizes microtubules. Both drugs preferentially inhibit rapidly-dividing cancer cells. We observe dose-response using motility as an effective contrast agent. This work opens the possibility for studies of three-dimensional motility as a multiplexed assay for drug discovery.   

Characterization and modeling of relative efficiency of optically stimulated luminescence Al$_{2}$O$_{3}$:C detectors exposed to heavy charged particles

Medical dosimetry of heavy charged particles (HCPs) and personnel space dosimetry are becoming important areas with the development of new facilities for cancer therapy of heavy ions and the increase of human activities in space. In particular, the measurement of dose in the space radiation field is one of the most challenging problems in personnel dosimetry due to the presence of a mixture of different particles with a wide range of energies. HCP creates a pattern of energy deposition around its path which is a characteristic of the type of particle and its energy. Due to different spatial distribution of dose around the HCP path, the response of the dosimeter can be significantly different for different types of particles and energies. This work characterizes the optically stimulated luminescence (OSL) response of Al$_{2}$O$_{3}$:C personnel dosimeter to different HCPs and energies. Also, a model based on track structure theory to predict the OSL response of the dosimeter is presented.   

Computed Tomography Measurements Using Optically Stimulated Luminescence of KBr:Eu In Real-Time.

Increasing complexity in modern scanning geometries invalidates the concept of computed tomography dose index (CTDI) for CT dosimetry. A real-time dosimetry system using optically stimulated luminescence (OSL) of KBr:Eu is evaluated in comparison with a pencil ionization chamber for CT dosimetry in this study. CT scans were measured over a relevant range of energies and tube currents using a GE LightSpeed Ultra scanner. Complete OSL signals were obtained before, during, and after the CT scans at a rate of 10Hz. Performance was determined in part by normalizing both the initial OSL intensity and the background-subtracted integral OSL to exposure reported by an ionization chamber. OSL response normalized to exposure shows good correlation with coefficients of variation of $\sim ${\%}5 or less. Results show that this OSL dosimetry system possesses great potential for faster, higher-resolution CT characterization and may prove a valuable alternative to CTDI.   

Session J22: Focus Session: Collective Dynamics of Self-Driven Particles

From cell extracts to fish schools to granular layers: the universal hydrodynamics of self-driven systems

Collections of self-driven or ``active'' particles are now recognised as a distinct kind of nonequilibrium matter, and an understanding of their phases, hydrodynamics, mechanical response, and correlations is a vital and rapidly developing part of the statistical physics of soft-matter systems far from equilibrium. My talk will review our recent results, from theory, simulation and experiment, on order, fluctuations, and flow instabilities in collections of active particles, in suspension or on a solid surface. Our work, which began by adapting theories of flocking to include the hydrodynamics of the ambient fluid, provides the theoretical framework for understanding active matter in all its diversity: contractile filaments in cell extracts, crawling or dividing cells, collectively swimming bacteria, fish schools, and agitated monolayers of orientable granular particles.   

Simulations of Interacting Magnetic Micro-swimmers

Following a recent realization of artificial micro-swimming (Dreyfus et. al., \emph{Nature}, \textbf{437}, 862-865), we conduct simulations of a swimmer whose mechanism of propulsion is the magnetically driven undulation of a flagellum-like tail composed of chemically linked paramagnetic beads. In our model, the tail is treated as a series of spheres tied together by inextensible, bendable links. The spheres interact magnetically through mutual dipole interactions, and hydrodynamic interactions are achieved by the force-coupling method. Building on our previous results, we examine the interactions between multiple swimmers employing a flagellum beating strategy as well as those using a rotary propulsion scheme. In addition to swimmer-swimmer interactions, the effects of a nearby surface on the behavior of a micro-swimmer will be discussed.   

Response and Fluctuations in an Active Bacterial Suspension

An active bacterial bath consists of a population of rod-like motile or self-propelled bacteria suspended in a fluid environment. In this talk, we present a two-fluid model for the dynamics of a bacterial bath, and show, in particular, that the non-equilibrium contribution to the stress arising from the swimming of the bacteria and the non-equilibrium couplings between the alignment tensor and bacterial density, lead to i) a $1/\sqrt{\omega}$ scaling in the power spectrum of the active stress fluctuations, and ii) anomalous density fluctuations in the bacteria themselves. These predictions are observed in a recent experiment.   

Collective dynamics of concentrated swimming micro-organisms

Approximately close packed populations of the cylindrical self-propelled bacteria Bacillus subtilis intermittently form domains of aligned, co-directionally swimming organisms. The velocities of these phalanxes are often ``high'' compared to the speed of individual swimmers. They vary with the depth of the suspension of organisms. Although the Reynolds number is $<$1, this collective dynamic phase, the ``Zooming BioNematic'' (ZBN), appears turbulent. Remarkable spatial and temporal correlations of velocity and vorticity, associated with the spontaneous appearance and decay of these surging phalanxes, were measured using appropriately modified Particle Imaging Velocimetry (PIV). These new data, together with measurements of the trajectories of individual cells, provide ingredients for a rational bio-fluid-dynamical theory of the ZBN.   

Large scale flows and density fluctuation in ensembles of swimming bacteria

We study experimentally self-organization of concentrated ensembles of swimming bacteria Bacilus Subtilis. Experiments are performed in a very thin (of the order of 1 bacterium diameter) fluid film spanned between four supporting fibers. Small amplitude electric field is used to adjust dynamically the density of bacteria inside the experimental cell. Our experiments revealed only gradual increase of the large scale flow correlation length with the increase in number density of bacteria, and no sharp transition. The fluctuation of density of bacteria as a function of thickness of the film was explored.   

Chemotaxis and Target Finding using Chemical Echolocation

Chemotaxis is usually modeled by cellular responses to an imposed, exogenous chemoattractant gradient. Here, we consider a scenario in which a single agent releases a chemical that diffuses and is converted to, or signals the production of another chemical upon contact with a target. This secondary chemical can diffuse back to the agent, which uses it as a chemoattractant. We show that this mechanism has interesting features depending on how the probe chemical is produced, and how the product chemoattractant is sensed. Although involving more steps than conventional chemotaxis that relies on a single chemoattractant, we show that this chemical ``pinging'' mechanism can provide cells with flexibility in regulating behavior and finding different targets.   

Dynamics of Gas-Fluidized Bipolar Rods

We study a driven, non-equilibrium two-dimensional system consisting of bipolar rods in a gas-fluidized bed. The rods have an aspect ratio of 4 and occupy an area fraction of 42{\%}, chosen both to minimize the effects of ordering as well as to ensure a uniform density of particles across the system. We are able to track the position and orientation of the particles as a function of time. From this, we measure the dynamics of the system with the advantage that our temporal resolution allows us to observe ballistic motion at the shortest time scales. We calculate the mean squared displacement (MSD) in both the lab frame and the particle's frame in which displacements are measured as either perpendicular or parallel to the rod's long axis. In contrast to a comparable system of isotropic particles in which the dynamics are thermal, our system exhibits distinctly athermal behavior. Specifically, the effective temperature along the parallel direction is greater than that along the perpendicular direction. Furthermore, the parallel MSD remains superdiffusive at the longest time scales we are able to measure before the particles have reached the wall whereas the perpendicular component experiences cross-over to diffusive motion. This is emphasized by the power law decay of the velocity autocorrelation function (VAF). In comparison to a thermal fluid, the parallel VAF decays much more slowly whereas the perpendicular VAF decays more rapidly. With these characteristics in mind, ours is a simple experimental system that could be used to compare to biological models of active particles as well as to generalize the framework of statistical mechanics to non-equilibrium, athermal systems.   

Simulation of suspensions of hydrodynamically interacting self-propelled particles

Simulations of large populations of hydrodynamically interacting swimming particles are performed at low Reynolds number in periodic and confined geometries. Each swimmer is modeled as a rod containing beads with a propulsion force exerted on one bead (with an equal and opposite force exerted on the fluid) and excluded volume potentials at the beads. At small concentrations, the swimmers behave analogously to a dilute gas in which the hydrodynamic interactions perturb the ballistic trajectories into diffusive motion. Simple scaling arguments can explain the swimmer behavior as well as the behavior of passive tracer particles. As the concentration increases, the hydrodynamic interactions lead to large-scale collective motion.   

Hydrodynamics of self-propelled hard particles.

Motivated by recent simulations and by experiments on aggregation of gliding bacteria, we study a physical model of the collective dynamics of self-propelled hard particles on a substrate in two dimensions. The particles have finite size, interact via excluded volume and are frictionally damped by the interaction with the substrate. Starting from a microscopic model of dynamics that includes non-thermal noise sources, we derive a continuum description of the system. The hydrodynamic equations are then used to characterize the possible steady states as a function of the particles' packing fraction and examine their stability. Research support by the NSF award number DMR-0305407.   

Traffic jams in driven intracellular transport on parallel lanes

Microtubules, the intracellular tracks for molecular motors like dynein or kinesin, are built of 12-14 parallel lanes. Although it has been revealed that the motor proteins typically remain on one track while proceeding on the microtubule, the statistics of deviations (random lane changes) is so far unknown. We investigate the effects of a small, but finite number of such lane changes by studying driven transport on two parallel lanes with simple site exclusion [1]. As a result, traffic jams emerge in the stationary density profiles, their location can be controlled by the particle fluxes at the boundaries. We obtain analytical results on the shape of the density profiles as well as resulting phase diagrams by a mean-field approximation and a continuum limit. \newline \newline [1] T. Reichenbach, T. Franosch, E. Frey, Phys. Rev. Lett. 97, 050603 (2006)   

DNA multi-ring formation via evaporation process

We present a study of multi-ring pattern formation of DNA aggregates during the solvent evaporation of a DNA droplet. When the contact line of a droplet is pinned at a solid substrate, a `coffee ring' pattern is often observed due to the outward flow during evaporation which carries the nonvolatile solute to the edge of the contact line. Here we report a remarkable observation of multiple rings of DNA stain, where stretched DNA molecules connect each ring. We use a high-speed confocal scanning microscope to investigate the kinetics of the multi-ring formation, when DNAs aggregate at the contact-line and cause a stick-slip receding process with periodic depinning of the contact line. A saw-tooth pattern in measured contact angle during droplet evaporation confirms the stick-slip receding dynamics, and a miscible viscous fingering pattern further confirms the stagnation flow responsible for the formation of consecutive rings. We also report a scaling behavior of the multi-ring wavelength with DNA concentration, droplet size and evaporation temperature, consistent with our proposed mechanism.   

Session J23: Metals: Actinides and Transport

Evolving Magnetism from self-damage in PuAm alloys

As a consequence of the unusual nature of plutonium's electronic structure, point- and extended-defects exhibit extraordinary properties. Low temperature magnetic susceptibility measurements on Pu and PuAm show that the magnetic susceptibility increases as a function of time, yet upon annealing the specimen returns to its initial value. This excess magnetic susceptibility arises from the $\alpha $-decay and U recoil damage cascades which produce vacancy and interstitials as point and extended defects and at low temperatures exceeds 10{\%} of the annealed value after about 1 month of damage accumulation. Isochronal annealing measurements of $\alpha $-Pu and stabilized $\delta $-Pu reveal that the damage is frozen in place below $\sim $30K and completely annealed away above 300K. The binary PuAm alloy follows a similar trend, but after warming to temperatures between 35 and 50K where defects are expected to begin moving, an enormous Curie like magnetic susceptibility arises with a Curie constant approximating 1 $\mu _{B}$/actinide atom. This large effective moment disappears after 60K as further annealing takes place. Work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract W-7405-Eng-48.   

Study of Phonons in $\delta $-Plutonium near the $\delta -\alpha $' Structural Phase Transition by X-ray Thermal Diffuse Scattering

The 5f electrons in Pu can be either bonding or localized, depending sensitively on the temperature, pressure, and impurity doping. As a result, Pu displays a rich phase diagram involving a large number of phases with substantially different atomic volumes. In a recent report of the phonon dispersion curves of Ga-stablized $\delta $-Pu at room temperature and ambient pressure, a pronounced deepening of the TA[111] phonon branch near the L point was discovered. This phonon softening was suggested to be related to a lattice shearing mechanism that could lead to the structural phase transition from the fcc $\delta $ phase to the monoclinic $\alpha $' phase at about 170 K. Here we report our measurements of x-ray thermal diffuse scattering from a $\delta $-Pu crystal (with 0.6 wt{\%} Ga) at temperatures from 307K to 200K. The results show no further softening of the phonons near the L point as the sample temperature decreases. The implications regarding the relationship between the soft mode and the phase transition will be discussed.   

Lattice dynamics of the light actinides

Despite general interest in $f$-electron elements, details about their phonon-dispersion relationships are very limited. But recently, a new hope has emerged with several works, using inelastic x-ray scattering, mostly on U and Pu. Nevertheless all this experimental issues show that theoretical works are needed to tackle the $f$ electrons systems elastic properties. Unfortunately these calculations are far from being straightforward. Theoretically, the most important problem comes from the difficulty to treat correctly the $f$-electrons and the relativistic effects needed in such heavy materials. The lattice dynamics of $\alpha$-uranium are known for almost 30 years, but until now any theory has been able to successfully reproduce these experimental data. Here we present the first \textit{ab-initio} phonon spectrum of $\alpha$-U[1]. We compare our spectrum obtained at 0 K with the neutron-scattering data obtained at room temperature with a particular attention to its anomalies. Then we predict the behavior of lattice dynamics of uranium as a function of pressure. We have also calculated the phonon spectra and the thermodynamic properties of Th, as the linear thermal expansion or specific heats[2], and the elastic properties of Th, Pa and U. [1]J. Bouchet submitted to Phys. Rev. Letter. [2]J. Bouchet, F. Jollet and G. Zerah, Phys. Rev. B 74 064637   

Density-Functional Calculations of $\alpha $-Pu-Ga (Al) Alloys

At atmospheric pressure plutonium metal exhibits six crystal structures. The least dense phase ($\delta $-Pu) has a 25{\%} larger volume than the ground-state ($\alpha $-Pu) phase and is thermodynamically stable at temperatures between 593 and 736 K. In order to extend the stability of $\delta $-Pu to ambient temperatures, plutonium is alloyed with a small amount of so-called '$\delta $-stabilizers', for example, Ga or Al. The $\alpha $-phase has no equilibrium solubility with any of these $\delta $-stabilizers but upon cooling of the $\delta $-Pu-Ga (Al) alloys, under certain conditions, Ga (Al) atoms can be trapped in the $\alpha $-lattice causing an expansion. First-principles methods are employed to study the ground-state atomic volumes of $\alpha $-Pu-Ga (Al) alloys. It was shown that a random distribution of Ga (Al) atoms in the monoclinic lattice of $\alpha $-Pu results in a maximum expansion of this lattice. Any kind of ordering of Ga (Al) on the monoclinic lattice results in shrinking of the lattice constant while the ordered $\alpha _{8}$-(Pu-Ga (Al)) configuration yields the smallest lattice constant which is very close to that of pure $\alpha $-Pu. In addition, energetics of the ordered and disordered configurations is discussed. 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 No. W-7405-Eng-48.   

The Elastic Moduli of Monoclinic and Orthorhombic Plutonium

Measurements were made of the bulk and shear moduli of high-purity polycrystalline Pu from 10K to 670K using resonant ultrasound spectroscopy. A simple dilatometer was employed to provide redundant detection of the phase transitions. We observed the expected phase transitions from monoclinic ($\alpha )$ to body centered monoclinic ($\beta )$ to orthorhombic ($\gamma )$ to face centered cubic ($\delta )$. Very accurate values were obtained for $\alpha $-Pu, $\beta $-Pu was very soft and difficult to analyze, as was $\delta $-Pu. Surprisingly, the $\gamma $-phase produced the high-Q resonances needed for accurate elastic modulus determination. We discuss also the unusual temperature dependences.   

Magnetotransport properties and the Fermi surface of high-quality single crystal VB$_{2}$

We have performed magnetotransport and dHvA measurements on high quality single crystals of VB$_{2}$ grown from a molten aluminum flux. At low temperature the magnetoresistance (MR) of VB$_{2}$ is very large ($\sim $1100{\%}) and is found to be extremely sensitive to sample quality. The field dependence of the MR is proportional to the applied field squared, as is expected from open orbits on the Fermi surface. In addition, we have performed full potential LAPW calculations with the WIEN2K band package using the GGA density functional to compute the bands and Fermi surface. The calculations suggest that the area of the Fermi surface is a strong function of the lattice constants. The results of the calculations will be compared to experiment.   

Nernst-Ettingshausen effect in Bismuth across the quantum limit

In elemental Bismuth, 10$^{5}$ atoms share a single itinerant electron. A magnetic field of the order of 10 T can confine electrons to the lowest Landau level. We report on the first study of metallic thermoelectricity in this regime. The thermoelectric response is mainly off-diagonal and peaks each time a Landau level hits the chemical potential. The associated maxima in the Ettingshausen coefficient saturates to a temperature-independent magnitude offering a notable constraint for theory.   

Hall Effect and Magnetoresistance in cubic helimagnets

An anomalous contribution to Hall Effect is observed in a variety of ferromagnetic conductors ranging from simple metals and oxides to dilute magnetic semiconductors, manganites, ruthenates and spinel compounds. Its microscopic mechanism (intrinsic versus extrinsic) is still under investigation. FeGe and MnSi are itinerant helimagnets, due to the lack of inversion symmetry of the crystal structure. A large anomalous contribution to the Hall Effect is observed in these systems from 2K up to 300K, with no significant sample-to-sample dependence. Moreover, the transverse magnetoresistance has a positive orbital part at low temperatures that involves the same anomalous contribution to the Hall angle. These results establish, on purely experimental grounds, the essentially intrinsic nature of the anomalous Hall Effect, associated in recent theoretical models with a finite Berry phase acting as an effective magnetic field on charge carriers.   

Quantum kinetics in layered systems

Electron transport in strongly anisotropic materials exhibit several unusual properties.In particular, the resistivity across the layers (c-axis) has either an insulating-like or non-monotonic temperature dependences, whereas the resistivity along the layers is metallic. It is generally believed that when the scattering rate, 1/$\tau$, becomes larger than the tunneling rate between the layers, J, the nature of transport changes from coherent (band-like) to incoherent (tunneling-like). By using a Prange-Kadanoff--like quantum Boltzmann equation for electrons coupled to phonons, we show that there is no coherent/incoherent crossover for any value of J$\tau$, as long as the usual ``good metal'' condition is satisfied, i.e., E$_F\tau\gg$ 1. In other words, a strongly anisotropic metal is as ``coherent,'' as an isotropic one. The situation is changed in the presence of resonant tunneling centers between the layers. We show that the unusual behavior of the c-axis resistivity can be explained by inelastic resonant tunneling through such centers.   

Giant Fluctuations of the Coulomb Drag

Coulomb drag has been shown to provide information about electron-electron interactions not available from conventional conductance measurements, e.g. [1]. For mesoscopic conductors, a spectacular interference phenomenon is UCF. There has been a prediction that Coulomb drag should also show similar fluctuations: with decreasing temperature the fluctuations in the drag are expected to become larger than the average drag, resulting in a random change of the sign of the drag with varying carrier density [2]. Contrary to the UCF, the origin of these fluctuations involves both quantum interference and electron-electron interaction effects. Here we report the first observation of reproducible fluctuations of Coulomb drag in a double-layer GaAs structure, as a function of both the carrier density and magnetic field. Surprisingly, the observed fluctuations are almost four orders of magnitude larger than originally predicted in the theory, which considered diffusive transport of interacting electrons. We explain the observed enhancement by the fact that in ballistic transport, realised in our structures, the Coulomb drag probes the local properties of the system. The latter are expected to show much larger fluctuations than the global ones. [1] Gramila, Eisenstein, et al PRL 66, 1216 (1991). [2] Narozhny and Aleiner, PRL 84, 5383 (2000).   

Phase coherence of conduction electrons below the Kondo temperature

The scattering of conduction electrons by magnetic impurities is known as the Kondo effect. This effect has been the subject of theoretical and experimental investigations for several decades. Until very recently [1, 2], however, there was no theoretical expression for the temperature dependence of the inelastic scattering rate valid for temperatures $T $not too far below the Kondo temperature, $T_{K}$. We present experimental measurements of the phase decoherence rate, $\tau _{\phi }^{-1}$, of conduction electrons in disordered dilute AgFe Kondo wires [3]. We compare the temperature dependence of the magnetic scattering rate, $\gamma _{m}$, with a recent theory of dephasing by Kondo impurities [2]. A good agreement with theory is obtained for $T$/$T_{K }>$ 0.1. At lower $T$, $\gamma _{m}$ deviates from theory with a flatter $T$-dependence. \newline \newline [1] G. Zarand, L. Borda, J. von Delft, and N. Andrei, Phys.Rev. Lett. 93, 107204 (2004). \newline [2] T. Micklitz, A. Altland, T. A. Costi, A. Rosch, Phys.Rev. Lett. 96, 226601 (2006). \newline [3] G.M. Alzoubi and N.O. Birge, Phys.Rev. Lett. in press (2006).   

Pre-Exponential factor and hopping criterion in the Efros-Shklovskii regime

We address the variable-range hopping regime in the range for which the measured temperature $T$ is of the order of the characteristic Efros-Shklovskii temperature $T_{ES}$. In such a range current theories imply $r_{hop}/\xi<1$, where $r_{hop}$ is the hopping length and $\xi$ is the localization length, clearly in contradiction with the standard criterion for hopping conduction. We consider impurity overlap wavefunctions of the form $\psi(r) \propto r^{-n}\exp(-r/\xi)$ and include the preexponential factor of the resistivity as a logarithmic correction in the Mott optimization procedure. From the general expression derived, the standard Efros-Shklovskii law is recovered for $T<1$, is found for $T_{ES} \geq T$. We argue that the new expression resulting from an interplay between preexponential and exponential factors is a consistent extension of the classical Efros-Shklovskii argument.   

Interaction Effects in a High-Mobility Two-Dimensional Electron Gas in a Nonquantizing Magnetic Field

Two dimensional electron gas in a perpendicular nonquantizing magnetic field, $B$, is considered. We demonstrate \footnote{preprint cond-mat/0611111.} that the anomaly in the polarization operator, $\Pi(q)$, near $q=2k_F$, where $k_F$ is the Fermi momentum, gets smeared with $B$ in a peculiar fashion: slowly decaying modulation, periodic in $(2k_F-q)^{3/2}$, emerges. The period of modulation sets a spatial scale, $p_0^{-1}\propto B^{-2/3}$, which is much smaller than the Larmour radius, but much larger than the de Broglie wavelength. This scale manifests itself, {\em e.g.,} in lifting the periodicity of the Friedel oscillations, $\delta \rho (r)$ in magnetic field, namely we find that $\delta \rho (r)\propto \sin \left[2 k_F r-(p_0 r)^3/12\right]/r^2$. The corrections to the interaction-induced characteristics of the $2D$ gas, such as relaxation rate and the tunnel density of states, coming from the distances $\sim p_0^{-1}$, are shown to be strongly singular (as $B^{1/3}$) in magnetic field.   

Electric-Field-Induced Hopping Conductivity in Polymers

The resistivity of highly insulating polymers exhibits a dependence on electric field strength. Mott and Davis as well as Poole and Frankle describe theoretically the resistivity of disordered semiconductors, when subject to a changing electric field, in terms of hopping conductivity models. Although such models have often been applied to polymers, there is little direct experimental evidence to confirm the validity of these theories when applied to polymers. We present such results for a newly-developed block co-polymer Hytrel, a highly insulating material. The constant voltage resistivity test method has been used to study Hytrel for a range of electric fields approaching electrostatic breakdown. Previously taken preliminary measurements are suggestive that Hytrel validates hopping conductivity models. With additional data we consider whether the Hytrel results are consistent with existing models of electric-field induced hopping conductivity.   

Dephasing in the semiclassical limit is system-dependent

We investigate dephasing in open ballistic chaotic systems in the limit of large system size to Fermi wavelength ratio, $L/\lambda_{\rm F} \gg 1$. Using the trajectory-based semiclassical theory, we calculate the weak localization correction $g^{\rm wl}$ to the conductance for a quantum dot coupled to (i) a dephasing voltage probe and (ii) an external closed quantum dot. In addition to the universal algebraic suppression $g^{\rm wl} \propto (1+\tau_{\rm D}/\tau_\phi)$ with the dwell time $\tau_{\rm D}$ through the cavity and the dephasing rate $\tau_\phi^{-1}$, we find an exponential suppression of weak localization by a factor $\propto \exp[-\tilde{\tau}/\tau_\phi]$, with a system-dependent $\tilde{\tau}$. In the dephasing probe model, $\tilde{\tau}$ coincides with the Ehrenfest time, $\tilde{\tau} \propto \ln [L/\lambda_{\rm F}]$, in both cases of perfectly and partially transparent dot-lead couplings. In contrast, when dephasing occurs due to the coupling to an external dot, $\tilde{\tau} \propto \ln [L/\xi]$ depends on the correlation length $\xi$ of the coupling potential instead of $\lambda_{\rm F}$.   

Session J24: Frank J. Padden Award Symposium

How ideal are the ideal-like polymers

The previously unknown long range correlations in the conformations of linear polymers in a $\theta$-solvent were found using analytical calculations and molecular dynamics simulations. Long range power law decay of the bond vector correlation function $\langle\cos\phi\rangle\sim s^{-3/2}$ dominate the standard exponential decay $\langle\cos\phi\rangle = e^{-s/l_p}$, where $\phi$ is the angle between the two bonds, $s$ is their separation along the chain and $l_p$ is the persistence length. These long-range correlations lead to significant deviations of polymer size from ideal with mean square end-to-end distance $\langle R^2 \rangle - b^2N \sim \sqrt N$, where $N$ is the number of Kuhn segments of size $b$. This new phenomena is explained by a fine interplay of polymer connectivity and the non-zero range of monomer interactions. Moreover, it is not specific for dilute $\theta$-solutions and exists in semidilute solutions and melts of polymers. Our results show good agreement with the experimental data on Flory characteristic ratio.   

Self-Assembly of Block Copolymers in a Nematic Liquid Crystal Solvent

Solutions of side-group liquid crystal polymers (SGLCPs) in a liquid crystal (LC) solvent are governed by rich thermodynamics resulting from the competition between the solvent's orientational order and the polymer's conformational entropy. Solutions of SGLCP-random coil block copolymers in LC solvent have an additional layer of complexity deriving from lyophilic/lyophobic interactions between the blocks and the solvent. Dissolving a triblock copolymer yields an LC gel where synergistic coupling between polymer and solvent results in novel properties; the orientational order of the nematic LC solvent imparts electo-optic and mechano-optic properties that are forbidden by symmetry in isotropic gels, and the polymer network provides memory via long-time relaxation processes that do not exist in the bulk LC. The gels are thermoreversible because the random coil block becomes soluble above the solvent's isotropic transition temperature. The exceptional sensitivity of these block copolymers' self-assembled structures to temperature and polymer architecture is demonstrated by small-angle neutron scattering and rheometry. NMR and neutron scattering give a detailed understanding of the coupling between the solvent order and SGLCP conformation.   

Threading Synthetic Polyelectrolytes through Protein Pores

We have measured the ionic current signatures of sodium poly(styrene sulfonate) as its single molecules translocate through an alpha-hemolysin pore embedded into a bilayer in a salty aqueous medium under an externally applied electric field. As in the previous experiments involving DNA and RNA, the pore current, which is a measure of the ionic conductivity of the low molar mass electrolyte ions, is significantly reduced when the polymer molecule translocates through the pore. By studying thousands of single molecule events, we have constructed distribution functions for the extent of the reduced current and for the translocation time. By investigating over two orders of magnitude in the molecular weight of the polymer, the average translocation time is found to be proportional to the molecular weight and inversely proportional to the applied voltage. Our experiments open up many opportunities to systematically explore the fundamental physical principles behind translocation of single macromolecules, by resorting to the wide variety of synthetically available polymers.   

Order and Disorder in Polydisperse Block Copolymer Melts

Utilizing creative strategies for the synthesis of model controlled-polydispersity poly(ethylene-\textit{alt}-propylene)-$b$-poly(\textsc{d,l}-lactide)(PEP-PLA) and polystyrene-$b$-polyisoprene(PS-PI) block copolymers, the effects of increased breadth in the molecular weight distribution on block copolymer self-assembly were investigated. Small-angle x-ray scattering and rheological measurements were carried out to characterize the morphological details of these self-assembled materials as a function of their polydispersity, interaction strengths, and compositions. A number of surprising consequences of increased breadth in the molecular weight distribution emerged; the domain spacing of the ordered structures increased, changes in morphology occurred, and the degree of segregation at the order-disorder transitions changed as well, particularly for asymmetric block copolymers. The change in the degree of segregation at the order-disorder transition as the polydispersity was increased was found to be dependent on the block copolymer composition, e.g., for PEP-PLA and PS-PI at asymmetric compositions, when the polydispersity was increased in the minority component, the degree of segregation at the order-disorder transition decreased, whereas when the polydispersity was increased in the majority component, the degree of segregation at the order-disorder transition increased.   

Morphological Evolution of Poly (caprolactone) Dendrites during Isobaric Relaxation of Metastable Monolayers at the Air/Water Interface

Isobaric crystallization at constant surface pressures of 11, 10.5, 10.3, 10, 9.5, and 8.5 mN/m were performed for a poly (caprolactone) (PCL)/poly (t-butyl acrylate) (PtBA) blend with a PtBA mole fraction of 0.14 to study the effect of surface pressure on the morphological evolution of PCL dendrites at the air/water interface. At 11 mN/m, corresponding to a higher degree of undercooling, morphological studies indicate that the side-branches in the two (100) sectors grow faster than those in the four (110) sectors, possibly because molecular diffusion effects arising from a limited material reservoir is more spatially constrained in the (110) sectors. During isobaric crystallization at 10.3 and 10 mN/m, four-arm dendrites observed show side-branches in the four (110) growth faces are better developed. Furthermore, isobaric crystallization at 9.5 mN/m causes a seaweedlike crystal morphology showing less branched and more compact structures with larger tip radii. Finally, PCL crystals grown at 8.5 mN/m show compact structures without side-branches.   

How Polymers Diffuse in Molecularly-Thin Films

We explore the fundamental question, how polymer diffusion in molecularly-thin films differs from that in isotropic melts comprised of the same polymer. To explore this, a new surface forces apparatus was developed to enable, for the first time to the best of our knowledge, spectroscopic measurement in addition to force measurements of the traditional kind. Here we describe experiments using fluorescence correlation spectroscopy (FCS) and fluorescence recovery after photobleaching (FRAP). Melts of polydimethylsiloxane (PDMS) were confined between atomically-smooth mica surfaces at carefully controlled separations. Translational diffusion of the polymer was measured as a function of film thickness and molecular weight. We show that adsorption slows surface diffusion by over three orders of magnitude and that this effect propagates to thickness roughly 3 times the unperturbed radius of gyration (R$_{G})$. Confinement between two opposed surfaces further reduces the diffusion coefficient (D) by at most a factor of 5. Spatially-resolved measurements elucidate how D furthermore depends on the local pressure that squeezes the thin film, as well as on molecular weight.   

Phononics and elastic moduli in polymeric and biological nanostructures

Mechanical stability of nanostructured materials is of paramount importance for any nanotechnology. However, measuring mechanical properties at such small scales and understanding their dependence on confinement effects remain a great challenge. We use Brillouin light scattering to study the vibrational spectra and mechanical moduli of polymeric and biological nanostructures. We demonstrate that elastic properties neither change nor appear anisotropic in polymeric lines as narrow as 80 nm. As biological nanomachines, viruses may be useful for various applications ranging from gene therapy and drug delivery to photonic crystals. Analysis of phononic spectra of viral structures indicates strong mechanical coupling between viruses, helping to explain the stability of structures formed by viruses. Spectral analysis further indicates that DNA cores of viruses, not their protein shell, dominate elastic properties of viruses, in contrast to traditional model assumptions. This new finding will impact not only nano-patterning, but also biomedical applications involving viruses and possible treatments of viral infections.   

Surface Dynamics of Glassy Polymer Films and Its Effect on Glass Transition Temperature

The surface dynamics of thin polymer films is believed to be different from that of bulk and is cited to be the source of Tg anomalies in thin films. In this study ellipsometry is used to measure the cooling rate dependence of Tg in thin polystyrene films. It is shown that as the temperature decreases below bulk Tg, the relaxation time behavior changes from Vogel-Fulcher to Arrhenius, with an activation energy that decreases linearly with film thickness, indicating that the relaxation time of the surface (limit of zero thickness) is also Arrhenius. To measure the relaxation time of the surface directly, a novel technique is used to produce nanometer size holes with well defined shapes and driving forces on the film surface. AFM is used to monitor the depth of the holes as a function of time at different temperatures. The relaxation times are obtained from the exponential decay of the depths. It is shown that at the low temperature limit the behaviour is Arrhenius with an activation energy similar to the one predicted from ellipsometry measurements. By combining these results the Tg of the surface can be estimated. These results can explain various contradictions in the literature and can provide a framework for a new theory to explain Tg reductions in thin polymer films.   

Thermodynamic and Kinetic Control of Charged Triblock Copolymer Assembly into Complex Nanostructures

Self-assembly of poly (acrylic acid)-block-poly (methyl acrylate)-block-polystyrene triblock copolymers produces various ordered nano-domains in THF/water solution through the interaction with organic counterions. These assembled structures include classic micelles (spheres, cylinders and vesicles), and non-classic micelles (disks, toroids, branched micelles and segmented micelles). Each micelle structure is stable and reproducible at different assembly conditions depending on not only solution components (thermodynamics) but also mixing procedure and consequent self-assembly pathway (kinetics). The key factors that determine the thermodynamic interactions that help define the assembled structures and the kinetic assembly process include THF/water ratio, PS block length, the type and amount of organic counterions, and the mixing pathway. The complex phase behavior and controlled morphology production have been studied via in-situ cryogenic transmission electron microscopy in combination with scattering techniques (small angle neutron scattering and light scattering). Delicate control of the interplay of thermodynamics with slow chain kinetics of block copolymers in solution offers a new strategy to create unique, functional nanostructures.   

Structured Interfaces of Surface Wrinkles for Adhesion, Optics and Sensors

From the adhesion of the gecko to the optics of the dragonfly's eye, nature provides numerous examples that utilize optimized microstructures to control interfacial properties. Inspired by the natural world, many research groups have adopted similar approaches to generate devices that are based primarily on top-down strategies -- i.e. lithography. An attractive alternative for creating surface structures is surface wrinkling. Surface wrinkling has been observed by many researchers over the past several decades and is associated with the onset of an elastic instability. The wavelength and critical stress of formation are determined by a combination of geometry and materials properties. Here, we present a new approach to generating stable surface wrinkles based on swelling of an elastomer with a photocrosslinkable monomer formulation. We explore dimensional, orientational and spatial control of the wrinkled structures by creating polymer surfaces with defined regions of contrasting elastic moduli. Specifically, we present an experimental phase diagram of various wrinkle structures, which highlights the discovery of two morphologies that have not been previously observed. We demonstrate how these unique structures can be used for enhanced control of adhesion in soft polymers and the simple fabrication of microlens arrays and compound lens structures.   

Session J25: Focus Session: Hybrid Organic, Inorganic Nanomaterials: Synthesis, Assembly

Directing and Orienting Nanoparticles and Nanorods at Fluid Interfaces, within Templates, and on Substrates

This presentation will center on the functionalization of nanoparticles and nanorods with organic and polymer ligands that influence the behavior of such particles in solution, at interfaces, and on substrates. When nanoparticles, whether quantum dots, gold nanoparticles, or bionanoparticles (such as tobacco mosaic virus or ferritin), are surface-functionalized with reactive ligands, they not only can form interesting assemblies, but also can be converted to robust structures through performing chemistry on the ligands. Nanoparticle-based capsules and sheets arise from such chemistries, which can be considered for potential applications in for example functional membranes and coatings.   

Formation of Giant Meso-Polymers from Magnetic Nanoparticles Using Fossilized Liquid Assembly

We report the ability to directly image the self-organization of polymer-coated ferromagnetic nanoparticles into one-dimensional mesostructures at a liquid-liquid interface. When polystyrene-coated Co nanoparticles (15 nm) are deposited at an oil/water interface under zero-field conditions, long ($\sim$ 5 $\mu$m) chain-like assemblies spontaneously form, where the precise morphology depends upon particle concentration, temperature, and assembly time. The assembly process was examined using ``Fossilized Liquid Assembly,'' a recently developed platform consisting of a biphasic oil/water system in which the oil phase can be flash-cured upon ultraviolet light exposure. The nanoparticle assemblies embedded in the crosslinked phase were then imaged using atomic force microscopy. Noting the dependence of chain length on the assembly conditions, we observed striking similarities between nanoparticle self-assembly and polymer synthesis. Previous reports on the mechanism and kinetics of equilibrium polymerization provide a useful framework for performing quantitative image analysis on the AFM-visualized mesostructures.   

Nanoparticle Alignment and Repulsion During Failure of Glassy Polymer Nanocomposites

We investigate crazing and failure in a model nanocomposite of surface modified nanoparticles (cadmium selenide, diameter is 5 nm) blended into polystyrene. We demonstrate that nanoparticles undergo three stages of rearrangement during craze formation and propagation in glassy polymer nanocomposites: 1) Alignment along the precraze, 2) Expulsion from craze fibrils, and 3) Assembly into clusters entrapped between craze fibrils. At an optimal volume fraction of nanoparticles, the failure strain of the nanocomposite is increased by nearly 100{\%} relative to unfilled polystyrene. This optimal volume fraction is related to the balance of two mechanisms: 1) the decrease in cross-tie fibril density for craze structures, and 2) the decrease in the probability of craze widening at higher tensile strain by decreasing the number of polymer entanglements at small interparticle lengths. These results offer a clear and detailed understanding of failure mechanism of glassy polymer-nanoparticle composites, and provide predictions for the future design of nanoparticle-based materials.   

Solvent-Mediated Plasmon-Tuning in a Nanoparticle-Poly(Ionic Liquid) Organogels and Hydrogels

The design, synthesis and characterization of hierarchically ordered composites whose structure and optical properties can be reversibly switched by adjustment of solvent conditions are described. Solvent-induced swelling and deswelling is shown to provide control over the internal packing arrangement and hence, optical properties of \textit{in situ} synthesized metal nanoparticles. Specifically, metal nanoparticle-containing ionic liquid-derived polymers are synthesized in a single step by UV irradiation of a metal ion precursor-doped, self-assembled ionic liquids, 1-decyl-3-vinylimidazolium chloride or 1-(8-(acryloyloxy)octyl)-3-methylimidazolium chloride, physical gels. Small-angle X-ray scattering (SAXS) studies are used to monitor the nanostructure of the polymers in the deswollen and swollen states. Optical spectroscopy of the dried composites reveals plasmon resonances positioned in the near-infrared. Upon swelling in alcohol or water, the materials undergoes a structural conversion to a disordered structure, which is accompanied by a color change and a blue shift in the surface plasmon resonance. These results demonstrate the far-field tuning of the plasmonic spectrum of gold nanoparticles by solvent-mediated changes in its encapsulating matrix, offering a straightforward, low-cost strategy for the fabrication of nanophotonic materials.   

Immobilizing Au Nanoparticles with Polymer Single Crystals, Patterning and Asymmetric Functionalization

Considerable attention has been paid to nanoparticle (NP) research due to their fascinating properties and potential applications in nanotechnology and biotechnology. Asymmetrically functionalizing NP is of particular interest since it could directly lead to controlled patterning of NPs into complex structures for a variety of applications. Here we report, for the first time, using 2-dimensional polymer lamellar single crystals as the solid substrate to create patterned functional (thiol) surface and immobilize AuNPs. We demonstrated that patterning of AuNPs could be achieved and the AuNP area density could be easily controlled by polymer molecular weight. Furthermore, this unique technique also enables asymmetric functionalization of AuNPs. Bilayer AuNPs/polymer hybrids were obtained. Dissolving PEO single crystals led to free asymmetric binary AuNP complexes. This approach provides a novel means to pattern AuNPs and synthesize asymmetrically functionalized AuNPs. We also anticipate that this methodology could be applied to other metallic or semiconducting NPs.   

High-Energy Density Capacitors using Nanoparticle-Polymer Composite Dielelectrics

By combining a polymer with a high dielectric strength and nanoparticles with an even higher dielectric constant, we can to explore exchange coupling between the two materials that will result in a material with an optimized dielectric constant and dielectric strength. [J. Li, \textit{Phys. Rev. Lett.} \textbf{90}, 217601 (2003)] We report the results of dielectric characterization of composites consisting of barium titanate and other dielectric nanoparticles embedded in a matrix of copolymers of vinylidene fluoride with trifluoroethylene. Basic measurements are made by creating parallel plate capacitors with aluminum electrodes on glass substrates. Capacitors made by solvent spin coating contain 0{\%} to 50{\%} nanoparticles by weight and have thickness of approximately 100 nm. Dielectric studies examine the relationship between capacitance and electric field, capacitance and temperature, and the pyroelectric response. This work is supported by the Office of Naval Research and the Nebraska Research Initiative.   

Fabrication of Patterned Mesoporous Silica Films Templated From Chemically Amplified Block Copolymers

Mesoporous metal oxide films have been the subject of intense research, for numerous applications including sensors, microfluidics, microelectronics, optoelectronics, microelectromechanical systems and catalysis. Many applications require precise patterning of the films to incorporate device features necessary for intended application. Here, patterned mesoporous silica films are obtained by performing domain selective condensation of precursors within self-assembled block copolymer templates by using supercritical CO$_{2}$ as a delivery medium. The domain selectivity is imparted by the segregation of acid catalyst into hydrophilic domains. Further, by using a photo acid generator, the presence of acid within the film can be controlled spatially via photolithography. Thus patterns at two different length scales i.e., at nanoscale from self-assembled block copolymer and microscale from photolithography can be generated simultaneously. Chemically amplifiable polymers, including poly (tertiary-butoxy carbonyloxy styrene-b-styrene), were used as block copolymer templates. Triphenyl sulfonium triflate was used as a photo acid generator.   

Metal nanocrystals incorporated within pH-responsive microgel particles

Cross-linked latexes of approximately 250 nm in diameter are synthesized by emulsion polymerization of 2-(diethylamino)ethyl methacrylate using a PEO-based macromonomer as the stabilizer at pH 9. These particles exhibit reversible swelling properties in water by adjusting the solution pH: at low pH they exist as swollen microgels due to protonation of the tertiary amine units whereas deswelling occurs above pH 7. The swollen microgels can be used as nanoreactors for the in situ synthesis of Pt nanoparticles. The effects of the method of Pt nanoparticle formation on the size of the microgel particles are studied by DLS. Polymer-metal interactions are investigated by UV-visible absorption spectroscopy, which confirms that the Pt salt is completely reduced to zero-valent Pt using NaBH$_{4} $. TEM and XRD verify the formation of nanometer-sized Pt nanocrystals within the microgels, which can be used as recoverable colloidal catalyst supports for various organic reactions.   

Self-Assembly of Magnetic Nanoparticles at the Surface and Within Block Copolymer Films

We investigate the self-assembly of magnetic Fe$_{3}$O$_{4}$ nanoparticles in thin films of a symmetric block copolymer of poly(styrene-$b$-methyl methacrylate), PS-$b$-PMMA (75 kg/mol). The Fe$_{3}$O$_{4}$ nanoparticles (4nm) are grafted by poly(methyl methacrylate) (PMMA) (2.7 kg/mol) brushes to improve their compatibility. The weight percent of Fe$_{3}$O$_{4 }$in PS-$b$-PMMA is 1, 4 and 10. The Fe$_{3}$O$_{4 }$reside at the intermaterial dividing surface and also form small disk-like aggregates within the PMMA phase. The addition of Fe$_{3}$O$_{4}$ slows down the transition from perpendicular to parallel lamellae morphology at the surface and slowing down increases as weight percent Fe$_{3}$O$_{4 }$increases. Using cross-sectional TEM, nanoparticles are found to be rejected from the parallel lamellae and gather preferentially within the perpendicular lamellae. These studies demonstrate that the Fe$_{3}$O$_{4}$ particles influence thin film morphology and visa versa. Because of widespread interest in nanodevices, this study shows that arrays of functional nanoparticles can be formed using block copolymer templates.   

Effect of Nanoparticles on the Phase Morphology of Block copolymers.

In this study we show that addition of nanoparticulates to copolymer self-assembling molecular templates causes variations in the phase morphologies. We report the results of the bulk phase behavior of poly(styrene-dimethyl siloxane) (PS-PDMS) and poly(styrene-butadiene-styrene) (SBS) block copolymer systems with inclusion of different percent loadings of 1-50 nm particles of gold and endohedral fullerenes. The copolymer samples (both with and without nanoparticles) have been prepared and characterized using AFM, TEM, SAXS and $^{13}$C-NMR measurements. We present results which show that even at relatively low concentrations nanoparticle inclusions (less than 2 wt./vol.{\%}) the block copolymer phase morphology is altered from that of the native copolymer.   

Limitations of electric field assisted patterning of nanoparticle filled polymers.

Among the many challenges as Polymer Nanocomposites move beyond commodity plastic applications, hierarchical morphology control is paramount as random arrangements of nanoparticles (NPs) will not provide optimized electrical, thermal or optical performance for many potential high-tech applications. Two general approaches to this challenge are emerging, i.e.: external-in (directed patterning of NP dispersions) and internal-out (mesophase assembly of NPs). To better understand limitations of electric field assisted patterning of NPs, the impact of inherent (particle shape, matrix viscosity, complex dielectric properties of the constituents) and external (temperature, frequency and magnitude of applied field) parameters on the torque generated on an isolated NP in a uniform electric field is calculated. An applied electric field induces a dipole in a dielectric particle when it has different permittivity from that of the media. When the particle shape is anisotropic, orientation of the induced dipole does not necessarily coincide with the applied field. The tendency of the dipole to align along the field causes a torque for the particle to rotate. Similarly, conductivity difference of particle and medium causes charges to build up at the interface. This free charge dipole can also generate a torque. These theoretical predictions are compared to experimental observations.   

Separation of Ionic Solutes Using Nanoparticle-Crosslinked Polymer Hydrogels

Polymer hydrogels are usually made by crosslinking a monomer such as N-isopropylacrylamide (NIPAAm) with a multifunctional crosslinker. Recently, gels have been shown to be formed even in the absence of monomer by using clay nanoparticles as crosslinkers. These particle-crosslinked gels tend to have larger pore sizes and higher gel strengths compared to conventional NIPAAm gels. In this talk, we will show that particle-crosslinked gels are also suited for use as separation matrices. In particular, we will describe the extraordinary ability of these gels to soak up a cationic solute from a solution. We speculate that cationic molecules can be adsorbed on the anionic surface of the clay platelets inside the gel, akin to a process of ion exchange. An additional unique property of these gels is that they can be disassembled in the presence of organic solvents -- due to the non-covalent interaction between polymer and particles (there is no counterpart for this behavior in conventional covalently-linked gels). By exploiting this property, cationic solutes adsorbed on the particles within our gel can be released and recovered.   

Molecular Dynamics Simulations of Cubic Phases in Pluronics Systems and Their Role in Templating Nanoparticles

We discuss molecular dynamics simulations aimed at predicting phase diagrams in Pluronic systems. Crystalline phases with cubic symmetries are particularly challenging to simulate. A general method that is able to obtain these phases is presented. As an example, we show our results for a system of ABA triblock polymers where each hydrophilic A block contains 10 beads and the hydrophobic block B contains 7 beads. These values match the ratio of PEO to PPO in Pluronic F127. Numerous simulation runs are carried out with differing initial conditions, which consistently produce textbook bcc and fcc lattices of micelles along with two other distorted bcc lattices. We find that the formation of a lattice is sensitive to the system's preparation and depends mainly on the kinetic temperature and equilibration time. Examination of the distorted lattices shows that they are related to the finite size of the simulation box. We conclude with some discussion on using these crystals as a template for nanoparticles or biomineralization.   

Session J26: Focus Session: Non-adiabatic Molecular Dynamics and Control at Conical Intersections II

Aspects of conical intersections: Dynamics, bound states embedded in the continuum and short-lived electronic states

Conical intersections are omnipresent in polyatomic molecules and their presence gives rise to the most severe breakdown of the Born-Oppenheimer approximation. Several general aspects of conical intersections and of the dynamics through them will be addressed. Particular attention will be paid to the question what happens to the potential energy surfaces if the electronic states are metastable. In addition, it is shown that nuclear dynamics on coupled potential surface can lead to bound states embedded in the continuum. Non-Born-Oppenheimer effects are responsible for the binding of these states. Once the Born-Oppenheimer approximation is introduced, these states at best become resonances which decay via potential tunnelling. The tunnelling is completely suppressed by the coupling between the electronic states. Another important issue which will be touched upon is dynamics in the presence of conical intersections in macrosystems. Here, the number of modes is extremely large and, nevertheless, their impact close to the intersections cannot be neglected. It is shown that effective modes can be derived which reproduce exactly the short-time dynamics of the whole macrosystem at low cost. Numerical examples are given. \newline \newline References: \newline \textit{H. K\"{o}ppel, W. Domcke and L.S. Cederbaum, Adv.Chem.Phys. }\underline {\textit{57}}\textit{, 59 (1984)} \newline \textit{G.A. Worth and L.S. Cederbaum, Annu-Rev.Phys.Chem. }\underline {\textit{55}}\textit{, 127 (2004)} \newline \textit{L.S. Cederbaum, R.S. Friedman, V.M Ryaboy and N. Moiseyev,} \newline \textit{Phys.Rev.Lett. }\underline {\textit{90}}\textit{, 013001 (2003)} \newline \textit{S. Feuerbacher, T. Sommerfeld and L.S. Cederbaum, J.Chem.Phys. }\underline {\textit{120}}\textit{, 3201 (2004)} \newline \textit{L.S. Cederbaum, E. Gindensperger and I. Burghardt, Phys.Rev.Lett. }\underline {\textit{94}}\textit{, 113003 (2005)}   

Semiclassical Description of Non-Adiabatic Dynamics - Part I

Molecular dynamics simulations of systems that involve non-adiabatic transitions has always been challenging as this involves following the coupling between quantum states of the system. The behavior of such systems can best be modeled by making sure that the method used can not only provide a way to incorporate quantum effects, but can also ensures the equivalent treatment of the electronic and nuclear degrees of freedom. We use the classical electron model (Meyer-Miller) to do the latter ; while time evolution using the semiclassical initial value representation (SC-IVR) ensures the inclusion of quantum effects. We are currently studying the viability of this approach with a few test systems. In order to further study the effectiveness of this approach, we are working on several variations of the SC-IVR. For instance, the Forward Backward IVR (FB-IVR) is a variation of the SC-IVR that presents a simplified formulation for correlation functions with a double propagator. The Linearized IVR (LSC-IVR) is yet another variation which results in a more `classical' formulation of the problem. Our observation and results obtained will be presented.   

Semiclassical description of non-adiabatic dynamics - Part II

Semiclassical IVR is a classical trajectory based method that is used to incorporate quantum effects into classical MD simulations. This, in combination with the classical electron analog model of Meyer and Miller allows us to describe the dynamics on coupled electronic states. The classical electron analog model treats the relevant electronic and nuclear degrees of freedom of a system on the same footing. This achieves dynamical consistency which mixed quantum-classical approaches tend to lack. In joint work with Ananth and Miller, we are studying different IVR approaches to describe non-adiabatic dynamics. One such formulation is ``The Exact Forward-Backward IVR''; this has no other approximation other than the semiclassical description of quantum dynamics. Applications of this approach to various model problems will be presented.   

Non-adiabatic effects in photoelectron spectroscopy

Recent developments in the construction of approximately diabatic second-order Hamiltonians in the vicinity of conical intersections have been employed to study photoelectron spectra of molecules in which nonadiabatic effects are preeminent. Our current approach explicitly includes all non-adiabatic coupling terms through second order, while requiring ab initio data at only (N[int] + 3) or (N[int] + 15) points for two and three-state intersections, respectively, where N[int] is the number of internal coordinates. This scaling allows very accurate wave functions to be used. Since the Hamiltonian is determined at a point of conical intersection, the method is ``self-policing'' in that the ability of the resultant surfaces to reproduce the vicinity of seams of intersection, as well as energy minima and the Franck-Condon region, is easily verified. We will report photoelectron spectra determined from these diabatic representations employing a harmonic oscillator basis and a Lanczos solver algorithm to diagonalize the resultant vibronic Hamiltonian matrices. The results of some initial applications will be discussed, with emphasis on the previously studied five membered heterogeneous ring systems, pyrazolyl (C3H3N2) and pyrrolyl (C4H4N) doublet radicals. These systems are of particular interest since they display low-lying conical intersections adjacent to both the neutral ground state geometries and the Franck-Condon region.   

Fast and accurate self-interaction-free methods for calculating electronic excitations

Time-dependent density functional theory methods achieve both success and disaster in describing electronic excitations in molecules. Most disasters, such as catastrophic failures for charge-transfer excitations, arise due to self-interaction errors. In this talk, I discuss recent progress on the development of self-interaction free methods for calculating electronic excitations. These methods are based on low-order many-body theory, using auxiliary basis expansions to obtain high computational efficiency. To obtain satisfactory accuracy from non-self-consistent treatment of electron correlations, the use of one or two empirical parameters is explored to scale same-spin and opposite-spin correlations. Using opposite-spin terms only yields a reliable and efficient method which is applicable to molecules in the 100 atom regime, with asymptotically fourth order scaling of computation with molecular size. While calibrated for good performance in the Franck-Condon region, the prospects for extension to conical intersections will be briefly mentioned.   

Using quantum dynamics simulations to understand motion around a conical intersection

Quantum dynamics simulations provide a key support in understanding laser spectroscopy measurements. To do this, a model must first be able to reproduce, or be associated with, an observation. The model can then be analysed to provide a picture at the molecular level. Unfortunately the wavepacket propagation methods used in many quantum dynamics calculations are unable to treat more than a few degrees of freedom: a major bottleneck in photochemical systems where the dynamics is dominated by internal conversion through a conical intersection. The multi-configuration time-dependent Hartree (MCTDH) method is one approach that has been very successful in accurately treating non-adiabatic polyatomic systems. Combined with the vibronic coupling model Hamiltonian we have been able to study in detail the dynamics of a number of molecules as they pass through a conical intersection. Recent work, to be covered in this talk, focuses on the complex photochemistry of benzene, showing how a time-resolved photo-electron spectrum can be calculated and interpreted in terms of the underlying molecular dynamics.   

Direct dynamics using variational Gaussian wavepackets. Application to the intelligent control of benzene photochemistry

The direct dynamics variational multi-configuration Gaussian wavepacket (DD-vMCG) method is based on the multi-configuration time-dependent Hartree (MCTDH) algorithm. It uses a time-dependent basis set of parameterised Gaussian functions, which are coupled so as to variationally provide the best possible representation of the wavepacket. This approach is designed to treat quantum effects in large molecules with on-the-fly calculation of the potential energy surface performed by an interfaced quantum chemistry program. Here, we apply this method to the study of the non-adiabatic photochemistry of benzene. Our aim is to rationalise how the way the wavepacket crosses the $S_{1}$/$S_{0}$ seam may modify the branching ratio Dewar benzene : benzvalene and enhance their production rather than non-radiative decay back to benzene. This study is intended to identify realistic non-radiative decay pathways that lead to alternative photochemical reactivity and to find corresponding targets that can be reached by optimal control experiments.   

Towards Modeling Coherent Control in Ab Initio Multiple Spawning Methods

Ab initio multiple spawning (AIMS) dynamics has been developed as a method to solve the nuclear and electronic Schr\"{o}dinger equations simultaneously. In this talk, we present new extensions to the AIMS method which allow modeling light absorption with shaped laser pulses for the purposes of achieving coherent control. The new methods are tested on a variety of low-dimensional problems by comparison to numerically exact wavepacket dynamics.   

Session J27: Focus Session: DNA Translocation / Nanopores - Experiments

A tunable DNA spring in a nanochannel

dsDNA becomes linearized when it is confined to nanofluidic channels with a cross-section of (100 nm)$^2$ or less, which has made them interesting for genomic DNA analyses. DNA is typically manipulated by means of electric fields. We have found that DNA undergoes a phase transition to a condensed state if an a.c. electric field is applied along the channel direction. The molecule collapses to about 1/4 of it's initial contour length. We will discuss how the effect depends on parameters such as frequency, field strength, channel dimensions, and will discuss the origin of the effect. Interestingly, DNA behaves like an artifical muscle that can be triggered by an a.c. electric field. Since the interaction is expected to hold for any solubilized polyelectrolyte, we speculate that the mechanism may lead to a new class of polymer-based mechanical actuators. These would not suffer from depolarization like piezo transducers.   

dsDNA and nanobubble studies using solid-state nanopores

DNA transport through fabricated solid-state nanopores is studied at various salt concentrations. dsDNA translocation at 1M KCl results in current blokkades, whereas by contrast current enhancements are observed at low salt concentrations. These current changes can be understood by taking both the volume and the counter ions of the molecule into account. Nanopore conductance and noise is studied as a nanopore is moved through a laser beam. The resulting conductance profiles show strong variations in the magnitude of the conductance and the low-frequency noise. In addition, we measure an unexpected double-peak conductance profile. A nanometer-sized gaseous bubble (nanobubble) explains this profile. Our data suggest that such nanobubbles act as the dominant source of low-frequency noise and conductance variability. Currently, translocation of RecA-coated DNA is employed to detect local protein structures and test translocation models. We will report on the latest status of these experiments.   

Protein translocating as unfolded chains through solid-state nanopores

We have detected translocation of the protein shrimp alkaline phosphatase (SAP) through a solid-state nanopore. The nanopores were fabricated in a silicon nitride membrane using a highly focused electron beam in a transmission electron microscope. Once formed, the nanopore was wet with an electrolytic solution and current was driven through it by application of an electric potential. When introduced to the negative side of the nanopore, the negatively charged SAP produced current blockages as the protein molecules were driven through the pore by the electric field. No current blockages occurred when protein had not been added to the electrolytic solution nor when polarity of the applied electric field was reversed. Furthermore, this globular protein does not appear to translocate as a sphere as might be expected, but rather goes through as an unfolded chain. Our current blockage events are similar to signals produced by lambda DNA translocating through a nanopore significantly larger than the DNA's diameter. This has implications for future experiments using nanopores to probe proteins.   

Fabrication of sealed nanofluidic channels with single wall carbon nanotube electrodes for electronic DNA detection and analysis

Detection of entropically elongated polymer molecules such as DNA in nanotubes by electronic means is a challenging task. SWCNT's are attractive nanoelectrode detection elements but cannot withstand many nanofabrication techniques commonly used in making nanochannels, such as dry etching. We have used near room temperature parylene deposition to create self-sealed nanochannels which pass over SWCNTs on the substrate surface. The process is totally e-beam compatible, and therefore allows us great flexibility in addressing problems and opportunities in nanoscale electronics. We will demonstrate applications such as electronic length measurement of elongated dsDNA molecules in the sealed nanochannels.   

Capturing and Expulsion Processes of DNA Translocations in Solid-State Nanopores

We study the DNA translocation dynamics through voltage biased solid-state nanopores. Our study examines the capturing and expulsion process of translocation events at various conditions, and compares them to artificial events. For events with translocation time on the order of 100 $\mu $s a significant portion of the translocation event corresponds to the transitory process of the DNA entering and exiting the nanopore, which is normally included in the overall translocation time. Our study reveals that DNA enter the nanopore with a higher speed than on exit. The limitations of the electronic response of the measurement system will also be discussed.   

Manipulating DNA molecules inside nanopores using magnetic tweezers

There has been intense interest recently in using solid-state nanopores for DNA sequencing. A key to this goal is to develop the capability to control the motion or translocation of DNA molecules through the pore. Magnetic tweezers provide the possibility for manipulating multiple DNA molecules through addressable nanopore arrays. We will report our experimental design as well as the preliminary results on manipulating DNA molecules inside nanopores using magnetic tweezers.   

Controlling DNA translocation through nanopores using optical tweezers

One of the key questions regarding DNA translocation studies is the ultimate limit to the spatial resolution of using ionic conductance measurement. We propose a method to improve on the spatial resolution by holding DNA under tension during translocation using optical tweezers. We will discuss the experimental setup and preliminary results.   

Metastability and capillary condensation hysteresis in nearly ideal cylindrical alumina nanopores

Nanoporous materials can be used as chemical and biological sensors. Anodized alumina, in which ordered cylindrical nanopores can be tuned in size, is a nearly ideal system to study gas adsorption and capillary condensation occurring in mesopores. Porous alumina with tunable pore diameters in the 10 to 60 nm range and a narrow distribution ($<$20{\%}) were dosed with several organic vapors. Capillary \textbf{\textit{evaporation}} occurs at equilibrium pressure for all pore sizes and gases, as predicted by the Kelvin equation. On the other hand, capillary \textbf{\textit{condensation}} occurs within a range of metastability of the gas phase, in agreement with theoretical models. Such a hysteresis in the condensation-evaporation process is a signature of metastability and depends on the gas adsorbed. Isopropanol (with stronger surface interactions) always condenses at the same pressure, whereas for toluene (with weaker interactions), the condensation pressure is less reproducible.   

Natural Gas Storage on Nanoporous Carbon.

Powdered and monolithic activated carbons have been made that have a large methane storage capacity (Alliance for Collaborative Research in Alternative Fuel Technology, http://all-craft.missouri.edu). The current best performer stores 115-119 grams methane per liter carbon at ambient temperature and 34 bar, compared to the DOE target of 118 g/L. Results are reported for the structure of the pore space (small angle x-ray scattering, nitrogen adsorption isotherms, methane adsorption isotherms, scanning and transmission electron microscopy), the methane binding energy (methane adsorption isotherms), and computer simulations of pore formation (probabilistic cellular automata). Most pores are centered about a width of 1.1 nm. At length scales larger than 100 nm, the samples are surface fractals with fractal dimension 2.4-2.6.   

Anisotropy of photoluminescence from dye molecules and zeolite-dye composites

The dynamics of photoluminescence from dye molecules in solvents and dye-containing zeolite rods were studied using polarized photoluminescence spectroscopy. We used nanoporous zeolites and pyronine dyes as the host and guest materials, respectively. The effects of concentration of dye molecules and zeolite-dye composites in various solvent were studied systematically. The anisotropy value ($\sim $2.8) reached the theoretical value ($\sim $3.0) in a highly viscous solvent (glycerol), whereas the anisotropy value is $\sim $1 in a low viscosity solvent (DMSO). The PL peak also shows a blue-shift in strongly polar solvents. In the case of zeolite-dye composites, we obtained a lower anisotropy value ($\sim $2.2) in glycerol. This result is interpreted in terms of energy transfer from dye molecules inside the zeolite pores to dye molecules on the surface of zeolite crystals. We also prepared a more advanced system, dye-containing zeolite rods in uniform orientations, using pyronine B and Y and zeolite L. The polarized PL spectra from vertically oriented monolayer of zeolite rods containing dye molecules show that the anisotropy ratio is $\sim $9 when the polarization direction of excitation light and the c-axis of zeolite rods are parallel.   

How conductive polymer/nano-conductive filler composites can be?

How conductive can polymer/filler composites be? It was thought the conductivity of composites could be increased by reducing the sizes of the fillers or increasing their aspect ratios, for example, by using carbon nanotubes. Invention of numerous conductive nanomaterials provides opportunity to verify this idea and to achieve higher conductivity. However, the highest conductivity of composites achieved was just a few percents of that of bulk materials of the fillers, regardless whether the filler was silver micron particles, platinum nano particles, carbon nano particles, or carbon nano tubes. The conductivity of filler-based composite is intrinsically limited by the micro-contact between the conductive fillers. Reducing the filler size or increasing aspect ratio did not yield significant improvements in conductivity although percolation may occur earlier.   

Free-volume anomaly in confined glycerol.

Glycerol is a small molecule glass-former which exhibits relatively high viscosity due to its extensive hydrogen bonding. Here we report the first measurements of local free volume and local mobility of glycerol confined in Vycor: a mesoporous silica glass with pores 70 Angstroms in diameter. We find that the lower molecular mobility in confinement (measured here using quasi-elastic neutron scattering) is accompanied by a higher mean free-volume size between molecules (as measured using positron annihilation lifetime spectroscopy). The strong wetting between glycerol and the glass surface appears to perturb the glycerol to such an extent that the normally observed free-volume/mobility relationship is reversed. Previous studies have come to similar conclusions (high glass transition temperature, low density) but this is the first to show that these effects originate locally. This is expected to have significant ramifications for the study of hydrogen-bonding liquids in confinement, for example water -- a topic of much current interest due to its application in hydration water in biological material.   

Induced Thermal Dynamics in Aerosil Dispersed Glass Forming Liquid

A high-resolution calorimetric spectroscopy study has been performed on pure glycerol and colloidal dispersions of an aerosil gel in glycerol covering a wide range of temperatures from $300$ to $380$~K, deep in the liquid phase of glycerol. The colloidal glycerol+aerosil samples with $0.07$, $0.14$, and $0.32 $~grams of silica per cm$^{3}$ of glycerol reveal activated energy (thermal) dynamics at temperatures well above the $T_g$ of the pure glycerol. The onset of these dynamics appears to be due to the frustration or pinning imposed by the silica gel on the glycerol liquid. Since this behavior occurs at relatively low silica density (large mean-void length compared to the size of a glycerol molecule), this induced dynamics is likely due to a cooperative mode of glycerol molecules with the aerosil gel via mutual hydrogen-bonding. However, the exact nature of these energy dynamics is not known. The study of such frustrated colloids may provide a unique avenue for illuminating the physics of glasses.   

Session J28: Focus Session: Carbon Nanotube Optics III

Resonance Raman of Single-Wall Carbon Nanotubes

The use of resonance Raman spectroscopy to study and characterize single-wall carbon nanotubes (SWNTs) will be discussed. The achievements and limitations of the technique for metrology purposes will be presented, addressing the importance of the excitonic nature of the optical transitions. We use the technique do understand the effect of carbon nanotube doping. The efforts to extend the Kataura plot to larger tube diameters and higher optical transitions not only extend our characterization capability, but also sheds light into the nature of the optically active levels. Experimental results that have not been predicted by solid state approaches are understood on the basis of quantum chemical calculations. It is also interesting to discuss some results on nano-ribbons and their relations to carbon nanotubes.   

Raman Studies of Exciton-Phonon Coupling in Carbon Nanotubes: Quantitation of Bundled vs. Isolated Behavior

Exciton-phonon and electron-phonon coupling are important for a number of carbon nanotube optical and transport behaviors and have recently drawn attention for their role in chirality- dependent intensities observed in radial breathing mode (RBM) Raman spectra. Given the importance of these effects, there is a need to quantitate the magnitude of the exciton-phonon coupling. We present a Raman transform analysis of RBM fundamental and overtone intensities that yield the magnitude of coupling for five specific nanotube chiralities. These results agree with values predicted through quantum chemical calculations and indicate that non-Condon effects may be important in describing nanotube transitions. We extend the analysis of the coupling to bundled nanotube samples and find it decreases significantly in these sample types. We also discuss the coupling behavior of a new class of intermediate frequency modes (IFMs) that display step-wise dispersive behavior. These IFMs are associated with coupling between the E11 and E22 transitions. Bundling is found to increase the coupling observed for these modes.   

Exciton-phonon interaction and Raman intensity of carbon nanotubes

Using extended tight binding framework, the exciton states and exciton-phonon interaction are calculated for understanding optical properties of single wall carbon nanotubes. Resonance Raman intensity for first and second order Raman processes are calculated as a function of $(n,m)$ with use of exciton wavefunctions. Chirality, type and diameter dependence of Raman intensity is now fully given. In particular, the dark exciton plays an important role for second-order, intervalley, resonance Raman processes. Although the exciton-phonon interaction is not so different from the electron-phonon interaction, the optical absorption (emission) is enhanced significantly by the localized exciton wavefunctions.\\ \ \\ References: J. Jiang et al, Phys. Rev. B, in press.   

Tunable Electron-Phonon Coupling in Isolated Metallic Carbon Nanotubes Observed by Raman Scattering

Metallic single-walled carbon nanotubes can exhibit significant broadening of the high-energy (G) mode Raman features. In contrast to narrow Raman widths for semiconducting nanotubes, full widths in excess of 50/cm are commonly observed in metallic nanotubes. Different possible physical origins have been proposed in previous literatures. In this paper, we demonstrate the ability to modify the Raman linewidth by electrostatic gating. Using measurements of individual suspended nanotubes, we find that either a positive or negative shift in the Fermi energy by an applied electrostatic field can reduce the linewidth by more than a factor of two. The results can be understood in terms of blocking vertical electronic transitions (electron-hole pair generation) possible for the zone-center phonons in an unperturbed nanotube, but not in a nanotube with a sufficiently shifted Fermi level. A simple model is presented to explain the experimental results.   

Ultrafast Spectroscopy of Phonons in Single-Walled Carbon Nanotubes

Recently, we observed coherent phonons (CPs) of the radial breathing mode (RBMs) in semiconducting single-walled carbon nanotubes (SWNTs) suspended as individuals in aqueous surfactant (1). We demonstrated CP spectroscopy as a powerful method for determining phonon and exciton energies in an ensemble of SWNTs with different chiralities. Here, we extend these ultrafast optical studies on various types of nanotube samples including films and solutions. In order to provide new insight into CP decay mechanisms, we systematically investigated the temperature dependence of CP amplitude, frequency, and lifetime from 4 -300 K while changing the pump/probe photon energy. We also investigated how bundling affects CP line widths. Furthermore, we compared the intensity dependence of CPs resonant with the $E_{11}$ and $E_{22}$ transitions by studying the excitation profile for specific RBMs, focusing particularly on the excitation line width and shape. 1) Y. S. Lim \textit{et al}., Nano Letters, published electronically November 2, 2006.   

Theory of coherent phonons in carbon nanotubes

We develop a general theory for the generation of coherent phonons in single wall carbon nanotubes or arbitrary chirality. Coherent phonons are generated in the nanotube via the deformation potential electron-phonon interaction with photogenerated carriers. In our theory the electronic states are treated in a third nearest neighbor tight binding formalism which gives a good description of the states over the entire nanotube Brillouin zone while the nanotube phonon states are treated in a valence force field model that includes bond-stretching, in-plane and out-of-plane bond-bending, and bond-twisting interactions. In the tight-binding electron-phonon interaction, all two center integrals out to fourth nearest neighbors are retained. The equations of motion for the coherent phonon amplitudes are obtained in a density matrix formalism and we find that the coherent phonon amplitudes satisfy driven oscillator equations for each value of the phonon wavevector. We will discuss excitation strengths for different coherent phonon modes and compare to recent experiments.   

Intrinsic BWF-lineshape Observed by Raman Scattering in Isolated Metallic Carbon Nanotubes

Broadened and asymmetric lineshapes for Raman scattering in the high-energy (or G) modes of metallic carbon nanotubes have been reported for many years. There remains, however, controversy about whether this behavior is an intrinsic feature of metallic nanotubes or is induced by perturbations. To address this issue, we have examined isolated metallic nanotubes suspended in air, with chiral indices determined independently by Rayleigh scattering and Raman measurements of the radial breathing mode. Our data show that strong broadening (to FWHM $>$ 50/cm) and weak asymmetry are typical of the high-energy Raman modes, with lineshapes describable by a Breit-Wigner-Fano (BWF) form. Significant variation in peak width and Raman shift is, however, observed as a function of the nanotube chiral index. Indeed, some metallic nanotubes have lineshapes and widths that are very similar to those of semiconducting nanotubes. We will discuss the observed variation and the origin of the BWF lineshape.   

Bundling and Electronic Effects on the BWF Feature for Doped and Undoped Carbon Single-wall Nanotubes

In this contribution we examine the role of bundling and electronic effects on the Breit-Wigner-Fano (BWF) Raman component for dispersions of undoped and boron-doped (p-type) SWNTs in various surfactants. Interestingly, we find that the intensity of the BWF component is sensitive to the degree of SWNT debundling, solution pH, doping level, charge transfer with redox active molecules, and differences in the SWNT-surfactant interactions, all of which lead to varying degrees of charge localization at the nanotube surface. In several cases, we observe a strong BWF component in the metallic Raman spectrum even for dispersions of highly isolated SWNTs. In general, our results, coupled with results from the literature, suggest that the presence and intensity of the BWF feature is sensitive to any changes in the magnitude of dielectric screening, whether from tube-tube interactions in bundles, from charge injection or depletion, or from charge polarization from tube-molecule interactions. These results suggest that, contrary to practice in some recent studies, the existence or lack of a BWF feature should not be used alone as a measure of SWNT aggregation. They also provide information regarding the nature of surfactant-nanotube interactions, SWNT redox chemistry, and nanotube separations.   

First-principles study of resonant Raman spectroscopy in graphite and carbon nanotubes

Resonant Raman spectroscopy is an increasingly used experimental tool for the characterization of carbon nanotubes (CNTs). It explores the coupling of optical, electronic, and vibrational modes in these quasi-one-dimensional systems. Using first-principles methods we calculate the electron-photon and -phonon matrix elements necessary to estimate the first-order Raman cross-section. For graphite, the non-interacting quasiparticle spectrum is sufficient, however, for CNTs, the excitonic spectrum and wave functions require an accurate description of electron-hole correlation. We calculate excitonic effects by solving the Bethe-Salpeter equation, using as input the quasiparticle spectrum obtained within the GW approximation to the electron self-energy. We analyze the exciton-phonon coupling in CNTs and its impact on the resonant Raman cross-section.   

Signature of the electron-phonon interaction in the electron spectral function of graphene

The spectral function of graphene has been measured with high energy and momentum resolution by angle-resolved photoelectron spectroscopy. It has been proposed that the measured spectral function exhibits combined signatures from electron-phonon, electron-electron, and electron-plasmon interactions. We here present a first-principle investigation of the contribution to the electron self-energy of graphene arising from the electron-phonon interaction. We compute the electron self-energy treating the graphene bandstructure within density functional theory, the lattice dynamics within density functional perturbation theory, and the electron-phonon interaction within the Migdal approximation. Due to its peculiar cone-shaped bandstructure, the electron-phonon contribution to the electron self-energy of graphene shows qualitative differences as compared to the case of ordinary bulk metals.   

1st and 2nd order Raman scattering from n-Graphene Layer (nGL) Films on Silicon Substrates.

Results of room temperature Raman scattering experiments on graphene and n-graphene layer films (nGLs) will be presented [1]. We find that the G band at $\sim $ 1582 cm-1 exhibits an interesting upshift in frequency with 1/n which we tentatively assign to a surface strain phenomenon connected with surface roughness of the substrate and compensated by the increase in stiffness of the nGL with increasing n. Interesting n-specific bands are observed in the $\sim $1350 cm-1 (or D-band) region which may correlate with deviations from planarity of the nGL. The second order scattering is very interesting and for small n (n$<$4) the (2D' or G') band intensity at $\sim $ 2700 cm-1 is actually higher than the first-order G-band scattering. The shape of this band is sensitive to n and thus can be used to identify n without an AFM measurement. Whereas, the 2D' band is sensitive to n, the 2nd order 2G band $\sim $ 3248 cm-1 is independent of n. These observations will be discussed in terms of the phonon and electronic dispersion of nGLs. 1 A. Gupta, G. Chen, P. Joshi, S. Tadigadapa and P.C. Eklund, `` Raman Scattering from High-Frequency Phonons in Supported n-Graphene Layer Films'' NanoLett (in Press).   

Raman Scattering Study of the Graphene-Substrate Interaction.

We report on Raman scattering studies of graphene and few graphene layer films (i.e., $n$GLs, where $n $is the number of graphene layers in the film). $n$GL films (n=1-3, 25) were prepared by mechanical transfer from HOPG to various substrates (SiO2:Si, Au, Ag, cleaved Mica, and free-standing films). For metallic substrates we observed a clear G-band frequency downshift relative to that observed when the $n$GL is on SiO2:Si. This downshift is interpreted in terms of a chemical charge transfer of electrons from the metallic substrate to the nGL. Interestingly, the position and shape of the 2D' (or G') band at $\sim $ 2700 cm$^{-1}$ is found insensitive to the substrate interaction.   

Raman Spectroscopy of single and double layer graphene

Dirac fermions and novel quantum Hall effects. Graphene also holds the promise of one day replacing silicon in microchips. Early Raman scattering has identified basic features of the G-band and D-band, where the former shows intensity dependence associated with addition of single layers, and the later displays significant intensity only for the single and double layer systems. We have performed room temperature Raman scattering with a spatial resolution of 0.5 microns consistent with this work. More recently, Pinczuk and Kim and co-workers have shown low-temperature Raman scattering that displays evidence of electron density dependent screening. We investigate the interlayer hopping with temperature-dependent Raman scattering and in our low-temperature Raman, we specifically investigate the novel coupling and edge states predicted by Castro Neto and co-workers.   

Session J29: Focus Session: Granular Flows I

Lateral stripe state in rapid granular flow on an inclined plane

Recently longitudinal vortices were reported in a rapid, dilute flow of sand down a rough inclined plane [1]. We present experimental results, showing that a robust stripe state develops at moderate plane inclinations in denser flows, with a structure substantially different from the one observed in dilute flows. We characterize this new type of stripes by measuring velocity profiles, height profiles, light transmission, and average density of the flow. As opposed to the stripes observed in the dilute regime, here the fast moving region corresponds to the maximum of the height profile. The stripe state is detected in the flow of various materials such as sand of different sizes, glass beads of different sizes, and copper particles of various shapes. We show that by increasing plane inclination we get back the dilute regime and the previously reported stripe structure. For sand particles with the diameter of d=0.4 mm the flow properties were extensively measured at six downstream locations. For this case we find an explicit correspondence between the accelerating nature of the flow and the formation of stripes in the dense regime. \vskip 5pt\noindent [1] Y. Forterre and O. Pouliquen, Phys. Rev. Lett. 86, 5886 (2001).   

Universality of granular impact dynamics

We dropped projectiles into granular media from various heights, and measured the dynamics by an optical method with 100 nanometer and 20 microsecond resolution. Data were obtained for 11 different projectiles (including cylinder as well as spheres) and 4 different granular media. The results can all be explained by a stopping force consisting of the sum of two terms: an inertial drag, proportional to velocity squared and independent of depth, and a frictional drag, proportional to depth and independent of speed. The latter scales as the square-root of projectile density and hence is not simply Coulomb friction. We also demonstrate that this stopping force law can explain seemingly-contradictory penetration and dynamics data reported by other researchers.   

The Liquid Nature of a Granular Jet Hitting a Fixed Target

We perform the granular analog to the 'water bell' experiment [1]. A column of dry spherical glass beads is accelerated by pressurized air through a glass tube to form a high-speed granular jet. When this jet collides with a stationary target disc, we observe the formation of granular sheets and cones enveloping the target similar to those seen when water jets hit a target and subsequently form water bells. The opening angle of the cones is measured as a function of the speed and diameter of the initial granular column and the diameter of the target disc. Under these conditions, dry granular material behaves similarly to a fluid with zero surface tension, i.e., a fluid with infinite Weber number. By decreasing the flux and increasing the size of the granular particles, we observe that the structure formed by the jet becomes more diffuse and the dynamics changes as the particulate nature of the material becomes more apparent. Furthermore, we measure the force impulse exerted on the target during the collision and relate it to the granular ripples formed on the thin ejected granular sheet. [1] C. Clanet, J. Fluid Mech. 430, 111 (2001).   

Rapid Granular Flows: From Kinetic Theory to Hydrodynamics

Rapid granular flows are defined as flows in which the time scales for the particle interactions are small compared to the inverse of the strain rate, so that the particle interactions can be treated as instantaneous collisions. We first show, using Discrete Element simulations, that even very dense flows of sand or glass beads with volume fraction between $0.5$ and $0.6$ are rapid granular flows. Since collisions are instantaneous, a kinetic theory approach for the constitutive relations is most appropriate, and we present kinetic theory results for different microscopic models for particle interaction. The significant difference between granular flows and normal fluids is that energy is not conserved in a granular flow. The differences in the hydrodynamic modes caused by the non-conserved nature of energy are discussed. Going beyond the Boltzmann equation, the effect of correlations is studied using the ring kinetic approximation, and it is shown that the divergences in the viscometric coefficients, which are present for elastic fluids, are not present for granular flows because energy is not conserved. The hydrodynamic model is applied to the flow down an inclined plane. Since energy is not a conserved variable, the hydrodynamic fields in the bulk of a granular flow are obtained from the mass and momentum conservation equations alone. Energy becomes a relevant variable only in thin `boundary layers' at the boundaries of the flow where there is a balance between the rates of conduction and dissipation. We show that such a hydrodynamic model can predict the salient features of a chute flow, including the flow initiation when the angle of inclination is increased above the `friction angle', the striking lack of observable variation of the volume fraction with height, the observation of a steady flow only for certain restitution coefficients, and the density variations in the boundary layers.   

Kinetic theory of hydrodynamic response functions for a dense granular fluid

The general response functions characterizing the response of a homogeneous isolated granular fluid to small spatial perturbations in the hydrodynamic fields have been described recently [1]. These response functions are time correlation functions for the Homogeneous Cooling State. Special cases of this class of time correlation functions are the Green - Kubo expressions for the hydrodynamic transport coefficients. In this work, these functions are expressed in terms of reduced singe particle functions that are expected to obey a linear kinetic equation. The functional assumption required to obtain such a kinetic equation and its relationship to the well studied Boltzmann and Enskog kinetic theories of a granular fluid are illustrated in the particular context of the shear viscosity of this fluid. [1] J. W. Dufty, A. Baskaran and J. J. Brey, J. Stat. Mech. L08002 (2006).   

Velocity correlations in dense granular flows observed with internal imaging

We measure the velocity fluctuations in uniform dense granular flows inside a silo using a fluorescent refractive index matched interstitial fluid. The measurements are made in the uniform plug flow region where the flow is dominated by grains in enduring contacts and fluctuations scale with the distance traveled, independent of flow rate. The distributions of the horizontal and vertical displacements for short time scales show fat tails compared to a Gaussian indicating large fluctuations in particle displacements and possible cage breaking. The mean square displacements show an inflection point supporting the presence of caging dynamics. The velocity autocorrelation function of the grains in the bulk shows a negative correlation at short time and slow oscillatory decay to zero similar to simple dense liquids. Weak spatial velocity correlations are observed in the bulk over several grain diameters. The observed correlations are qualitatively different at the boundaries where significant structural ordering in the flowing granular layer is observed.   

Lubrication forces in dense granular flow with interstitial fluid: A simulation study with Discrete Element Method

Using three-dimensional molecular dynamics simulations, we study steady gravity-driven flows of frictional inelastic spheres of diameter $d$ and density $\rho _g$ down an incline, interacting through two-body lubrication forces in addition to granular contact forces. Scaling arguments suggest that, in 3D, these forces constitute the dominant perturbation of an interstitial fluid for small Reynolds number \textit{Re} and low fluid density$\rho$. Two important parameters that characterize the strength of the lubrication forces are fluid viscosity and grain roughness. We observe that incline flows with lubrication forces exhibit a packing density that \textit{decreases} with increasing distance from the surface. As the incline angle is increased, this results in a severely dilated basal layer that looks like ``hydroplaning'' similar to that observed in geological subaqueous debris flows. This is surprising since the model explicitly disallows any buildup of fluid pressure in the base of the flow, and suggests that hydroplaning might have other contributing factors besides this traditional explanation. The local packing density is still determined by the dimensionless strain rate $I\equiv \dot {\gamma }{\kern 1pt}d\sqrt {\rho _g /p} $, where $p$ is the average normal stress, obeying a ``dilatancy law'' similar to dry granular flows.   

Geometrical Mechanism for Solid-Fluid transition in a Granular system

We report an experimental investigation of the geometrical mechanism for solid-fluid transition in a quasi-two dimensional granular system. We demonstrate the presence of geometrical structures resembling plane tilings composed of squares and equilateral triangles in our quasi-2D granular fluid. We further show that this tiling structure manifests itself in distinct features in the bond-length and bond-angle distribution functions. These experimental observations coupled with a number of previously reported theoretical and simulation studies strongly support the proposed square-triangle tiling mechanism for 2D melting. These findings present a possible way to explain the observed phase transitions in non-equilibrium granular systems using entropic-like arguments similar to those used for equilibrium hard sphere/disk systems.   

Swirling Motion in the System of Vibrated Elongated Particles

We study large-scale collective motion emerging in a monolayer of vertically vibrated elongated particles. The motion is characterized by recurring swirls with the characteristic scale exceeding several times the size of individual particle. Our experiments identified small horizontal component of the oscillatory acceleration of the vibrating plate in a combination with orientation-dependent bottom friction as a source for the swirls formation. We developed a continuum model operating with velocity field and local alignment tensor which is in a qualitative agreement with the experiment.   

Angle of Repose of Small, Conducting and Non-Conducting Plates

We have investigated the behavior of granular collections consisting of laser-cut shapes from conducting and non-conducting paper, with various cross-sectional shapes (square, rectangular, triangular, circular) and in several sizes and aspect ratios. In particular we have measured the Angle of Repose of piles consisting of large numbers of these particles. While the shape of these particles would suggest that these should behave as thin plates, making quite shallow piles, instead we find that the piles are not shallow, and that the piling is remarkably robust to external disturbances. We will compare our results for various types of materials in various shapes, and also compare these results with what we have observed for larger, symmetric particles.   

Large scale surface flow generation in driven suspensions of magnetic microparticles: Experiment, theoretical model and simulations

Nontrivially ordered dynamic self-assembled snake-like structures are formed in an ensemble of magnetic microparticles suspended over a fluid surface and energized by an external alternating magnetic field. Formation and existence of such structures is always accompanied by flows which form vortices. These large-scale vortices can be very fast and are crucial for snake formation/destruction. We introduce theoretical model based on Ginzburg-Landau equation for parametrically excited surface waves coupled to conservation law for particle density and Navier-Stokes equation for water flows. The developed model successfully describes snake generation, accounts for flows and reproduces most experimental results observed.   

The Behavior of Ultrafine Particles in the Absence and Presence of External Fields

Length scales of particles and their surrounding medium strongly determines the nature of their interactions with one another and their responses to external fields. We are interested in systems of ultrafine particles (0.1 - 1.0 micron) such as volcanic ash, solid aerosols, or fine powders for pharmaceutical ihalation applications. We develop a numerical model for these systems using the Derjaguin-Muller-Toporov (DMT) adhesion theory along with the van der Waals attraction between the particles and their contact mechanical interactions. We study the dynamics of these systems in the absence and presence of gravity by controlling the particle size, and thereby, the surface properties of the particles. Finally, we explore the response of these systems to external fields by studying the evolution of the internal microstructure under contant load and shear strain.   

Quasi-equilibrium in tapered chains

The approach to equilibrium in 1d lattices is interesting for granular media since temperature is not well-defined and various authors have reported a violation of equipartition. We extend our previous work on shock mitigation in tapered chains to look at energy sharing among spheres and how the system appraoches a so-called quasi-equilibrium. An overlap potential of adjacent particles is used to model the elastic response of spheres under loading and has the form, $V\sim\delta^n$. For spheres, $n=5/2$ and is known as the Hertz potential. We can also compare results when $n=2$ which resembles spring-like behavior. It should be noted however, that in both cases the potential has no restoration term and vanishes when adjacent spheres lose contact. We present the velocity statistics for a variety of Hertzian chain configurations as well as fluctuations for the system's total kinetic energy for both $n=2$ and $n=2.5$. We find that most particles in these systems exhibit Gaussian velocity distributions and that the kinetic energy fluctuations of the system depend strongly on system size and weakly on tapering of the spheres. Fluctuations do not damp out over long time however, indicating that the steady-state is a type of quasi-equilibrium. Mathematical fits of the mean fluctuations are further provided as functions of system size, tapering, and $n$.   

Session J30: Focus Session: Characterizing Spatio-Temporal Complexity in Fluids and Materials

Topological Analysis of Spatial Temporal Patterns

It is fairly easy to collect large amounts of high dimensional data describing the time dependent spatial structures of materials or fluids either through experimentation or numerical simulation. In this talk I will describe how techniques from computational topology can be used to reduce both the size and dimension of the data sets and still provide useful statistics for parameter identification, model selection, and quantification of the spacio-temporal complexity of the dynamics. These ideas will be presented in the context of experimental working involving spiral defect chaos for Rayleigh- Benard convection and numerical simulations of stochastic and deterministic Cahn-Hilliard equations.   

Coarse-grained velocity gradients in turbulence

In a turbulent flow, energy cascades from large length and time scales, where it is injected into the flow, to small scales, where it is dissipated by the action of molecular viscosity. At small scales, this energy dissipation is characterized by the velocity gradient tensor. At larger scales, however, different dynamics must apply. We therefore present measurements of a velocity gradient tensor coarse-grained over inertial-range scales in an intensely turbulent laboratory water flow. We discuss the potential of these coarse-grained gradients as a probe of the scale-to-scale energy transfer in the turbulent cascade and their relation to Large Eddy Simulation. This work was supported both by the National Science Foundation and by the Max Planck Society.   

Turbulent-Laminar Patterns in Shear Flows

We study computationally turbulent-laminar patterns in very-large-aspect-ratio plane Couette flow. These states consist of large-scale alternations of turbulent and laminar flow oriented obliquely to the steamwise direction. Such flow patterns are now believed to be typical of many transitional shear flows when observed on long length scales. For a fixed pattern orientation of $24^{circ}$, suggested by experiment, the basic scenario observed in computations as the Reynolds number is decreased is the following: From uniform turbulence there is a transition to intermittent patterns at $Re\simeq 420$, then to steady, spatially periodic patterns at $Re\simeq 390$. The wavelength increases as the Reynolds number is decreased until $Re\simeq 310$, where the flow consists of localized turbulence within a laminar background. This scenario can depend on pattern orientation -- at $90^{circ}$ with respect to the flow direction, we observe spatio-temporal intermittency in which turbulent patches that repeatedly disappear abruptly and then re-nucleate gradually. We present an analysis of these flows in terms of mean quantities and discuss the difficulties of determining critical bifurcation parameters for such turbulent-laminar systems.   

Turbulence structures and unstable periodic orbits

Recently found unstable time-periodic solutions to the incompressible Navier-Stokes equation are reviewed to discuss their relevance to near-wall turbulence and isotropic turbulence. It is shown that the periodic motion embedded in plane Couette turbulence exhibits a regeneration cycle of near- wall coherent structures, which consists of formation and breakdown of streamwise vortices and low-velocity streaks. In phase space a turbulent state wanders around the corresponding periodic orbit for most of the time, so that the root-mean-squares of velocity fluctuations of the Couette turbulence agree very well with the temporal averages of those along the periodic orbit. The Kolmogorov universal-range energy spectrum is observed for the periodic motion embedded in high- symmetric turbulence at the Taylor-microscale Reynolds number $Re_\lambda=67$. Spatio-temporal structures of the periodic solution in high-symmetric flow are investigated to characterize the dynamics of coherent structures which appear in the energy cascade process.   

Computational Homology in Rayleigh-Benard convection experiments

Computational homology is used to analyze the spiral defect chaos (SDC) state in Rayleigh-Benard convection. Image time series of flows visualized by shadowgraphy are used as input; the homology analysis yields Betti numbers, which counts the number of connected components and holes in the flow patterns. Probability distributions and entropies derived from the Betti number measurements are used for identifying and characterizing different states in the SDC regime.   

State and Parameter Estimation of Spatio-Temporally Chaotic Systems: Application to Rayleigh-Benard Convection

Data assimilation refers to the process of obtaining an estimate of a system's state from a time series of incomplete and noisy measurements along with a model (possibly approximate) for the system's time evolution. Here we demonstrate the applicability of a recently developed data assimilation method, the Local Ensemble Transform Kalman Filter (LETKF), to Rayleigh-Benard convection, a non-linear, high dimensional, spatio-temporally chaotic fluid system. Using this technique we are able to extract the full temperature and velocity fields, including the mean flow, from experimental images of shadowgraphs. In addition, we describe extensions of the algorithm for estimating fluid parameters.   

Geometric Diagnostics of Complex Patterns: Spiral Defect Chaos in Convection

Motivated by the observation of spiral patterns in a wide range of physical, chemical, and biological systems we present an approach that aims at characterizing quantitatively spiral-like elements in complex stripe-like patterns. The approach provides the location of the spiral tip and the size of the spiral arms in terms of their arclength and their winding number. In addition, it yields as topological information the number of pattern components (Betti number of order 1), as well as their size and certain aspects of their shape. We apply the method to spiral defect chaos in thermally driven Rayleigh-B\'enard convection and find that the winding number of the spirals, but not their arclength, is non-monotonic in the heating. The distribution function for the number of spirals is significantly narrower than a Poisson distribution. The distribution function for the winding number decays approximately exponentially. For small Prandtl numbers the analysis reveals a large number of small compact pattern components. Including non-Boussinesq effects, we find that they not only break the up-down symmetry but also strongly increase the number of small, compact convection cells.   

Pattern selection and control via localized feedback

Many theoretical analyses of feedback control of pattern-forming systems assume that feedback is applied at every spatial location, something that is often difficult to accomplish in experiments. We consider an experimentally more feasible scenario where feedback is applied at a sparse array of discrete spatial locations. We use generalized linear stability analysis to determine how dense the actuator array needs to be to select or maintain control of a given pattern state in the presence of noise. The one-dimensional Swift-Hohenberg equation is used to illustrate our theoretical results and explain earlier experimental observations on the control of the Rayleigh-B\'enard convection.   

Structural analysis of particulate suspensions under simple shear flow

A new simulation platform that takes the interaction between fluid and particle has been developed. We analyzed three-dimensional microstructures of repulsive and weakly aggregating suspensions under simple shear flow. Two-dimensional Fourier Transform of the particle images and pair distribution functions were used for microstructure analysis. Particles are well aligned in repulsive suspension while there is an anisotropic configuration of particle clusters in weakly aggregating suspension. We could observe a vorticity-directional motion even in a simple shear flow for aggregating particle suspension, which was recently reported by scattering techniques. Helical motion towards the vorticity direction appears because the flow field is disturbed by the extra stress of the particles. High local shear rate regime is also observed near the fast helical streamlines. This result will provide a clear outlook for the simple shear flow of particulate suspensions.   

Light Propagation in Quasi-Ordered Media

We evaluate the use of light to probe patterns in optical media whose index of refraction is modulated in the direction of propagation over length scales large relative to the optical wavelength. First, we show how a quadratic index waveguide with periodic axial variations induces a parametric instability in the geometric optics limit. A fundamental scaling allows us to examine a wide range of physical conditions and explore nonlinear behavior such as resonance and chaos. Second, we show that a periodic array of cylinders acts as a waveguide and also shows resonances. We consider the possibility for using these results to probe order-disorder phenomena in systems as widely different as fluid flows and living tissues.   

Oscillons and reciprocal oscillons

Formation of spatially localized oscillations in parametrically driven systems is studied, focusing on the dominant 2:1 resonance tongue. Both damped and self-exciting oscillatory media are considered. The forced complex Ginzburg-Landau equation is used to identify two types of such states, small amplitude oscillons and large amplitude reciprocal oscillons resembling holes in an oscillating background. In addition a variety of front-like states with nonmonotonic profiles is described. A systematic analysis of the origin and stability properties of these states is provided. In many regimes all three states are related to the presence of a Maxwell point between finite amplitude spatially homogeneous phase-locked oscillations and the zero state, leading to a large multiplicity of coexisting stable states of different types.   

Session J31: Transport in Carbon Nanotubes: Experiment

Transport in Carbon Nanotube -- Polymer Field Emission Cathodes.

Embedding carbon nanotubes in host polymer matrices is an attractive way to control the nanotube density and provides a way to protect the nanotube emitter. Field emission from individual carbon nanotubes is usually discussed in terms of the field enhancement factor and electrostatic screening. The field enhancement factor can be regarded as the most important factor for efficient emission when transport of electrons is not the rate limiting step. Large area emission characterisation of cathodes tends to produce an ensemble average of the enhancement factor with sites with the lowest local turn on field emitting. We show that in nanotube polymer composites charge transfer through the composite and the effects of fluctuation induced tunneling due to variable nanotube-nanotube separation are important considerations.   

A comparative study of transport properties between low- and high-resistance nanotube field-effect transistors

A large amount of fundamental work has been done in demonstrating the possibilities of using carbon nanotubes in future nanoscale devices. A primary concern in these devices has been the nature of transport mechanism in carbon nanotubes, especially semiconducting nanotubes where the conductance can be modulated significantly using a gate voltage, giving rise to a nanotube field-effect transistor. The quality of the metal-nanotube interface plays a significant role in determining the characteristics of field-effect transistors fabricated using single-wall carbon nanotubes. When contact-resistance is high due to a large Schottky barrier, the transistor characteristics are dominated by this barrier. When the barrier height is low, the intrinsic nanotube electronic properties determine the transistor characteristics. In this work, we compare and contrast the significant features of the transistor characteristics for the two types of devices.   

Transport properties of metallic nanocluster impregnated multi wall carbon nanotubes

Artificially engineered low-dimensional heterostructures form a class of extremely exciting new materials for fundamental and applied research. One such very interesting system is the formation of arrays of metallic nanoclusters confined to one-dimension, within the scaffolding of another one dimensional material. A good example of this kind of a system is metal-nanocluster-impregnated carbon nanotubes. In this work, we report the fabrication of nanoscale heterostructures in the form of ferromagnetic metal nanocluster array impregnated multi wall carbon nanotubes. The nanoclusters can be impregnated into the nanotubes by a simple electrochemical technique. Two and four terminal devices with such individual nanoarchitectures have been fabricated using a combination of photo- and focused ion beam lithography. The systems form extremely exciting platforms for investigating charge and spin transport in confined geometries. We present preliminary data on the electrical properties of these novel systems at room and low temperatures.   

Electronic transport in loops comprised of individual carbon nanotubes

We discuss electronic transport in loops comprised of individual carbon nanotubes. The conductance versus gate voltage shows oscillations with a number of periods. These oscillations persist up to temperatures $\sim $50 K. We compare our results with a model [Dear APS organizers: please note the theoretical talk on the same subject given by Gil Refael. We would like to reference this talk if possible in this abstract.] that accounts \newline for the interference of counterpropagating electron waves around the loop, analogous to a Sagnac interferometer in optics. In this model, the different velocities for right and left movers in the two carbon nanotube bands produce large energy scale interference oscillations. We find semi-quantitiative agreement between our data and the theory. These results may enable phase coherence in nanotubes to be studied up to temperatures much higher than the cutoff imposed by thermal smearing.   

Sagnac interference in Carbon nanotube loops

Some of the most pronounced manifestations of electron interference effects occur in carbon-nanotubes. When a nanotube is made to form a loop, a new large-period mode of interference appears in the conductance measured as a function of gate-voltage or source-drain voltage. The periodicity of the fringes is determined by the velocity detuning between right- and left-moving electrons, thus the nanotube loop forms a Sagnac interferometer which precisely measures this detuning. In our work we explore this effect in strongly interacting Carbon nanotubes, and carefully consider the response to the gate and source-drain voltages.   

Electronic transport across a Carbon nanotube Heterojunction: Experimental observations of high temperature Coulomb Charging and level spacing.

We present electrical transport measurements of individual a single-wall carbon nanotube in which the chiral indices (n, m) are not fixed along the nanotube length. These kinds of structures are known to show strong rectifying behavior in current voltage characteristics. At low temperatures the device essentially behaves like a quantum dot with very high charging energy and level-spacing. These features can be seen till 70K also, making it a high temperature single electron transistor.   

Thermomagnetic Measurements of Transport in Single Walled Carbon Nanotubes

The thermomagnetic transport properties of single walled carbon nanotubes bundles and mats in high magnetic fields have been measured in vacuum and in the presence of noble gases. They are used to determine mechanism responsible for change in thermopower and resistivity in the presence of gases with respect to one measured in high vacuum. The thermopower and its change in a magnetic field is recorded in Ne, Ar, Xe atmospheres. The variation of the zero-field thermopower with the presence of noble gases is consistent with that observed recently [1]. The magnetothermopower in a saturating magnetic field is only 0.2{\%} larger than the zero-field thermopower. As the magnetothermopower in high field is independent of the scattering mechanism, this result argues in favor of diffusion mechanism as responsible for variations in transport properties, and against the recently suggested concept that collisions between gas molecules and the nanotubes are responsible for the changes in thermopower. \newline \newline [1] H. E. Romero, K. Bolton, A. Tosen and P. C. Eklund, Atom Collision-induced Resistivity of Carbon Nanotubes, Science 307 89 (2005)   

Observation of unusual structure in the low-temperature conductance of carbon nanotubes

Carbon nanotubes grown by chemical vapor deposition on a oxidized silicon substrate were contacted to form a gated sample consisting of a pair of tubes in parallel. The sample was tested at low temperature and high magnetic field using a dilution refrigerator and superconducting magnet. The current versus bias voltage graph shows a general trend consistent with the linear relationship except at low voltage. Further investigation is done by computing the differential conductance (dI/dV) and investigating how it varies with bias voltage. Here we see some intriguing behavior including a substantial increase in conductivity near zero voltage and a pronounced asymmetry with bias voltage. The temperature dependence of the zero field peak and asymmetry show that they appear at low temperatures and receding quickly by 3.0K. However, the magnetic field dependence is less intuitive. There are apparent shifts and possible splits that seem to develop but they are not followed in a manner consistent with simple theory.   

Low temperature electron transport measurements of dielectrophoretically assembled single wall carbon nanotube

Dielectrophoretic (DEP) assembly of carbon nanotube (CNT) has attracted tremendous interests because of its usefulness in assembling CNT at selected positions in nanoelectronic circuits with high yield. Although DEP technique has been used to fabricate nanoelectronic devices, the effect of contact resistance and nanotube buckling at the electrode-substrate interface has not been examined. Here, we present electronic transport measurements of DEP assembled single wall carbon nanotubes from room temperature to 1.5 K to examine the contact resistance and nanotube buckling. The nanotubes were suspended in dimethylformamide (DMF) and assembled between gold or palladium electrodes by the application of an AC electric field. A fundamental understanding of the contact resistance and buckling behavior will lead to improved designing of DEP assembled devices.   

Electrical transport through tunable pn junctions in suspended carbon nanotubes

Energy barriers play a crucial role within many carbon nanotube electronic devices, but their properties are often complicated by disorder induced by a substrate. Here we study well-controlled pn junctions in suspended carbon nanotubes whose transparency and position can be tuned using electrostatic gates. To achieve this we suspend individual single walled nanotubes above two separate gate electrodes and monitor the tube's conductance while changing the gate voltages in various ways; by varying the voltage difference between both gates we can tune the pn junction's width, whereas offsetting both gates by a common voltage controls its position. In small bandgap nanotubes we can additionally tune the junction's transparency by axial magnetic fields. At low temperature, these devices allow measurements of quantum dots with a tunable length and coupling to the leads.   

Scaling of Resistance with Channel Length in Single Walled Carbon Nanotubes.

We report on the scaling of resistance with channel length in single walled carbon nanotubes (SWNTs) with multiple Pd Ohmic contacts. The channel lengths range from 100nm to 400um. The intrinsic 1-D resistivity of individual SWNTs are measured from the slope of the linear dependence of resistance and length. The temperature dependent electron mean free length can be obtained from these data. While the mean free length ranges 100-500 nm at room temperature, the low temperature saturated value shows values as high as $\sim $10um. In addition, we will discuss possible mechanisms for the deviation from linear scaling behavior, as seen in long length scales ($>$100um). Finally, we will report unusual strong suppression of conductance outside of the band gap regions of several SWNTs deviating from typical small and large band gap semiconducting SWNTs.   

Magnetic and Transport Properties of Carbon Nanotube-Based One-Dimensional Nanocomposite Materials

Carbon nanotubes (CNTs) prove to be extremely well suited for the investigation of electron conduction in one-dimensional materials. CNTs have been shown to conduct in the ballistic regime according to Landauer's formula. When placed in electromagnetic fields and at low temperatures where the mean free path is smaller than the diameter of the tube, CNTs been shown to be the perfect platform to study Luttinger liquid behavior, the Kondo effect and quantum fluctuations, electron coherence, and the Aharonov-Bohm effect. These unique properties can be further enhanced by inserting materials into the cavities of the CNTs. In this work, we use anodized porous alumina templates as a substrate for the controlled growth of CNTs by means of chemical vapor depostion. AC electrodeposition is then used to deposit Fe, Ni, Co, as well as semiconductor nanowires inside the tubes. The magnetic and electrical properties of such nanotube-nanowire composites (both single and bundled) in the presence of applied magnetic fields upto 5.5T and at low temperatures down to 4.2K are studied and preliminary results will be reported.   

One-Dimensional Carbon Nanotube Electrodes

The study of the transport properties of organic semiconducting materials has been limited by the lack of suitable electrical contacts. Inappropriate charge injection at the electrode - organic semiconductor interface results in large contact resistances that often dominate device performance. We describe a strategy for circumventing charge injection barriers by using 1D metallic carbon nanotube electrodes. The favorable electrostatics at the tip of an individual carbon nanotube allows for efficient field assisted charge injection into organic semiconducting layers. A detailed finite element numerical study has allowed us to determine the scaling parameters required to optimize the performance of carbon nanotube electrodes. We present experimental results for pentacene organic thin-film transistors connected using individual metallic carbon nanotube source and drain contacts. Different gate oxide thicknesses (20 - 100 nm), channel lengths ( 2 - 100 nm) and carbon nanotube diameters (1 - 3 nm) were explored using experimental and numerical techniques.   

Heat Transport in Carbon Nanotubes Measured using Raman Spectroscopy

We investigate thermal transport in suspended carbon nanotubes on micromachined heating devices using Raman spectroscopy. Individual carbon nanotubes suspended between two membranes with integrated heaters and thermometers are characterized using micro-Raman spectroscopy. The temperature dependent shifts of the Raman mode frequencies are used to quantify the heating along the length of the nanotube. We find that results vary depending on the number of defects in the nanotube. The results are understood on the basis of a diffusive thermal transport model.   

Breakdown of Fourier's law in nanotube thermal conductors

We present experimental evidence that the room temperature thermal conductivity ($\kappa )$ of individual multiwall carbon nanotubes and boron-nitride nanotubes does not obey Fourier's law as do ordinary thermal conductors. By varying the length (L) of the nanotube, we find that $\kappa $ diverges as L$^{0.6\sim 0.9}$. Our results show that Fourier's law is violated despite the fact that the ballistic phonon condition is not satisfied and large isotopic disorder is present.   

Session J32: Quantum Computing in AMO Systems

Super-fluid assisted quantum computation with group II atoms

We investigate the possibility of using super-fluid immersion in order to suppress diabatic transitions in a system governed by a time-dependent Hamiltonian. A simple model has been used to study the question where quantum information is stored in the nuclear spin of a group II atom which is trapped in a harmonic oscillator that is traveling at a constant velocity inside of a stationary BEC. While the motion of the trap acts to heat the atom in the trap to higher vibrational levels, the motion of the trapped atom creates excitations in the BEC and carries the energy away in the form of phonons and decreases the effective heating.   

Interfacing Collective Atomic Excitations and Single Photons

A variety of quantum communication and computing schemes rely on the storage and transfer of single photonic excitations. We demonstrate generation, storage, and adiabatic transfer of such excitations, using ensembles of Cs atoms within a low finesse optical resonator as a storage medium. We explore theoretical and practical limitations on read-out, experimentally realizing a peak atomic-photonic conversion efficiency of .84(11). The storage in the system exhibits two doppler times, which can be understood in terms of long- and short- wavelength spin gratings simultaneously written into the atomic ensemble. We demonstrate cavity mediated transfer of a quantized atomic excitation between atomic ensembles within the same optical resonator. These results pave the way towards a practical single photon generation and storage apparatus, useful in quantum communication, computation, and beyond. This work was supported in parts by the NSF, DARPA, and ARO.   

Anyonic Braiding in Optical Lattices

Topological quantum computation proposes to use braiding of collective excitations implanted in topologically protected coherent quantum states of many particles, as opposed to a single particle, to aid in or even perform quantum computation. Here we explicitly work out a realistic experimental scheme to create, braid and detect topological excitations in the Kitaev model built on a tunable robust system, a cold atom optical lattice. A key feature of topological excitations is their braiding statistics, how they behave when one excitation is taken around another. An observation of the non-trivial braiding statistics described in this Report would directly establish the existence of anyons, quantum particles which are neither fermions nor bosons. Demonstrating anyonic braiding statistics is tantamount to observing a new form of matter, topological matter. Once created, excitations in quantum topological matter, as opposed to delicate single particle quantum states, can provide a robust way to encode and manipulate quantum information.   

Turning back time in the optical lattice: How to measure the fidelity of a quantum simulation.

I show how to perform a Loschmidt echo (time reversal) in the Bose-Hubbard model implemented with cold bosonic atoms in an optical lattice. The echo is obtained by applying a linear phase imprint on the lattice and a change in magnetic field to tune the boson-boson scattering length through a Feshbach resonance. I discuss how the echo can measure the fidelity of the quantum simulation, and also the intensity of an external potential (e.g. gravity), or the critical point of the superfluid-insulator quantum phase transition.   

Characterization of scalable ion traps for quantum computation

We discuss the experimental characterization of several scalable ion trap architectures for quantum information processing. We have developed an apparatus for testing planar ion trap chips \footnote{S. Seidelin \emph{et al.}, Phys. Rev. Lett. \textbf{96}, 253003 (2006).}$^,$\footnote{ J. Kim, \emph{et al.}, Quantum Inf. Comput. \textbf{5}, 515 (2005).}, which features: a standardized chip carrier for ease of interchanging traps, a single-laser Raman cooling scheme, and photo-ionization loading of Mg$^+$ ions. The primary benchmark for a given trap is the heating rate of the ion motional degrees of freedom, which can reduce multi-ion quantum gate fidelities. As the heating rate depends on the ion trap geometry and materials, our testing apparatus allows for efficient iteration and optimization of trap parameters. With the recent ability to fabricate planar traps with sufficiently low heating rates for quantum computation $^2$, we describe current results on the simulation and fabrication of planar traps with multiple intersecting trapping zones for versatile ion choreography.   

Entangling operations and rapid measurement of clock-state qubits in Yb or Sr for quantum information processing

The optical clock-transitions in Yb and Sr are prime candidates for encoding qubits for quantum information processing applications. Electric dipole one- and two-photon transitions between the long-lived $^1$S$_0$ and $^3$P$_0$ states are angular momentum and parity forbidden, respectively. This results in a highly desirable low decoherence rate. In this work, we investigate the challenges involved in using these prime candidates. We devise entangling operations for Yb and Sr atoms trapped in optical microtraps, as well as determine the feasibility of rapid qubit rotation and measurement of qubits encoded in this desirable low-decoherence clock transition. We propose ultracold collisions for entangling operations and a recoil-free three-photon transition [1] for fast rotation of qubits, followed by ultrafast readout via resonant multiphoton ionization. The rapid control of atomic qubits is crucial for high-speed synchronization of quantum information processors, but is also of interest for tests of Bell inequalities. We investigate rapid measurement of clock-state qubits in the context of a Bell inequality test that avoids the detection loophole in spacelike separated entangled qubits. [1] T. Hong, C. Cramer, W. Nagourney, E. N. Fortson, Phys. Rev. Lett. 94, 050801 (2005)   

Quantum spin relaxation in mixtures of spinor cold atoms

Recently spin relaxation becomes an extensively studied subject in the field of spintronics. One of the most important mechanism of electron spin relaxation in a semiconductor quantum qubit results from the hyperfine interaction with nuclei spins. Due to limitations in solid state experiments, the effects of nuclei spins to the electron spin relaxation is still not fully understood yet. Here we propose that such electron-nuclei system can be modeled by a mixture of two species of spinor cold atoms (say $^{7}$Li and $^{87}$Rb), loaded in a bi-frequency optical lattices of large wavelength difference. We use exactly diagonalization method to study how an initially spin polarized ``electron" atom relaxs in a spin bath of ``nuclei" atoms. Our calculation shows that the spin relaxation are strongly sensitive to the polarization of ``nuclei'' atoms, while for the fully unpolarized case the relaxation is mainly determined by the density of states. Our theoretical results can be also applied in studying the electron spin relaxation dynamics in the solid state quantum qubit.   

Microfabricated surface-electrode ion traps for scalable quantum information processing

We confine individual atomic ions in an rf Paul trap with a novel geometry where the electrodes are located in a single plane and the ions confined above this plane [1,2,3]. This device is realized with simple fabrication procedures, making it a potential candidate for a scalable ion trap for quantum information processing using large numbers of ions. We confine laser-cooled ions 40 micrometers above planar electrodes. These electrodes are fabricated from gold on a fused quartz substrate. The heating rate of the ions is low enough to make the trap useful for quantum information processing. [1] J. Chiaverini et al., Quantum Inf. Comput. \textbf{5}, 419 (2005). [2] S. Seidelin et al., Phys. Rev. Lett. \textbf{96}, 253003 (2006). [3] J. Britton et al., quant-ph/0605170.   

A Point Paul Trap for Quantum Information Experiments

The point Paul trap is a surface electrode ion trap that provides three-dimensional confinement using a single radiofrequency ring electrode, with ground inside and outside. Such a trap may have applications in the development of ion trap lattices for quantum simulation and quantum computation, and has previously been demonstrated using charged microspheres. Here we present a point trap for strontium ions with a ring radius of 1.6 mm and a ring width of .3 mm, consisting of gold electrodes on a laminate susbstrate and loaded by laser ablation. Typical operating parameters are a trap depth of .8 eV and secular frequencies of 250 kHz, for a trap drive of 800 V at 2.5 MHz. Numerical models of the trap are compared to experimentally measured motional secular frequencies of the ions, and the efficiency of laser ablation loading as a function of trap depth is studied. An experiment to measure the heating rate of a single ion as a function of height in the trap is also discussed.   

Optical MEMS Based Beam Steering for 2D lattice

Most scalable quantum computation approaches using arrays of trapped ions or individual neutral atoms in optical lattices require the experimental capability to address individual qubits in the large array. It is difficult to achieve such flexibility with traditional optical systems utilizing bulky components aligned on optical tables. Optical micro-electromechanical systems (MEMS) technology can provide a flexible and scalable solution for this functionality. We have developed simple beam steering optics using controllable MEMS mirrors that enable one laser beam to address multiple qubit locations in a linear trap or 2D trap lattice. The system can individually address 25 different positions on a 5 x 5 square array. MEMS mirror settling times of $<$ 5$\mu $s were demonstrated which allow for fast access time between qubits. Characterization of beam quality and optical power throughput is also presented. This system has the advantage of providing multiple individually addressed spots of different colors simultaneously without any frequency shifts.   

surface-electrode ion trap loaded by laser ablation

traps operated at liquid helium temperatures offer many advantages \newline for exploring new physics, such as quantum interactions between ions \newline and superconductors; cooling may also reduce anomalously high ion \newline heating rates currently observed and attributed to surface charge \newline fluctuations. However, cryogenic traps are traditionally \newline experimentally challenging to realize, due to the careful attention \newline required to thermally anchor the trap, and due to the incompatibility \newline of standard high-temperature ion sources with a cryogenic environment. \newline We demonstrate a new approach to these challenges using a millimeter \newline scale printed-circuit board trap with surface electrode geometry, \newline operated in a liquid helium bath cryostat, to trap and cool strontium \newline 88 ions. The planar aspect of this trap simplifies anchoring to the \newline helium baseplate, and provides clear access for loading ions from an \newline ablation plume produced by $<$7 mJ pulses of a Q-switched Nd:YAG laser \newline incident on a Sr/Al alloy target. We are able to load traps with \newline depths as low as 0.7 eV, and with laser cooling we observe small ion \newline crystals with between one and twenty six optically resolved ions, with \newline individual ion lifetimes averaging 2 hours. Initial estimates based on \newline the observed residual gas collision rates are consistent with a vacuum \newline pressure below 10\^{}{\{}-9{\}} torr, and the true pressure is likely much lower.   

Sideband cooling and anomalous heating of trapped $Sr^+$ ion.

Many schemes for entangling and quantum processing with trapped ions require cooling the ions close to motional ground state, and the anomalous heating of the ion can be the limiting factor in gate fidelity. We developed a simple laser system, based on external cavity diode lasers with optical feedback to a running-wave cavity to investigate this heating. Without the use of a high finesse cavity, or fast active feedback, we have achieved $<30$ kHz linewidths and $\approx 1$ MHz long term stability. This system was used to sideband cool a single $Sr^+$ ion to a motional ground state with $> 90\%$ probablity and observe Rabi oscillations on the $5S_{1/2} \rightarrow 4D_{5/2}$ transition. We present our results on heating and cooling rates of the ion in room temperature and cryogenic ion traps.   

One-way quantum computing in optical lattices with many atom measurements.

In one-way quantum computation single qubit measurements on a highly entangled state, known as a cluster state, are sufficient to perform universal quantum computation. One of the most promising approaches for generating the cluster state is to manipulate ultracold atoms in optical lattices. Unfortunately, the small lattice spacing places severe constraints on the ability to sequentially measure the states of individual atoms by external lasers, a crucial requirement for one-way computing. With current technology, we are generally limited to many atom measurements. We have developed a deterministic protocol for one-way quantum computing based on many atom measurements on an optical lattice cluster state, requiring only polynomial classical overhead. Our scheme opens the way toward concrete experimental quantum computing in neutral atom systems.   

Session J33: Focus Session: Superconducting Qubits II

Two Superconducting Charge Qubits Coupled by a Josephson Inductance

When the quantum oscillations [Pashkin {\it et al.}, Nature {\bf 421}, 823 (2003)] and the conditional gate operation [Yamamoto {\it et al.}, Nature {\bf 425}, 941 (2003)] were demonstrated using superconducting charge qubits, the charge qubits were coupled capacitively, where the coupling was always on and the coupling strength was not tunable. This fixed coupling, however, is not ideal because for example, it makes unconditional gate operations difficult. In this work, we aimed to {\it tunably} couple two charge qubits. We fabricated circuits based on the theoretical proposal by You, Tsai, and Nori [PRB {\bf 68}, 024510 (2003)], where the inductance of a Josephson junction, which has a much larger junction area than the qubit junctions, couples the qubits and the coupling strength is controlled by the external magnetic flux. We confirmed by spectroscopy that the large Josephson junction was indeed coupled to the qubits and that the coupling was turned on and off by the external magnetic flux. In the talk, we will also discuss the quantum oscillations in the circuits.   

Kinetics of quasiparticle trapping in a Cooper-pair box.

We study the kinetics of the quasiparticle capture and emission process in a small superconducting island (Cooper-pair box) connected by a tunnel junction to a massive superconducting lead. At low temperatures, the charge on the box fluctuates between two states, even and odd in the number of electrons. Assuming that the odd-electron state has the lowest energy, we evaluate the distribution of lifetimes of the even- and odd-electron states of the Cooper-pair box. The lifetime in the even-electron state is an exponentially distributed random variable corresponding to a homogenous Poisson process of ``poisoning'' the island with a quasiparticle. The distribution of lifetimes of the odd- electron state may deviate from the exponential one. The deviations come from two sources - the peculiarity of the quasiparticle density of states in a superconductor, and the possibility of quasiparticle energy relaxation via phonon emission. In addition to the lifetime distribution, we also find spectral density of charge fluctuations generated by capture and emission processes. The complex statistics of the quasiparticle dwell times in the Cooper-pair box may result in strong deviations of the noise spectrum from the Lorenzian form.   

Narrow band microwave radiation from a biased single-Cooper-pair transistor

We have spectroscopically measured narrow-band microwave radiation emitted from a single-Cooper-pair transistor (SCPT) electrometer biased in its sub-gap region. This radiation was detected by photon-assisted quasiparticle tunneling in a nearby SCPT, in a configuration that closely mimics a qubit-electrometer integrated circuit. In addition to the usual Josephson radiation generated by the electrometer, we also find emission lines whose frequency depend on both the gate charge and bias voltage of the electrometer, and attribute these lines to radiative Cooper-pair transport processes in the biased transistor. Our results suggest that the dissipative operation of an SCPT electrometer, when used as a qubit readout device, may severely disrupt the system it attempts to measure. This radiative coupling between Josephson charge devices, which dominates when coupling in the charge channel is negligible, may impose design constraints on a large scale multi-qubit quantum computer.   

Measurement of the Excited State Lifetime of a Cooper-Pair Box

We have used a radio frequency superconducting single electron transistor (rf-SET) biased around the double Josephson quasiparticle peak to measure the lifetime T$_{1}$ of the excited state of an Al/AlO$_{\mbox{x}}$/Al Cooper-pair box (CPB) qubit. The CPB had a charging energy E$_{C}$/k$_{B}$ = 0.78 K and a maximum Josephson coupling energy E$_{J}$/k$_{B}$ = 0.70 K and all measurements were made at about 40 mK. T$_{1}$ was found by sending a pulse of microwaves to the gate of the CPB and then using the rf-SET to observe the decay rate of the charge signal on the CPB. Near the degeneracy point of the CPB, we observed T$_{1}$ of approximately 100 ns, which was near the limit of the rf-SET bandwidth. As we move away from the degeneracy point, T$_{1}$ varies, reaching a maximum of approximately 400 ns. We examine whether these changes in T$_{1}$ are commensurate with the quantum noise spectral density from the rf-SET.   

Efficient one- and two-qubit pulse gates for an oscillator stabilized Josephson flux qubit

We present schemes for one- and two-qubit dc pulse gates for the IBM qubit. As reported previously by our group, the qubit consists of three Josephson junctions, three loops, and a superconducting transmission line. The qubit-qubit coupling is assumed to be inductive. We show that there are settings of the flux control parameters for which the effective qubit-qubit coupling can be made negligible, allowing one to perform high fidelity single qubit gates. Assuming the presence of no decoherence processes, we are able to reach gates of 99$\%$ fidelity for pulse times in the range of 20-30$\rm{n}s$. Our $T_2$ estimates indicate coherence times long enough to perform approximately 50 of those gates. The control of leakage plays an important role in the design of our dc pulses, preventing shorter pulse times. Also, we look for schemes which may alleviate the errors due the $1/f$ noise. In addition, we show how to perform two-qubit gates in the system, demonstrating a controlled phase operation.   

A Quantum Computer based on Tunable Flux Qubits

Based on the experimental and theoretical results on the different types of superconducting qubits, we feel there are several features which are desirable for the development a scalable quantum computer: (1) Tunable qubits, (2) Tunable coupling, and (3) Storage. We present two and three junction versions of the IBM tunable flux qubit coupled to a harmonic oscillator which exhibit all these features. We can adiabatically move the information from the flux qubit into the harmonic oscillator for storage and back. When the information is in the harmonic oscillator, coherence times are limited by the quality factor of the harmonic oscillator which is known to be high. When the information is in the flux qubit, one and two-qubit gates are implemented using microwave pulses with gate times of about 10-20ns. Coherence times at the operating point are limited by 1/f flux noise with estimated dephasing times of about 100ns. Using shaped pulses simulations show that the overall unitary gate can be made relatively insensitive to frequency drifts, resulting in errors of about 10$^{-3}$ even for total gate lengths similar to the dephasing time. Further improvements will most likely involve reducing flux noise.   

The voltage-controlled superconducting flux qubit

We study a voltage-controlled version of the superconducting flux qubit [Chiorescu \textit{et al.}, Science {\bf 299}, 1869 (2003)] and show that full control of qubit rotations on the entire Bloch sphere can be achieved. Circuit graph theory is used to study a setup where voltage sources are attached to the two superconducting islands formed between the three Josephson junctions in the flux qubit. Applying a voltage allows qubit rotations about the $y$ axis, in addition to pure $x$ and $z$ rotations obtained in the absence of applied voltages. The orientation and magnitude of the rotation axis on the Bloch sphere can be tuned by the gate voltages, the external magnetic flux, and the ratio $\alpha$ between the Josephson energies via a flux-tunable junction. We compare the single-qubit control in the known regime $\alpha<1$ with the unexplored range $\alpha>1$ and estimate the decoherence due to voltage fluctuations.   

Microwave-Induced Cooling of a Superconducting Persistent-Current Qubit

We present the experimental demonstration of microwave-induced cooling of a persistent-current qubit. Our qubit is a multi-level artificial atom. Thermal population of the first-excited qubit state is driven to a higher-excited state, from which it preferentially relaxes to the qubit ground state. Cooling is realized, because the driving-induced population transfer to the ground state is faster than the thermal repopulation of the excited state. We achieve effective qubit temperatures < 3 mK, a factor 10x-100x lower than the dilution refrigerator ambient temperature. This talk will present and discuss these experimental results. [1] S.O. Valenzuela, W.D. Oliver, D.M. Berns, et al., Science (2006).   

Coherent Quasiclassical Dynamics of a Superconducting Persistent-Current Qubit

A new regime of coherent quantum dynamics of a qubit is realized at low driving frequencies in the strong driving limit. Coherent transitions between qubit states occur via the Landau-Zener process when the system is swept through an energy-level avoided crossing. The quantum interference mediated by repeated transitions gives rise to an oscillatory dependence of the qubit population on the driving field amplitude and flux detuning. These interference fringes, which at high frequencies consist of individual multiphoton resonances, persist even for driving frequencies smaller than the decoherence rate, where individual resonances are no longer distinguishable. A theoretical model that incorporates dephasing agrees well with the observations. \newline [1] D.M. Berns, W.D. Oliver, S.O. Valenzuela et al., PRL 97, 150502 (2006).   

Adiabatic Evolution of a System of Four Coupled Flux Qubits

We report upon experimental results from a system consisting of four flux qubits linked via in-situ sign and magnitude tunable coupling elements. The device was operated as an adiabatic quantum computer to solve NP-complete problems whose solutions are encoded in the groundstate configuration of the qubits. Each qubit was coupled to its own dedicated dc-SQUID in order to measure the state of each qubit, thus allowing for unambiguous identification of the groundstate of the coupled qubit system at the end of a computation. Results will be compared to a quantum model of the system's evolution.   

Studies of decoherence in a large area Nb flux qubit

We report measurements using pulsed microwaves to investigate the decoherence mechanisms in a large area Nb based flux qubit. Our qubit uses an rf-SQUID in a gradiometer configuration and has independent, in situ, controls for the relative positions of levels in different fluxoid wells and the barrier height between the wells. We present measurements of decoherence times from coherent oscillations and microwave spectroscopy. These measurements are well suited to evaluate potential improvements in the materials and the fabrication process of both flux and phase qubits based on different flux states.   

Measurement of relaxation time in a large-inductance superconducting flux qubit

Superconducting flux qubits with large geometric inductance are promising candidates for scalable quantum computing because it is easier to couple many of them together to form a quantum circuit sufficiently large for useful computational task. Recently, it has been shown that the dominant decoherence mechanism in superconducting flux qubit carefully isolated from environment is energy relaxation. It is therefore important to understand various relaxation mechanisms. We report here a systematic study of relaxation time as a function of flux bias, temperature, and operating point of the readout circuit in an rf SQUID flux qubit. Determination of the qubit's parameters via microwave and resonant tunneling spectroscopies allow quantitative comparisons to theoretical models. Implications of the result will be discussed.   

Long-range coupling mechanism and architecture for superconducting flux qubits

Devising a scalable mechanism enabling long-range interaction of qubits in a solid-state quantum computer is an important open problem. With only nearest neighbour interactions, gate error rates of order $10^{-7}$ or lower would be required to perform an arbitrarily large computation. If the right kinds of long- range interactions are available, gate error rates of order $10^ {-4}$, and possibly higher, would be acceptable. We discuss exactly what kinds of long-range interactions are required, present a simple mechanism for superconducting flux qubits, a scalable architecture based on this mechanism, and discuss the challenges on the road to physical realisation.   

Session J34: In vivo Imaging and Biomolecular Fibrils

Crystallographic Properties of Physiological Hydroxyapatite as a Function of Age

Hydroxyapatite with 4-6 wt {\%} B-type carbonate substitution is the major mineral component in our teeth and bones. Crystal structure properties of human teeth as a function of age between 17 and 91 years are investigated. X-ray powder diffraction reveals a partial phase transition from the hexagonal Ca$_{5}$(PO$_{4})_{3}$OH (Hydroxyapatite) to the triclinic Ca$_{4}$H(PO$_{4})_{3}$.2H$_{2}$O (Calcium Hydrogen Phosphate Hydrate) at the 70 year old tooth. This phase becomes predominant in the diffraction pattern of a 91 year old tooth. Correlation of such transition with physical properties of synthetic hydroxyapatite could provide useful insights in dentistry and medicine.   

Single fluorescent nanodiamond as a cellular biomarker

Type Ib diamonds emit bright fluorescence at 550--800 nm from nitrogen-vacancy point defects, (N-V)0 and (N-V)-, by high-energy ion beam irradiation and subsequent thermal annealing. The absence of fluorescence intermittency and photobleaching in addition to its non-cytotoxicity and the easiness of surface functionalization make the fluorescent nano-sized diamonds (FND) a promising fluorescent probe for single-particle tracking in heterogeneous environments. We investigated the basic photophysical properties of surface-functionalized single FND particles with average diameter of 35-nm using single-photon and two-photon excitation. The application of tracking single FNDs in HeLa cells was also demonstrated. We found that the photostability of FNDs is not deteriorated by the surface treatment and the brightness of the fluorescence emitted by FNDs is much higher than typical organic dyes. The absorption and emission wavelength of FND, which are well separated from that of the intracellular components, further ensures the good signal to noise ratio for its application as a cellular biomarker.   

Intracellular Osmolyte Distributions Assessed by $^{1}$H and $^{23}$Na Magnetic Resonance Microscopy

Recently, Magnetic Resonance Microscopy (MRM) has been applied to the high resolution imaging and localized spectroscopy of isolated cells$^{1,2}$. With resolutions $<$40 $\mu $m, these efforts have demonstrated the diverse intracellular environments that can be probed by proton MRM to provide insight into the compartmental diffusion and relaxation of intracellular water and metabolites. In this study, the intracellular distribution of the inorganic osmolyte sodium in isolated single neurons is assessed by MRM through the acquisition of three-dimensional (3D) microimages by direct observation of $^{23}$Na. These efforts are made possible through (a) the use of a specially constructed, double-tuned Radio Frequency (RF) microcoil and (b) the application of a unique, ultra-widebore 21.1-T magnet. Results show an increased sodium signal in the nucleus of the L7 neuron of \textit{aplysia Californica}. These $^{23}$Na findings are compared with MR data that display a heterogeneous distribution of the organic osmolyte betaine, which appears to be localized at high concentrations to the cytoplasm. The link between the intracellular distributions of sodium and other osmolytes in this single neuron model may shed light on intracellular osmoregulatory processes, particularly in response to toxic or pathological perturbations. $^{1}$S.C.Grant, \textit{et al.}, Magn. Reson. Med. 2000. $^{2}$S.C.Grant, \textit{et al.,} Magn. Reson. Med. 2001.   

Harmonic Generation Spectroscopy of Enzymatic Activity in Live Organisms

We report on measurements of harmonics generated by whole cells, chloroplasts, and whole plants in response to applied sinusoidal electric fields. The frequency- and amplitude-dependence of the induced harmonics exhibit features that correlate with physiological processes. In particular, we find that harmonics generated by whole plants and suspensions of chloroplasts are dramatically increased by the presence of light. Systematic studies of the second and third harmonic generation spectra of chloroplast suspensions indicate the following: 1) a broad peak, centered around 10 kHz applied frequency (20 kHz for the second harmonic) appears when the photosynthetic electron transport chain is activated by light in the presence of a suitable electron acceptor, such as ferricyanide; changes observed in the time-dependent harmonic response for fixed frequency are correlated to the presence of light activation of photosynthetic election transport activity. 2) This feature correlates with oxygen evolution activity of photosynthesis. In whole plants, multiple peaks in the light-activated harmonic generation spectra suggest that the method may be able to selectively probe specific photosynthetic activity in plants.   

Near-Infrared Fluorescence of the NBT/BCIP Chromogenic Stain

We demonstrate the previously unreported near infrared (NIR) fluorescence of the dark purple stain formed from 5-bromo-4-chloro-3-indolyl phosphate (BCIP) and nitro blue tetrazolium (NBT). Although the product is a solid with strong optical absorption, its fluorescence enables high cellular resolution imaging of gene expression. We use spectrofluorometry to identify NBT diformazan as the component of the stain that is the fluorophore exhibiting the strong fluorescence signal. The fluorescence shows an intense emission signal (780-910 nm) that is well separated from excitation (645-685 nm). The NBT diformazan fluorescence is also photostable. Because NBT/BCIP is a widely used chromogenic stain, existing staining protocols can also be applied to fluorescence imaging techniques to increase the resolution of gene expression patterns.   

Development of a `Protein Microscope' to Map Peptide Distributions in Cells

We report on the development of a new instrument, dubbed a `Protein Microscope,' that uses near-field optical techniques to increase the spatial resolution of atmospheric pressure matrix-assisted laser desorption and ionization (AP-MALDI). This functions as a novel front-end for time-of-flight mass spectrometry. Standard protein identification techniques involve homogenization of a tissue sample, which destroys all spatial and temporal information about the expressed proteins. Our new NSOM-based instrument will allow the identification and mapping of proteins expressed in intact cells and tissues, which is of great interest as protein expression connects genomic information with the functioning of an organism.   

The fate of cells in skin: from clonal analysis to cell kinetics

Biologists are keen to understand the mechanisms of development and maintenance of tissues in mammals. As well as its intrinsic scientific interest, an understanding of the kinetics of cell division has important implications for mechanisms of aging and cancer development. Analysis of cell populations (clones) resulting from progenitor cells provides indirect access to the laws governing cell division and fate. Yet, until recently, the quality of clonal fate data acquired \emph{in vivo} has inhibited reliable quantitative analysis. By addressing a recent, detailed, and extensive experimental study of mammalian skin, we develop a general theoretical framework which shows that the wide range of clonal fate data are consistent with a remarkably simple cell kinetic model. As well as overturning the accepted paradigm for skin maintenance, the analysis introduces a general framework for analysing clone fate data in future experiments. We now have a robust platform to study the effect of drug treatments and the influence of cell mutations on the epidermis.   

Photo-ionization Potential Threshold of Single Human Fibrinogen Molecule Absorbed onto Silicon Surfaces

Human Plasma Fibrinogen (HPF), which is a protein involved in haemostasis and thrombosis, is known to readily adsorb onto artificial surfaces. Therefore, understanding the absorption process for specific surfaces is critical to establish biocompatibility. In this study, the photo-ionization potential of single HPF molecules adsorbed onto oxidized p-type silicon substrates was studied by photoelectron emission microscopy (PEEM). PEEM, using the spontaneous emission output of the Duke OK-4 free electron laser (FEL), were illuminated at tunable wavelengths between 248 and 310 nm. The photo-ionization potential threshold for single HPF molecules was found to be 4.6 $\pm $ 0.2 eV. The electronic states of the molecule were related to the electronic states of the oxidized Si surfaces. The deduced alignment of the electronic states is consistent with negative charge transfer from the adsorbed fibrinogen to the p-type silicon substrates which would proceed by tunneling through the thin oxidized layer.   

In vitro, interaction of homotrimers with heterotrimers of type I collagen

The dominant mutations in type I collagen cause a group of diseases, often termed collagen, or connective tissue, diseases: for example, Osteogenesis Imperfecta (OI) characterized by bone fragility and skeletal deformity. The mechanism in which collagen mutations affect on the diseases is still unknown. To understand the fibril assembly and their interactions might provide a key to approaching the cause of the collagen diseases. This study demonstrates that the self-assembly, termed fibrillogenesis, of type I collagen homozygous mutations revealed substantial differences in the kinetics with the absence of lag time and in the morphology of 3D fibril network structure. The heterotrimers (normal) and homotrimers (mutant) in mixtures were segregated within the same fibrils during fibrillogenesis, in correspondence between confocal microscopy and thermodynamic measurements. The efficiency for self-assembly of the homotrimers into fibrils was markedly reduced, while that of the heterotrimers was not affected by the presence of homotrimers with no change in solubility.   

Determining Beta Sheet Crystallinity in Fibrous Proteins by Thermal Analysis and Infrared Spectroscopy

We report a study of self-assembled beta pleated sheets in \textit{Bombyx mori} silk fibroin films using thermal analysis and infrared spectroscopy. Crystallization of beta pleated sheets was effected either by heating the films above the glass transition temperature (Tg) and holding isothermally, or by exposure to methanol. The fractions of secondary structural components including random coils, alpha helices, beta pleated sheets, turns, and side chains, were evaluated using Fourier self-deconvolution (FSD) of the infrared absorbance spectra. As crystalline beta sheets form, the heat capacity increment from the TMDSC trace at Tg is systematically decreased and is linearly well correlated with beta sheet content determined from FSD. This analysis of beta sheet content can serve as an alternative to X-ray methods and may have wide applicability to other crystalline beta sheet forming proteins.   

Resonance Effects in the Ultraviolet Raman Spectroscopy of Collagen in Mineralized Tissues

Ultraviolet resonance Raman spectroscopy (UVRRS) was used to investigate type I collagen in solid tissues including tendon, dentin, and bone. With 244 nm excitation, spectral features from both the amide backbone (amide I, II, and III) and resonance-enhanced side-chain vibrations (Y8a, tyrosine) were observed. This contrasts with reported Raman spectra of proteins in solution excited with similar UV wavelengths, where side chain vibrations, but not strong amide features, are observed. The height of the dominant amide I feature in teeth and bone can be reversibly increased/decreased in dentin by dehydration/rehydration cycles. Also, the amide I peak is relatively stronger in both human bone and dentin from older donors. The strong intensity of the amide I UVRRS feature in these mineralized tissues is attributed to an increase in the width of the $\pi\rightarrow$ $\pi$$^{\textrm{*}}$ amide resonance in collagen compared to the solution phase. These findings suggest that UVRRS can be used as a specific probe of the collagen environment in bone and dentin.   

Surface modification of cotton and silk fabrics by SF$_{6}$ plasma

Hydrophobic properties are of the interest in fabric and textile manufacturers. We have used SF$_{6}$ plasma to modify the surface of cotton and silk fabrics. We have found that SF$_{6}$ plasma enhances the hydrophobic property of both types of fabrics. The water contact angle of SF$_{6}$-treated fabrics increased from 20 degrees up to 140 degrees. The measured absorption time was found to depend upon the treatment time and RF power, only at the low SF$_{6}$ pressure of 0.005 and 0.05 torr. At higher pressure, all samples achieved high absorption time of about 200 min, regardless of the RF power and treatment time. The morphology changes of fabrics after plasma treatment were characterized by scanning electron microscopy and atomic force microscopy. After plasma treatment, the rms surface roughness of the fibre increased from about 20 nm to 40 nm. From X-ray photoelectron microscopy analysis, we found that the higher the F/C atomic ratio leads to the longer the absorption time or the improved hydrophocity of the fabric.   

Fundamental Investigations of the Extracellular Proteins Fibrin and Collagen in Microchannel Devices

Microfluidic structures are particularly amenable to controlled investigations of protein bundle and network formation. Hydrodynamic focusing is utilized to create a diffusion-controlled gradient of reactants, enabling non-equilibrium investigations. We present studies of the blood clotting protein fibrin, a three-dimensional network formed from the enzymatic cleavage of fibrinogen monomers by the protein thrombin. Fibrin is a vital component of blood clots, and has been implicated in a variety of diseases. Real-time fluorescence microscopy and x-ray micro-diffraction are used to quantify supramolecular assembly and provide snapshots of the evolution of fibrin network formation. We also show that collagen, a ubiquitous extracellular protein, can be bundled in situ through the use of a pH gradient. An outlook toward artificial blood vessels arises from the insight that both fibrin and collagen can easily be used to coat microchannel structures. The resulting mesh forms an ideal environment for red blood cells and other cell types.   

Biomolecule surface patterning by aqueous polymer nanografting (APN)

We have demonstrated a method to chemically pattern aqueous polymer layers on the nanoscale. An atomic force microscope (AFM) was used to mechanically remove positively charged polymers from silica and mica surfaces with submicron resolution in liquid. Polyallyl amine (PAA) and polylysine were both been patterned creating 10 and 20 micron boxes with nanometer scale edge transition lengths. These patterns can serve as templates for patterning lipid and protein layers in buffer environments where pH and concentration can be controlled.   

Mechanical Single Molecule Investigations of SNARE Protein Interactions

We used an Atomic Force Microscope (AFM) to perform single molecule investigations of the SNARE (soluble N-ethyl maleimide-sensitive fusion protein attachment protein receptors) proteins, syntaxin, synaptobrevin and SNAP 25. These proteins are involved in the docking and release of neurotransmitters. The rupture force and extension of the interactions were measured. Chemical reaction rate theory was applied to obtain the energy barrier width and lifetime. Their temperature dependence was also explored.   

Session J35: Protein Water Interactions

Oscillatory Growth of Ice Crystals Observed in a Solution of Antifreeze Glycoprotein

One-directional growth experiments of ice crystals in an aqueous solution of antifreeze glycoprotein (AFGP) were carried out using a growth cell made of thin glass capillaries. When the interface tips of ice crystals were constructed by prismatic planes, the interface position changed periodically with time. These phenomena were not observed for the growth of basal planes in the AFGP solution or for the growth of ice crystals in pure water. We first observed the oscillatory growth of ice crystals in the AFGP solution. Fluorescent labeled AFGP molecules were also used to observe the diffusion, incorporation, and segregation of the solute at the interface, in the solid and in solution. The periodic incorporation of AFGP molecules were clearly observed in conjunction with the growth rate changes.   

A model of self-oscillatory growth of ice crystals in antifreeze glycoprotein solutions

We discuss that~ an oscillatory crystal growth is observed not only in the growth of an ice crystal from AFGP solution but also in the motion of steps on the surface of ice crystals in the presence of AFGP molecules. Our model of the oscillatory growth of crystals accounts for two elementary processes relevant to the growth: 1) an interface kinetic processes for transformation into a crystalline phase at the interface, and 2) a diffusion process for the transport of latent heat liberated at the growing interface. In this talk, we propose the hypothesis of a hysteresis behavior of growth rate to explain the formation of periodic structures of a growing crystal without a change of external conditions.~ ~The self-oscillatory growth in the presence of AFGP adsorbed molecules can occur because of the coupling of interface kinetics to the transport of latent heat under constant growth conditions.   

Antifreeze Protein (AFP) and Antifreeze Glycoprotein (AFGP) Kinetics at the Ice/Solution Interface

AFPs and AFGPs found in some fish, plants and insects are a necessary tool for surviving sub-freezing environments. They occur in a wide range of compositions and structure, but to some extent they all accomplish the same functions: they suppress the freezing temperature, inhibit recrystallization, and modify ice crystal growth. Here, we observe the exact location of AFGPs, Type I and Type III AFPs by 1-directional growth experiments using fluorescence and phase contrast microscopy as well as free growth experiments using 3-d confocal microscopy. In all cases, the proteins clearly adsorb at the interface. By comparing the fluorescent image with the corresponding phase contrast image we find that AFGPs incorporate only into the solid in veins and not into the ice lattice structure. Type I AFPs show similar behavior as AFGPs, but type III AFPs adsorb to specific planes within the ice lattice. We have also calculated the diffusion constants and the surface adsorption concentration from both types of experiments. Our results indicated that the different types of AFPs or AFGPs accomplish essentially the same function in slightly different ways and that it is not necessary for the protein adsorption to the ice interface to be as rigid as once thought.   

Protein slaving to the solvent and the relation to hydrodynamics

Protein motions can be categorized by the nature of their coupling to solvent dynamics. Some protein motions, including the final ligand binding process in myoglobin (Mb), are largely independent of solvent fluctuations. Others, such as entry and exit of ligands from Mb require Debye-like $\alpha$ fluctuations in the solvent to proceed. A third class of motions, including the r.\ m.\ s.\ displacments of atoms are controlled by solvent $\beta$ fluctuations. We show that a slaving picture of protein dynamics, $k_{\mathrm{protein}} = k_\alpha/n$, where $n$ is a nearly T-independent factor, known to be as large as $10^5$, is consistent with an essentially hydrodynamic picture of $\alpha$-slaved protein motions. Consistency with hydrodynamics (i.\ e.\ the Stokes-Einstein equation) can be demonstrated by considering changes to protein stability caused by ordinary experimental protocols for measuring viscosity- and T-dependent protein dynamics data. The decomposition of protein dynamics into several discrete classes suggests modelling techniques to simplify the simulation of protein dynamics.   

Dynamics of Lysozyme in a Glycerol-Water system

Bio-preservation of proteins is of great commercial and academic interest. A variety of sugars have been found to be effective in preserving the structure of proteins. This has been attributed and in some cases proved to their ability to form strong hydrogen bonds with proteins thus restricting their motion. The work presented here explores the hypothesis that glycerol, a tri-alcohol curbs the motion of protein. \newline \newline We have carried out a 10ns Molecular Dynamics simulation to study the phenomenon. The structure of Lysozyme (PDB code 193L) has been studied in three solutions of 10, 20 and 30 {\%} by weight of glycerol in water. \newline \newline Glycerol molecules in all three solutions have shown a tendency to agglomerate around the protein. Strong hydrogen bonding has also been observed between glycerol molecules and the protein. With increasing time, the g(r) of glycerol molecules around proteins shows two peaks with increasing prominence suggesting the movement of glycerol cluster to positions closer to the protein surface.   

An extended dynamical solvation shell around proteins.

Water solvating biomolecules in organisms has different properties from the bulk. Such solvation shells can be characterized by a variety of structural and dynamical measures. The fundamental question of biomolecule hydration is: how far out into the solvent does the influence of the biomolecule reach? We use terahertz absorption spectroscopy of the five helix bundle protein Lambda Repressor 6-85, coupled with molecular dynamics simulations, to show that correlated water motion at a sub-psec time scale persists to distances of at least 20 angstrom. We show this by determining that bulk water, water molecules mainly interacting with a single protein molecule, and water molecules interacting with more than one protein molecule have different absorption signatures in the THz frequency range, leading to an experimentally detectable non-monotonic dependence of the absorption coefficient on protein concentration. This trend is supported in the calculations, which further show that long-distance hydration is a dynamical effect correlating many water molecules, not one that noticeably perturbs the structural distribution of one or a few water molecules from the bulk value.   

Structural and dynamical properties of water in hydrophobic confinement, as probed by \textit{ab-initio }molecular dynamics.

Unraveling the microscopic properties of water confined in small channels will help understand fluid flow and transport at the nanoscale, and will shed light on the solvation of biomolecules. To date most of the properties of confined water are poorly understood and, in many cases, controversial. We present a first principles computational study of prototype systems ---water confined between graphene sheets and inside carbon nanotubes-- which have received widespread experimental attention and for which, however, such basic questions as diffusion at the nanoscale, and characteristics of the hydrogen bonded network remain unanswered. Our simulations show that the liquid density substantially increases at the water/surface interface, and that water diffusion is faster in highly confined structures, due to a decrease of the dipole moment in interfacial water molecules and correspondingly a decrease in H-bond network strength. We propose that many effects attributed to confinement in the past are actually interfacial effects due to subtle electronic structure rearrangements, and that these are amenable to vibrational and x-ray absorption spectroscopy investigations.   

Basal Plane Affinity of an Insect Antifreeze Protein

sbwAFP is a powerful antifreeze protein (AFP) with high thermal hysteresis activity that protects spruce budworm (sbw) from freezing during harsh winters in the spruce and fir forests of USA and Canada. Different types of antifreeze proteins have been found in many other species and have potential applications in cryomedicine and cryopreservation. When an ice crystal is cooled in the presence of AFP below the non-equilibrium freezing point the crystal will suddenly and rapidly grow in specific directions. Hyperactive antifreezes like sbwAFP expand perpendicular to the c-axis (in the plane of the a-axes), whereas moderately active AFPs, like type III from fish, grow in the direction parallel to the c-axis. It has been proposed that the basis for hyperactivity of certain AFPs is that they bind and accumulate on the basal plane to inhibit c-axial growth. By putting fluorescent tags on these two types of AFPs we have been able to directly visualize the binding of different types of AFPs to ice surfaces. We do indeed find that the insect AFP accumulates on the basal plane of an ice crystal while type III AFP does not. Supported by CIHR and BNTI.   

Study of Hydrogen Bond and Dipolar Interaction in Water-like Fluid with Toy Model

Hydrogen bond and dipolar interaction, which originated from the high polarizability of asymmetric water-like molecules, give rise to anomalous properties. Anionic interface of water-like fluid is understandable as a result of hydrogen bond and excluded interactions of OH$^{-}$ and H$_{3}$O$^{+}$. Range of dipolar interaction reaches over several water-like molecule size. And, the interaction between dipole and ion affects on about 20 times longer than the size of water-like molecule. Therefore, the interaction between charged particles within this range shows different behavior compared to interaction in a uniform dielectric medium. Toy model gives physical insights and helps comprehensions to complex phenomena. In this study we give the numerical simulation to investigate these phenomena.   

Density and Structure of Water under Confinement as Determined using Monte Carlo Simulations

The structure and local density of water is thought to play an important role in phenomena such as protein adsorption. These properties of water under confinement between surfaces can be significantly different from those of bulk water. A change in the water's structure, which is coupled to a change in the local density of the confined water in equilibrium with the bulk water, can create an attractive or repulsive force between the planar surfaces. This force itself can dominate the mechanism of adsorption when adsorbing molecules are within close proximity from adsorbent. In order to probe the effects of confinement further, Grand Canonical ensemble Monte Carlo (GCMC) simulations of Single Point Charge Enhanced (SPC/E) water confined between two planar surfaces of differing hydrophobicity, ranging from hydrophobic to hydrophilic, have been performed. The dependence of the water's structure and local density on the hydrophobicity and distance between the two planar surfaces has been determined. Further, the effect of surface curvature will also be examined.   

The protein hydration transition

We previously reported the hydration transition in the THz dielectric response for native state hen egg white lysozyme (HEWL). As hydration increases the response slowly increases until at 0.25h (gm water/gm protein) the absorbance and index sharply increase. The hydration level coincides with the filling of the first solvation shell. The THz dielectric response arises from relaxational and resonant vibrational response, where the vibrational response corresponds to delocalized structural motions sensitive to the conformation and the environment. We examine the contribution of low frequency vibrational modes to the hydration transition by calculating the normal mode density as a function of solvent content using CHARMM. We find that the density of low frequency modes increases with the increasing solvent content, but this increase does not show the transition seen experimentally. We discuss that another source for the hydration transition in the THz response may be the hydration dependence of the activation energy for glass-like beta fluctuations that contribute to the relaxational response.   

Inverted Solubility of the Pro 23 to Val Mutant of Human $\gamma $D Crystallin-- Altered Phase Diagram from a Single Amino Acid Substitution and the Effect of PEG

Many genetic cataracts are the result of single point mutations in the amino acid sequence of lens crystallin proteins. The P23T mutation in human $\gamma $D-crystallin (HGD) is associated with several different cataract phenotypes. The solubility of the protein shows an inverse temperature dependence. This is in contrast with the native protein. The replacement of Thr23 with a Ser or a Val residue shifts the location of the inverted solubility line to higher concentrations [1]. We describe the phase diagram of the P23V mutant of HGD, which exhibits aggregation, crystallization and liquid-liquid phase separation (LLPS). We have used QLS to probe the interactions of the protein in the soluble region of the phase diagram. We have developed a model to describe the observed retrograde solubility of the protein. Using PEG we introduce a so-called ``depletion interaction'' to further investigate the origin of the retrograde solubility. [1] A. Pande, O. Anunziata, N. Asherie, O. Ogun, G.B. Benedek, J. Pande, \textit{Biochemistry} \textbf{44}, 2491-2500 (2005).   

Free energy study of uranyl complexes across water-oil and water-oil+tri-butyl phosphate (TBP) interfaces

Free energy profiles of heavy metal ion complexes, UO$_{2}$ (NO$_{3})_{2}$, UO$_{2}$ (NO$_{3})_{2}$TBP$_{2}$, and TBP, across the water-hexane and water hexane+TBP (50{\%}/50{\%}) interfaces, were calculated from molecular dynamics simulations. These complexes and interfaces are relevant to recently developed heavy-ion separation techniques. The solute complex with TBP, UO$_{2}$ (NO$_{3})_{2}$TBP$_{2}$, shows strong interfacial activity in contrast to the free energy barrier for UO$_{2}$ (NO$_{3})_{2}$ at the water-hexane interface. Increased TBP concentration in the oil phase reduces the interfacial activity and better solvates the ion complexes and their ligands. The solute complex with TBP oriented parallel to the water-hexane+TBP interface binds more strongly to the hexane+TBP phase than to the pure hexane phase. The (un-complexed) TBP orientational probability distribution shows the polar head buried in water, while the nonpolar tails are buried in the oil phase, and hence TBP exhibits interfacial activity. The calculated density profiles at the interface show that TBP acts not only as a carrier for uranyl transport across the interface, but also as an ``interface modifier''. Our simulation results are in agreement with the recent study of uranyl transport across chemically modified membranes with TBP based metal ion carriers.   

Differential Dielectric Spectroscopy of Protein Solutions: Observation of Protein Interactions

Observation of a protein-protein interaction is illustrated by dielectric measurements on rabbit IgG (190 $\mu $g/ml) and Protein A (19 $\mu $g/ml) by a homemade dielectric cell and HP 4194A impedance analyzer. Frequency shifts of ratios 2.0 and 1.6 with respect to the individual relaxation characteristics of IgG and Protein A were obtained by dielectric spectroscopy, which has historically been used to determine the properties of solvated biomolecules to measure the hydrodynamic and electrical properties of individual proteins and of solution. Dielectric relaxation theory predicts changes in the dielectric relaxation characteristics of proteins due to protein interactions resulting in larger hydrodynamic volumes. Experimentally, bovine serum albumin, protein A, and rabbit IgG were added sequentially to phosphate buffer and the incremental dielectric changes were measured. The differential dielectric response, as a biophysical technique, gives insight into the interaction of the added protein with biomolecules in solution and can indicate the presence of protein-protein interactions.   

Session J38: Focus Session: X-ray and Neutron Instruments and Sciences II

Picosecond X-ray Pulse Generation at the Advanced Photon Source

Synchrotron radiation from storage ring-based facilities typically has a pulse length of many tens to many hundreds of picoseconds. In an effort to improve the temporal resolution of the study of dynamic and transient properties, the APS has been exploring the possibilities of producing short (a few picosecond) pulses though transverse deflection of the particle beam via radio frequency cavities installed in straight sections of the storage ring. These cavities produce a longitudinally coordinated vertical momentum to particle bunch that, when passed through an insertion device, then emits radiation with similar properties. Slits can then be used to time slice the beam or crystal optics can be employed to temporally compress the chirped radiation beam. Several approached for the implementation of this capability at the APS will be discussed along with the expected performance.   

TEMPO Beamline at the Soleil Synchrotron Radiation Source

TEMPO is a soft X-rays beamline now opening to the user community at the French synchrotron radiation source Soleil.[1] The two experimental stations will be based on photoelectron spectroscopy and will be mainly devoted to kinetic and dynamic studies of the electronic and magnetic properties of surfaces and interfaces. The high flux coupled to the energy resolution of the electron energy analyzer equipped with a new time resolved detector will allow the user to perform the following kind of investigations using photoelectron spectroscopy: i) the evolution of the chemical environment (surface coordination, chemical bonding with different elements) of selected chemical species at the surface using spectroscopic signatures; ii) the dynamics of magnetization reversal in nanostructures, using the temporal characteristics of Soleil at the scale of tens of picoseconds; iii) excited states using synchrotron pulses in the temporal range of a picosecond with pump-probe experiments with two photons (laser + synchrotron radiation). The beamline design and the technical solutions adopted for time resolved experiments will be presented along with the first results. [1] http://www.synchrotron-soleil.fr/anglais/science-and-users/experiments/tempo/index.htm   

Using crystal optics as a guard aperture in coherent diffraction imaging experiments

A crucial issue in coherent x-ray diffraction imaging experiments is how to increase the signal-to-noise ratio when measuring relatively weak diffraction intensities from a nonperiodic object. To achieve such a goal, a guard aperture that can block the unwanted parasitic scattering from the beam-defining aperture is necessary. The conventional guard-edge-type aperture, however, is not easy to align and may produce secondary scattering from itself. In this presentation we present a novel crystal guard aperture concept, in which a pair of multiple-bounce crystal optics is employed [Xiao et al, Opt. Lett. 31, 3194(2006)]. Different from the guard-edge-type aperture, the crystal guard aperture does not produce secondary scattering and therefore guarantee super-clean incident beam. The effectiveness of the crystal guard aperture method has been verified by the theoretical analysis and simulations based on Fresnel propagations of a dynamically diffracted Bragg wave. Recent coherent diffraction experiment results also confirmed the validity of this new guarding scheme.   

Phase retrieval from coherent x-ray diffraction data utilizing pre-determined partial information

We developed a phase retrieval algorithm that utilizes pre-determined partial phase information to overcome insufficient oversampling ratio in diffraction data. Implementing the Fourier modulus projection and the modified support projection manifesting the pre-determined information, a generalized difference map and HIO (Hybrid Input-Output) algorithms are developed. Optical laser diffraction data as well as simulated x-ray diffraction data are used to illustrate the validity of the proposed algorithm, which revealed the strength and the limitations of the algorithm. Finally, the proposed algorithm is applied to reconstruct images from coherent x-ray diffraction data of Au patterns. The proposed algorithm can expand the applicability of the diffraction based image reconstruction.   

Nano-Scale Resolution Spectro-Microscopy by Coherent X-ray Diffraction.

Coherent x-ray diffraction microscope, with its spatial resolution limited only by signal-to-noise ratio, has paved a route to a generic nano-scope relieved from crystalline specimens and destructive sample preparation. We advanced it further as a versatile spectro-microscopy. By using stark contrast in x-ray scattering lengths in the vicinity of atomic absorption edges, we could identify elements distribution at a nanometer scale. As the element specificity is acquired from direct x-ray absorption, it provides full flexibility for \textit{ab initio} imaging. Successful results on elemental mapping of nano-structures and single biological cells from 1-3 keV range coherent x-ray source will be presented.   

Diffuse scattering due to nanoprecipitation in Ni-Al-Si alloys

Ni-Al-Si alloys demonstrate the tendency to the formation of L1$_{2}$ ordered coherent precipitates. Diffuse X-ray scattering around both the fundamental and superstructure reflections is analyzed both theoretically and experimentally for Ni-Al-Si single crystal alloys with coherent ordered precipitates after stress annealing. The shape function of the coherent precipitates is discussed. Diffuse scattering reveals precipitation induced strong lattice distortions in the matrix. Distortions of the lattice together with the changes of the scattering factor in the volume occupied by the precipitate cause asymmetry of the diffuse scattering distribution. Oscillations of the diffuse scattering intensity are observed. The shape of the coherent precipitates is asymmetric with a 15{\%} elongation along the stress annealing direction. Research at ORNL sponsored by the Division of Materials Sciences and Engineering, Office of Basic Energy Sciences U.S. Department of Energy, under Contract DE-AC05-00OR22725 with UT-Battelle and at APS under contract No. W-31-109-ENG-38.   

Studies of stressor effects on silicon nanostructures using synchrotron X-ray nanodiffraction

Scaling in the semiconductor industry has been accomplished by reduction in gate length and oxide thickness to enable large-scale decreases in device area and improved transistor performance. Strained silicon offers improved mobility at no significant additional costs. Fundamental understanding of the structural equilibrium between the silicon and the stressors at the device level is critical in manipulating properties for performance gains. The inhomogeneous strains in the silicon channel of nanotransistor devices due the epitaxy and lattice mismatch between Si and SiGe were individually studied using X-ray nanodiffraction at the Advanced Photon Source. Diffraction intensity from the strained silicon of less than 1x10$^{-4}$ $\mu $m$^{3 }$and the SiGe stressor were mapped in reciprocal space around the points of (004), (115), and (-115). Lattice bending up to a few degrees at both sides of the Si/SiGe interface were measured, and the associated strains were quantitatively extracted in functions of the lattice curvature. The effect of the size of stressors was also studied.   

X-ray photon correlation spectroscopy in microfluidic ssytems

We present a new experimental method that combines X-ray photon Correlation Spectroscopy (XPCS) and microfluidics and allows the direct measurement of the mesoscale dynamics of various soft matter systems (e.g. colloids, polymers, biological molecules like proteins, RNA, etc.) under flow conditions. Such a setup reduces the risk of beam damage and also allows time-resolved studies of various processes taking place in mixing flowcells. In the experiments reported here, we have used colloidal suspensions of hard-spehere systems, and studied their Brownian dynamics under laminar flow. Our experimental results and theoretical predictions show that the diffusive (Brownian) dynamics of the colloids can be decoupled from the flow-induced, convective dynamics.   

Development of new x-ray absorption spectroscopy measurement

The x-ray absorption spectroscopy (XAS) is a powerful tool to probe electronic structure of valence states. However, its conventional measurements such as total electron yields or fluorescence yields often restrict sample conditions due to surface sensitivity and charging effects in an insulator or self- absorption effects, respectively. As an alternative, we found to extract XAS spectra from soft x-ray reflectivity measurements for transition metal compounds. We performed the soft x-ray reflectivity measurements on reference transition metal oxides, CoO and NiO, at Co and Ni $L_{2,3}$-edges, respectively, and successfully extracted the XAS spectrum using Kramers-Kronig relation from the reflectivity data. In the measurements, the scattering angle was set to be in specular conditions. Considering that the reflectivity is a photon-in and photon-out experiment, this result suggests an alternative to obtain XAS spectra for systems, in which the conventional XAS measurements are not applicable.   

Overcoming Experimental and Intrinsic Broadening in Excited State Spectroscopies using Richardson-Lucy Deconvolution

The oscillatory signature of the photoelectron interference phenomenon central to core shell spectroscopies is frequently broadened by experimental or intrinsic (i.e., core-hole lifetime) energy resolutions, limiting the interpretation of the measurement.~ For example, this problem occurs in x-ray absorption fine structure (XAFS) measurements of heavy elements where the core-hole lifetime is very short ($\hbar /\tau _{core-hole} \ge 5\,\mbox{eV})$, and also in non-resonant x-ray Raman scattering measurements where the instrumental resolution (typically $\sim $1 eV) can be nearly an order of magnitude larger than the intrinsic energy resolution.~ Given the small statistical uncertainties in typical XAFS data and in recent XRS measurements using dedicated multielement spectrometers, the question naturally arises as to deconvolving the data with respect to the known instrumental or intrinsic resolutions.~ Here, we demonstrate that the Richardson-Lucy iterative algorithm provides a robust maximum likelihood method for addressing this issue in both XAFS and XRS. We demonstrate this method on core-hole broadened Ag XAFS data and experimentally broadened diamond and graphite XRS data.   

Organic-modified and biological silica studied by synchrotron x-ray pair distribution function measurements

Biomineralization is a process by which living organisms create composite organic/mineral tissues which have hierarchical structures on micron and submicron scales. Fine control over mineral phase and morphology make biomineralization an important inspiration for materials science. It is often not appreciated that even amorphous minerals such as silica can exhibit hierarchical structure and special properties. One difficulty is that the molecular structures of amorphous phases can be hard to elucidate. We are exploring the use of pair distribution function measurements from synchrotron x-ray scattering to study silica structures, comparing both synthetic organic-modifed silicas and germanium-containing biosilica from diatoms. The raw scattering patterns show clear differences. We will discuss how these data can be scrutinized to determine what differences may be created at the molecular level by different silicification processes.   

Non-Resonant Inelastic X-Ray Scattering and Energy-Resolved Wannier Function Investigation of \textit{d-d} Excitations in NiO and CoO

We have investigated dipole-forbidden $d-d$ excitations in the non-resonant inelastic x-ray scattering (NIXS) spectra of NiO and CoO. The spectral weight of these Mott-gap excitations vanishes at small q, but dominates the large-q NIXS spectra and is highly anisotropic with well-defined nodal directions. Theoretical analyses based on energy-resolved Wannier functions within LDA+$U$ have shown the origin of the anisotropy to be selection-rules reflecting the underlying cubic point group symmetry. The measured and calculated orientation anisotropies of the NIXS spectra will be discussed and the anisotropies for NiO and CoO will be compared to demonstrate that such measurements represent sensitive probes of weak symmetry breaking in particle-hole wave functions.   

Wave-optical simulation of hard X-ray nanofocusing by precisely figured elliptical mirrors

Computer simulations of nanofocusing by elliptical mirrors are presented wherein the diffraction and propagation of coherent hard X-rays are predicted using wave-optical calculations. Surface height data acquired \textit{via} microstitching interferometry were used to calculate the complex pupil function of a mirror, taking into account the Fresnel reflectivity and treating the surface topography as an aberration to a perfect elliptical mirror. The reflected wavefield amplitude and phase downstream of the mirror were obtained by numerically evaluating the Fresnel-Kirchhoff diffraction integral. Simulated intensity profiles, and contours (isophotes) around the focal plane are presented for coherent illumination by a 15~keV point source, which indicate nearly diffraction-limited focusing at the 40~nm level. The effect of high spatial frequency microroughness on nanofocusing was investigated by low-pass filtering the Fourier spectrum of the residual height profile. Simulations using the filtered metrology data revealed that roughness length scales shorter than 0.1~mm have a minor effect on the focal spot size and intensity.   

Producing small size parallel x-ray beam by multi-plate crystal cavity with compound refractive lenses

To produce a coherent and extremely parallel x-ray source for advanced experiments, multi-plate crystal cavities consisting of compound refractive lenses were prepared on silicon wafers by lithographic techniques. The crystallographic orientation of the crystal is the same as that reported for x-ray resonators (Phys. Rev. Lett. 94, 174801, 2005). X-ray (12 4 0) back diffraction from these monolithic silicon crystal devices clearly showed interference fringes due to cavity resonance through the compound refractive lenses (CRL). However, the expected focusing effect from the CRL was not observed, but rather beam compression was detected. That is, the incident x--ray beam size of about 90$\mu $m across the CRL was reduced to 20$\mu $m. The beam size remained the same at different positions along the transmitted beam direction. Namely, a small sized parallel x-ray beam was produced. The origin of this beam compression mechanism is believed to be due to the competition between the multiple back reflection of the crystal cavity and the focusing of the CRL, in addition to crystal absorption.   

Alignment system for doubly curved crystal x-ray optics

Doubly curved crystal (DCC) optics efficiently diffract a large area beam from a laboratory point source to produce a monochromatic focus. DCC optics have application in crystallography, x-ray fluorescence, and imaging. In order to obtain maximum intensity from doubly curved crystal (DCC) optics, accurate alignment of the optic is crucial. A simulation model and alignment system have been developed which allow rapid optimization of the six axis position and angle of the optic in an x-ray system.   

Session J39: Focus Session: Hydrogen Storage I

Strategies for increasing hydrogen storage capacity and adsorption energy in MOFs

Storage of hydrogen in its molecular form is difficult and expensive because it requires employing either extremely high pressures as a gas or very low temperatures as a liquid. Worldwide effort is focused on storage of hydrogen with sufficient efficiency to allow its use in stationary and mobile fueling applications. DOE has set performance targets for on-board automobile storage systems to have densities of 60 mg H$_{2}$/g (gravimetric) and 45 g H$_{2}$/L (volumetric) for year 2010. These are system goals. Metal-organic frameworks (MOFs) have recently been identified as promising adsorbents (physisorption) for H$_{2}$ storage, although little data are available for their adsorption behavior at saturation: a critical parameter for gauging the practicality of any material. This presentation will report adsorption data collected for seven MOF materials at 77 K which leads to saturation at pressures between 25 and 80 bar with uptakes from 2{\%} to 7.5{\%}. Strategies for increasing the adsorption energy of hydrogen in MOFs will also be presented.   

Structure, lattice dynamics, and hydrogen adsorption properties of zeolitic imidazolate framework-8

Zeolitic imidazolate frameworks (ZIFs) are a new family of nanoporous metal-organic framework compounds that possess interesting zeolite-type structures with very high chemical stability [1,2]. We performed high-pressure isotherm measurements at various temperatures to characterize the H$_{2}$ adsorption properties of ZIF8, which consists of ZnN$_{4}$ clusters linked by 2-methylimidazole [H$_{2}$C$_{3}$N$_{2}$-(CH$_{3})$]. We find that the adsorption capacity is 1.2 wt{\%} at 77 K and 1 atm, while the maximal adsorption is 4.5 wt{\%} at 30~K and 3 atm. The initial heat of adsorption is $\sim $5 kJ/mol. Using neutron powder diffraction, we investigated the structure of ZIF8 and its associated hydrogen adsorption sites. These sites were directly determined using difference Fourier analysis and agree well with first-principles predictions. Furthermore, we studied the structural stability and lattice dynamics of ZIF8, combining inelastic neutron scattering and first-principles calculations. Several interesting phonon modes were identified. [1] X. C. Huang et al., Angew. Chem. Int. Ed. 45, 1557 (2006). [2] K. S. Park et al., Proc. Natl. Acad. Sci. U.S.A. 103, 10186 (2006).   

Quantum methyl rotations in zeolitic imidazolate framework-8: Inelastic neutron scattering and first-principles calculations

Zeolitic imidazolate framework-8 (ZIF8), which consists of ZnN$_{4}$ clusters linked by 2-methylimidazole [H$_{2}$C$_{3}$N$_{2}$-(CH$_{3})$], is a newly discovered framework compound with interesting hydrogen-adsorption properties. The presence of a single type of methyl group in its crystal structure renders ZIF8 an ideal system for studying quantum methyl rotations. Combining inelastic neutron scattering measurements and first-principles calculations, we studied the quantum rotational tunneling and phonons associated with the ZIF8 methyl groups. The rotational tunnel splitting is an extremely sensitive probe of the local potential. The measured tunnel splitting ($\sim $345 $\mu $eV at 1.4~K) indicated a nearly free quantum rotor ($i.e.$, a very low methyl rotational barrier), which is unusual for the solid state. With guest molecules adsorbed inside the framework, the rotational barrier was found to change significantly. Hydrogen adsorption decreased the barrier at low loading, yet increased it at higher loading. Methane adsorption nearly doubled the rotational barrier. These results provided clues for understanding the nature of the ZIF-guest molecule interactions.   

Hydrogen molecule binding to unsaturated metal sites in metal-organic frameworks studied by neutron powder diffraction and inelastic neutron scattering

Metal organic framework (MOF) materials have shown considerable potential for hydrogen storage arising from very large surface areas. However, the low binding energy of hydrogen molecules limits its storage capability to very low temperatures ($<$ 77 K), which is impractical for industrial applications. Using neutron powder diffraction (NPD), we have characterized the hydrogen adsorption sites in a selected series of MOF materials with exposed unsaturated metal ions. Direct binding between the unsaturated metal ions and hydrogen molecules is observed and responsible for the enhanced initial hydrogen adsorption enthalpy. The different metals centers in these MOFs show different binding strength and interaction distances between the hydrogen molecule and metal ions. The organic linker also affects the overall H$_{2}$ binding strength. Inelastic neutron scattering spectra of H$_{2}$ in these MOFs are also discussed.   

Infrared spectroscopy of trapped hydrogen in metal-organic-frameworks

We present a novel use of diffuse reflectance infrared spectroscopy to study the quantum dynamics of molecular hydrogen trapped within metal-organic-framework (MOF) hosts. This technique is particularly useful in the context of hydrogen storage since it provides detailed information about the intermolecular potential at the binding site. The spectra consist of quite sharp bands associated with the quantized vibrational and rotational motion of the trapped hydrogen. The vibrational bands are redshifted relative to the gas phase while the rotational sidebands contain an additional fine structure due to the orientational dependence of the binding potential. Results on MOF-5 reveal the presence of two primary binding sites. The first saturates at a loading concentration on the order of 4 {H$_{2} $} per Zn ion and has a binding energy of roughly 4 kJ/mole. The second has a somewhat lower binding energy. Both site produce an ortho to para conversion rate on the order of 30-50 \% per hour.   

Gas separation using novel materials: kinetics of gas adsorption on RPM-1 and Cu-BTC metal-organic frameworks~

We have measured the adsorption kinetics of two gases, freon and argon, on two microporous metal-organic framework materials, RPM-1 (or [Co$_{3}$(bpdc)$_{3}$bpy]$\cdot $4DMF$\cdot $H$_{2}$O, bpdc = biphenyldicarboxylate) and Cu-BTC (or [Cu$_{3}$(btc)$_{2}$(H$_{2}$O)$_{3}$], btc = benzenetricarboxylate). The measurements were conducted at comparable values of the scaled temperatures (T$_{isotherm}$/T$_{critical})$ for the respective gases. In our experiments, we monitor the pressure decrease as a function of time after a dose of gas is admitted into the experimental cell. The kinetics results obtained for both gases are similar on Cu-BTC, while they are significantly different in RPM-1. Our results indicate that RPM-1 has potential for gas separation for mixtures of species with dimensions similar to argon and freon; this is not the case for Cu-BTC MOF.   

High Pressure Volumetric Sorption Measurements On Small Samples At Low Temperatures

With recent efforts to develop new hydrogen storage materials needed for the hydrogen economy, fast and accurate hydrogen capacity measurements are needed to screen the numerous types of test samples. Having an ability to perform such measurements on small masses facilitates the research both by reducing the effort required to produce enough material for testing and by increasing measurement throughput since small samples have process faster. Here, we report on improvements to the volumetric method that allows measurements at high pressures ($\sim $ 80 bar) and low temperature (77 K). Critical components for the exact control of temperature gradients from room temperature to the LN2 bath as well as methods used to minimize the effect of the falling LN2 level will be described. Experimental procedure, instrument calibration, accuracy estimates, and instrument verification will be discussed. Finally, example data with samples will be used to show the functioning of the instrument.   

Novel H2 Sorption Measurements of Nanostructured Materials

To expeditiously develop nanostructured materials with high hydrogen sorption capacities, a novel volumetric measurement apparatus was designed and constructed that is suitable for rapid analysis of the small samples (milligram) typically available in the laboratory. The instrument enables both low temperature (down to $\sim $12K) volumetric measurements and high temperature (up to 1300K) sample processing without the need for sample transfers. The instrument has been used to study the hydrogen sorption behavior of chemically and thermally processed raw and purified nanostructured materials (e.g. nanotubes, activated carbons, polymers, aerogels). Hydrogen sorption, specific surface area, and binding energy results for different samples will be reported. The goal of these activities is to engineer hydrogen sorption materials that can ultimately meet the DOE's targets for vehicular fuel cell applications. Funding for this effort provided by the DOE's EERE Hydrogen Program within the Center of Excellence on Carbon-based Hydrogen Storage Materials, and by the Office of Science, Basic Energy Sciences, Materials Science and Engineering under subcontract DE-AC36-99GO10337 to NREL.   

Hydrogen Adsorption in Carbon-Based Materials Studied by NMR

Hydrogen adsorption in carbon-based materials such as boron-doped graphite and boron-doped single-walled carbon nanotubes (SWNTs) were investigated by nuclear magnetic resonance (NMR). $^{1}$H NMR is shown to be a sensitive and quantitative probe for detecting adsorbed gas molecules such as H$_{2}$, methane, and ethane. NMR measurements were carried out in-situ under given H$_{2}$ pressure up to a pressure of over 100 atm. From such $^{1}$H NMR measurement, the amount of adsorbed H$_{2}$ molecules was determined versus pressure. This gives an alternative method for measuring the adsorption isotherms where the H$_{2}$ signature is identified based on spin properties rather than weight or volume as in gravimetric and volumetric measurements. The measurement shows that boron doping has a favorable effect on increasing the adsorption enthalpy of H$_{2}$ in carbon-based systems. This work was done in collaboration with NREL and Department of Chemistry, University of Pennsylvania, within the DOE Center of Excellence on Carbon-based Hydrogen Storage Materials and is supported by DOE.   

In-situ electronic structure study of H$_2$ adsorption on HOPG

The storage of hydrogen in a both safe and compact manner is of great importance for, for example, hydrogen powered vehicles. Interesting candidates for dense storage of hydrogen are different types of carbon based nanomaterials: single (SWCNT) and multi-walled carbon nanotubes, C$_{60}$ and C$_{70}$. Various groups have reported different amounts of hydrogen stored using SWCNTs. Highly ordered pyrolytic graphite (HOPG) has similarities with the carbon systems mentioned above. Photon-in, photon-out techniques are well suited for measurements of the electronic structure of these materials under ambient hydrogen pressure. X-ray absorption (XAS) and emission spectroscopy (XES) measurements have been performed on HOPG under different hydrogen pressures. The measured partial density of states of this system will be presented.   

Hydrogen Adsorption on Nanoporous Biocarbon

As a part of the Alliance for Collaborative Research in Alternative Fuel Technology (http://all-craft.missouri.edu) we study activated carbons made from corncob, optimized for storing methane and hydrogen (H2) by physisorption at low pressure. We report here: (a) storage capacities of 73-91 g H2/kg carbon at 77 K and 47 bar, validated in three different laboratories (the 2010 DOE target is 60 g H2/kg system); (b) binding energies from H2 adsorption isotherms (c) temperature-programmed desorption data; (d) degree of graphitization of the carbon surface from Raman spectra; (e) pore structure of carbon from nitrogen and methane adsorption isotherms, and small-angle x-ray scattering. The structural analysis shows that the carbon is highly microporous and that the pore space is highly correlated (micropores do not scatter independently).   

Adsorption of supercritical carbon dioxide and propane in porous aerogel

We demonstrate that small-angle neutron scattering (SANS) can be used to determine the density and volume fraction of the adsorbed fluid phase in porous materials. The developed methodology is used to study the adsorption of near-critical CO2 and propane in aerogel as a function of pressure and temperature. For the first time the variation of the density and volume fraction of the adsorbed phase of near-critical fluids is reported and analyzed. These parameters are used to determine the absolute fluid adsorption without additional assumptions commonly used in the literature. The adsorption of CO2 and propane (8 g/g and 1 g/g, respectively) is found to be significantly higher in aerogels than in activated carbons and silica gels. The results provide new insights in the adsorption behavior of supercritical fluids, such as a non-monotonic variation of the density of the adsorbed phase and depletion of aerogel at high pressures.   

Session J40: QHE

Experimental observation of six valleys and an anisotropic integer quantum Hall effect on H-Si(111) surfaces

We have recently developed a new high mobility 2DES on a clean and atomically flat hydrogen-passivated Si(111) surface, where electrons are gated through an encapsulated vacuum dielectric. Our devices have exhibited peak scattering times, $\tau \sim $5ps, which exceed even the best Si(100) MOSFETs. We will discuss magneto-transport measurements made as a function of tilted magnetic fields in such a 2DES. The Si(111) surface is unique in that it has been calculated to have six valleys of equal energy in its ground state. Measurements at T=150mK and n$_{s}$= 6.5 x 10$^{11}$cm$^{-2 }$show clear signatures of the integer quantum Hall effect where contrary to predictions filling factors less than 6 are observed (i.e. $\nu $=6, 5, 4, 3, and 2). In addition, we have observed anisotropy in R$_{xx}$ with respect to the crystal orientation in a magnetic field range of 0$\le $B$\le $12T, which is also unexpected for an equally occupied six-fold degenerate system. As a result, an application of a non-interacting model whereby two valleys have a greater population than the remaining four has general agreement with the Landau level crossings at specific tilt orientations and with the anisotropy of R$_{xx}$ at low fields. At high fields, the presence of $\nu $=3,4 and 5 indicates that individual valleys are splitting but what is more interesting is the anisotropy of R$_{xx}$ at $\nu $=3 {\&} 4 toward in-plane magnetic fields.   

Anomalous Resistance Fluctuations in a Macroscopic 2DEG on a H-Si(111) Surface

We report the experimental observation of large ($\sigma (R)/\left\langle R \right\rangle \sim $15{\%}) resistance fluctuations as a function of electron density for a high mobility 2DES induced on a free H-passivated Si(111) surface in the strongly `metallic' regime. The observed fluctuations are reproducible and two orders of magnitude larger than the time-dependent noise. As the contact spacing ($\sim $1mm) is four orders of magnitude larger than the mean free path length ($\sim $100 nm), an explanation in terms of universal conductance fluctuations seems implausible. Because the dielectric is vacuum, the dominant scattering centers are located right at the surface. As discussed in [1], this 2DES has 6 unequally occupied valleys, which leads to an anisotropic longitudinal resistance. Interestingly, we note a strong anti-correlation between the fluctuations observed for orthogonal current directions. Furthermore, the fluctuations appear largely insensitive to small magnetic fields ($\vert $B$\vert <$ 2T). We present a systematic experimental characterization of this phenomenon, including temperature dependence (0.15 to 14K), I-V characteristics, and the response to perpendicular and parallel magnetic fields up to 12 T. [1] See talk ``Experimental observation of six valleys and an anisotropic IQHE on H-Si(111) surfaces'' K. Eng \textit{et. al.}   

Spin Susceptibility of an Anisotropic 2D Electron System in an AlAs Quantum Well

We report measurements of the spin susceptibility in dilute 2D electrons confined to a 150A wide AlAs quantum well. In the absence of in-plane strain, the electrons in this well occupy two degenerate in-plane valleys with an anisotropic mass. We lift this degeneracy by applying symmetry breaking strain in the plane, thus obtaining a 2D electron system in a single, anisotropic, in-plane valley. In this system we observe an enhancement of the spin susceptibility over the band value that increases as the density is decreased. Yet, the spin susceptibility is suppressed compared to the results of quantum Monte Carlo calculations. We attribute the suppression to the finite layer thickness of electrons and the anisotropic in-plane Fermi contour. Our measurements also show that the effective mass remains nearly constant and close to its value in bulk AlAs down to the lowest densities (r$_{s}$ = 10), in contrast to Si-MOSFET data.   

Effetcs of thickness and mass anisotropy on the spin susceptibility of the 2DEG in AlAs QWs

It has been demonstrated that device details, such as the transverse thickness, may affect in a substanial manner the spin susceptibility of the two dimensional electron gas (2DEG) which is realized in semiconducting heterostructures [1]. An important device detail in AlAs quantum wells(QW) is an in-plane mass anisotropy [2], which even in the regime with only one valley occupied is combined with a sizeable transverse thickness. For selected values of the well width appropriate to the experiments, we evaluate the effect of such thickness and, partly, of the mass anisotropy through a mapping of the 2DEG with mass anisotropy onto an `equivalent' isotropic 2DEG with effective mass $m^*=\sqrt{m_tm_l}$. We then critically compare our results with experimental meausurent and assess the importance of anisotropy effects that go beyond this simple minded mapping. \newline [1] S. De Palo et. al., Phys. Rev. Lett. 94, 226405 (2005). \newline [2] M. Shayegan et. al., phys. stat. sol. (b) 243, 3629 (2006)   

Scanning Probe Study of Edge States in a Two-Dimensional Electron System

The edge-state picture in two-dimensional electron systems (2DESs) in an applied perpendicular magnetic field successfully explains several properties observed in transport measurements in the quantum Hall regime. Although a handful of scanning probe experiments have resolved these states, a detailed understanding of the apparent edge-state width and resistance has yet to emerge. We apply Scannning Charge Accumulation Imaging, a cryogenic capacitance technique, to study the behavior of a 2DES in a GaAs/AlGaAs heterostructure sample. The sample contains a pattern of narrow metallic gates fabricated on the exposed surface, a 2DES 80 nm below the surface, and an underlying metallic electrode. By allowing charge to tunneling vertically from the underlying electrode our technique is especially sensitive to compressibility variations of the system. Here we report progress on resolving the edge state properties with this method.   

Characterization and modelling of one-dimensional states in a bent quantum Hall system

We study the transport properties of a one-dimensional (1D) wire state at the corner of a $90^{\circ}$ bent quantum Hall (QH) system. Such a system is formed in a corner-overgrown bent quantum well [1] by applying a tilted magnetic field $B$. The corner geometry itself serves as a sharp QH boundary and hosts strongly coupled 1D forward and reverse movers with no barrier in between. At different magnetic fields we measure a different conductance behavior of the 1D wire, depending on the QH filling factor $\nu$. In the integer QH regime, at equal filling factors $\nu = 1$ and $\nu = 2$ on both facets of the bent 2D system, we observe an insulating phase where the wire conductance decreases rapidly with decreasing temperature $T$ and DC bias Voltage $V_{DC}$. The integer filling factors $\nu>2$ show a critical behavior with only weak dependence on $T$ and $V_{DC}$. Spin-unresolved Hartree calculations of the dispersions of the corner states illustrate possible origins of the two observed phases [2]. The calculations also provide an insight into the electronic states in the bent QH system, which has no analogue in a planar structure. \noindent[1] M. Grayson, D. Schuh, M. Huber, M. Bichler, and G. Abstreiter, APL 86,); \noindent[2] M. Grayson, L. Steinke, D. Schuh, M. Bichler, L. Hoeppel, J. Smet, K. v. Klitzing, D. K. Maude, and G. Abstreiter, submitted;   

Temperature Limited Spectroscopy of the Quantum Hall Liquid

We present spectra of the tunneling density states of a two dimensional electron gas (2DEG) in GaAs over a range of 30 meV centered about the Fermi surface, revealing the beautiful structure present in these systems far from the Fermi energy. Using these measurements, we examine the dependence of the exchange-enhanced spin-gap on electron density (0-$4\times{}10^{11}\ \mathrm{cm}^{-2}$) and magnetic field, observe induced spin splittings in Landau levels away from the Fermi energy, and compare measured linewidths to the expected lifetime broadening from interactions. The measurements are performed using time domain capacitance spectroscopy which uses short pulses to drive electrons perpendicularly between the 2DEG and a bulk electrode while monitoring the induced image charge on an isolated electrode. Using a very low duty cycle maintains a 100 mK electron temperature even when injecting electrons at energies 1000 times larger than $\mathrm{k}_{\mathrm{b}} \mathrm{T}$, while the absence of in-plane current allows us to continue to measure when the 2DEG is fully depleted or has vanishing in-plane conductivity.   

Time-Dependent Conductivity in the Quantum Hall Effect

We analyze the quantum Hall effect in a 2D electron system with a periodic potential. We show that the conductivity begins to oscillate in time when an electric field is suddenly switched on. Assuming linear response, we obtain an analytical expression of the time-dependent conductivity. The time dependence comes theoretically from the Fourier components of the response function with nonzero frequencies. The amplitude of the oscillation gradually decreases as a function of time and the conductivity eventually approaches to its average, which is given by the Chern number according to the Kubo formula. We numerically calculate the temporal oscillation of the conductivity in the case of a superlattice in a semiconductor. We find that both the Hall and diagonal conductivities oscillate with a period of pico to nano seconds.   

Simple-layered high mobility field effect heterostructured two-dimensional carrier devices.

We present in this talk a two-dimensional electron heterostructure field effect device of simplistic design and ease of fabrication that displays high mobility electron transport. This is accomplished using a high efficacy contacting scheme and simple metallic overlapping gate, obviating dopant layers. The resultant devices demonstrate adjustable electron densities and mobilities larger than 8x10$^{6}$ cm$^{2}$/V-sec at the highest densities of 2.4x10$^{11}$/cm$^{2}$. This device type provides a new experimental avenue for studying electron correlations and may answer demands for routine fabrication of practical HEMTs. In one extension of this work, using the same basic heterostructure design with appropriate contacts diffused to the AlGaAs/GaAs interface, a 2D high mobility hole channel can be populated through field effect, resulting in transport with clearly resolvable quantum Hall states at high magnetic fields. Finally, we also present a method for producing mesoscopic structures in these field effect 2D electron systems, which takes advantage of the extensive electron density control available when both the bulk and mesoscopically defined electronic densities can be tuned via overlapping gate arrays.   

2D hole transport in GaAs MOSFETs

We have fabricated enhancement-mode p-channel GaAs MOSFETs on the (100) surface of undoped GaAs/Al$_{x}$Ga$_{1-x}$As heterostructures, using atomic-layer-deposited Al$_{2}$O$_{3 }$dielectric and Ti/Au gate, and measured their transport properties. The capacitively induced 2D hole density (p), determined from Shubnikov-de Haas oscillations, can be tuned from 9x10$^{9 }$to 3x10$^{11}$cm$^{-2 }$by applying a negative gate bias. Within this range, the effective capacitance is close to that of an ideal parallel plate capacitor, and the leakage current remains virtually zero. The highest possible density is limited by the heterostructure design, not by gate leakage. The 2D hole mobility at T=0.3K increases with p and saturates at 6.3x10$^{5}$ cm$^{2}$/Vs for p$>$2.3x10$^{11}$cm$^{-2}$. In this talk, we present data on transistor drain current-voltage characteristics, as well as magneto transport and the quantum Hall effects.   

Towards Novel Electron and Hole Structures: Characterizing n- and p-Type (110) GaAs/AlAs

The (110) facet of GaAs holds promise for new devices because it is the cleave facet, allowing cleaved edge overgrowth [1] and corner overgrowth structures [2], and because recent work demonstrates that Si can also function as an acceptor for high mobility p-type structures [3]. We present characterizations of p-doped GaAs on (110) wafers and cleave facets, which show an interesting spin-orbit coupling effect, resulting in a spin-index anticrossing in the lowest Landau level. n-doped AlAs on the same (110) facet shows a strong anisotropy, suggesting that only a single anisotropic-mass valley is occupied. Initial attempts at combining n- and p-type doped structures in coplanar 2D systems will be presented. [1] M. Grayson, APL 87, 212113 (2005); [2] M. Grayson, APL 86, 032101 (2005); [3] F. Fischer, APL 86, 192106 (2005).   

Electron correlations and single particle physics in the integer quantum Hall effect

Recently, the local inverse compressibility of an integer quantum Hall system has been measured (Ilani et. al., Nature {\bf 427}, 328(2004)) as a function of electron density $n$ and magnetic field $B$, a quantity which mimics the change of the chemical potential in the system with respect to the particle density. These compressibility patterns reveal signatures of charging in the quantum Hall system, which in general are attributed to Coulomb interaction in correlated systems and are incompatible with single-particle physics. We have developed a mean-field description for these charging patterns within a spin-unrestricted Hartree-Fock approximation, but allowing for charge rearrangement in the ground state with respect to changes in $n$ and $B$. Our results match the experimental observations at least in the localized regions and are compatible with the single-particle picture of the localization-delocalization transition. In agreement with experimental data we show that electron-electron interaction cannot be neglegted in a comprehensive theory of the integer quantum Hall effect.   

Experimental technique for the realization of multiple simultaneous Hall effects

The Hall effect is examined in multiply connected specimens through transport studies of a double-boundary geometry fabricated on two-dimensional (2D) GaAs/AlGaAs heterostructures. The study begins with the identification of a complement of the Hall bar, called the ``anti Hall bar,'' which helps to generate a Hall effect within interior boundaries, when current is passed via interior contacts. Then, a double current technique is applied in an `anti-Hall bar within a Hall bar' geometry, which includes the above-mentioned `anti-Hall bar' within the usual Hall bar configuration. The double current experiments show that (i) the Hall effect on a boundary depends only on the current injected via the same boundary, while (ii) the magnetoresistive voltages are insensitive to the origin of the current within the specimen. The experimental results are compared with the recent theoretical modeling of this configuration by Oswald et al. (Phys. Rev. B 72, 035334 (2005)).   

Session J41: Photonics/Optoelectronics

Chemically and Electrically Tunable Block Copolymer Photonic Gels: Exceptionally Large Tunability via Uniaxial Swelling

Many potential applications of photonic band gap (PBG) materials have been limited by their insufficient tunability and sensitivity as well as their difficulty of fabrication. Self-assembly of block copolymers could provide an unraveling route for addressing these problems. Here we report the preparation of chemically and electrically tunable PBG materials from self-assembly of PS-b-P2V. Because of the unique meso-lamellar gel structures, where the swellable P2VP gel layers are bound on the glassy PS layers, our photonic gels expand only uniaxially normal to the substrate, and which gives extremely large tunability from UV-VIS to NIR region. Using the osmotic deswelling, we demonstrate dynamically and reversibly tunable photonic gels in aqueous salt solution. We also demonstrate the electrical tunability for the same system by using electrophoretic control of domain spacing. We anticipate our material can be applicable to many novel applications including an active component of display devices, electrically tunable lasers, and electrically controlled photonic switches.   

Polarization conversion in a silica microsphere

Light transmitted through a waveguide coupled to a whispering gallery mode (WGM) resonator will experience a non-linear phase shift. Microsphere WGMs can be of two orthogonal polarizations, and they are non-degenerate. We can exploit the non-degeneracy of the modes and the induced phase shift to convert the incoming polarization to its orthogonal one with high efficiency. We have experimentally demonstrated a conversion efficiency of 75\% on a silica microsphere.   

Light Propagation in Colloidal Glass: enhancement of scattering at reduced coordination number

We measure the propagation of light through random films of strongly-scattering microspheres as a function of the mean number of contacts per particle (the coordination number, $Z)$. Two kinds of colloidal spheres are mixed to prepare dried films with random structure. Latex spheres coated with a high-index ZnS are mixed in various ratios with PMMA spheres and the PMMA spheres are then dissolved by acetone. The transport mean-free path of light $l$* is then extracted from measurements of coherent backscattering of light from the films. We found a minimum of $l*$ (maximum of scattering) occurs around $Z$=4, not in a close-packed film (Z$\sim $11), which is counterintuitive. In a simple mean field model, decreasing $Z$ reduces the local average refractive index and enhances the optical contrast of each scattering sphere with the effective background, thus reducing $l$*. These results may guide our understanding of the propagation of waves in random media in general and may lead to new photonic materials. This work is supported by the NSF-sponsored UMASS MRSEC. A.D.D is a Cottrell Scholar of the Research Corporation.   

Bio-functional subwavelength optical waveguides for chemical detection.

Compact, reusable biochemical sensors are highly desirable for rapid on-site analytical analysis of gas and liquid mixtures in the field. A key to miniaturizing devices and providing reliable quantitative chemical identification of small sample volumes hinges on the development of novel materials capable of multiple complementary sensing modalities. Here we build a bio-functional optical sensing platform that utilizes the evanescent field of a subwavelength nanowire waveguide to detect single biochemical molecules. The optical cavities are integrated into polymeric flow cells for rapid chemical functionalization, multiplex sensing and reusability. The biocompatibility of the waveguide is assured by assembling fluid lipid membranes tagged with receptor molecules within the evanescent field. With the advantage of carrying out multiple spectroscopy techniques such as absorbance, fluorescence and surface enhanced Raman spectroscopy (SERS) on sub-picoliter probe volumes, these evanescent field sensors offer a unique design for portable all-optical detection systems. This work was performed under the auspices of the U.S. Department of Energy by University of California, Lawrence Livermore National Laboratory under Contract W-7405-Eng-48.   

Focused Surface Plasmons for Enhanced Raman Scattering

Surface plasmon polaritons launched at concentric arcs can be focused into a sub-wavelength wide focal spot of high near-field light intensity. When increasing the number of arcs from one to eight the focused intensity increases by a factor of 30. The focused plasmons give raise to enhanced Raman scattering from R6G molecules placed in the focal area. By exploiting the polarization dependence of the focusing we establish an enhancement of the Raman signal by a factor of three. Our results show that focusing of propagating surface plasmons on flat metal surfaces may be an alternative to localized plasmons on metal nanostructures for achieving enhanced Raman scattering. In particular, a flat metal substrate enables better control over the local electric fields and the placement of analyte molecules, and, therefore, ultimately better fidelity of Raman spectra.   

Evanescent field response to small patterned features on a planar waveguide biosensor

A novel optical sensor is developed on the concept that the evanescent field surrounding the core of an appropriately designed waveguide can be very sensitive to the local refractive index of the cladding surrounding the core. The formation of a protein- or ssDNA-based adlayer via specific binding of an analyte target to one of several localized patches of immobilized biological molecule probes can be detected by measuring the change in the evanescent field of the waveguide. The biosensor studied is based on a waveguide fabricated from a high refractive index silicon nitride thin film surrounded by a lower cladding of silicon dioxide and an upper cladding of air or water. To detect small features, the thickness of the waveguide core is optimized to be a fraction of a micrometer. In this study, the response of the sensor to small polymer features (1$\times $1 $\mu $m$^{2}$, 20 to 80 nm thick) with indices of refraction comparable to biological material is evaluated using near field scanning optical microscopy (NSOM). The results have significant implications for the density of a sensor array, particularly in two-dimensional arrays. Issues such as transient interference and positional dependency are addressed.   

Surface-plasmon-polariton band structure of nanostructured multi-layer systems

We calculate the optical response of multi-layer systems where at least one layer contains arrays of metalic substructures of nanometer size. The dependence on lattice constant and excitation energy is studied systematically. An - at first glance non-intuitive - dependence of the plasmon intensity on the geometry of the metalic substructures is observed numerically. Generalizing the work of Park and Lee [PRL 95, 103902 (2005)], the results are interpreted in terms of plasmon-plasmon and plasmon-radiation couplings of different strength. A rich surface-plasmon-polarition bandstructure with ``gaps'' due to avoided crossings is seen. Super- and subradiant modes are found in the vicinity of those features.   

Photonic bandgap properties of gradient-index thin films with one-dimensional periodicity and anisotropic structure.

Glancing angle deposition is a single-step fabrication technique providing \textit{in-situ} control over the internal columnar structure of the deposited thin film. Using this technique, the density of thin TiO$_{2 }$films are varied sinusoidally along the substrate normal direction with a physical periodicity equivalent to wavelengths in the visible spectrum. As radiation propagates in the film, constructive and destructive interference effects lead to the observation of photonic bandgap properties. Optical characterizations of the films are performed to examine the properties of the photonic stopband. These results examine stopband behavior for light at non-normal incidence and of different polarization states. The incorporation of structural defects in the film to introduce and control optical states within the stopband is also studied. The observed properties are related to the structure of the films using effective medium theory and solution of Maxwell's equations in anisotropic media.   

Periodically bent porous metal oxide nanostructures as linear polarization selective Bragg filters

The periodically bent nematic liquid crystal (LC) phase is a theoretical arrangement characterized by uniaxial rod shaped mesogens in an s-shaped configuration. The periodically bent phase is predicted to yield strong linear polarization selective Bragg effects due to the modulation of the extraordinary refractive index throughout the sample. Unlike the analogous selective circular Bragg effects obtained from the readily achievable chiral nematic LC phase, the periodically bent nematic phase is comparatively more difficult to realize. Glancing angle deposition is a physical vapour deposition technique which allows for the fabrication of isolated columnar nanostructures, suitable for LC infiltration. Using advanced substrate motion control, s-shaped films and other designs that incorporate a modulated extraordinary index are easily fabricated. Experimental transmittance and reflectance spectra are presented to examine the strength and polarization selectivity of the stopband for as-deposited metal oxide films. The dependencies on film material and deposition angle are investigated. The observed results are compared to simulations obtained using a frequency domain electromagnetic mode solver and to earlier results obtained for the periodically bent nematic LC phase.   

Extraordinary optoconductance in InSb-Au thin film hybrid structures

Extraordinary optoconductance (EOC)\footnote{K. A. Wieland \textit{et al}., Appl. Phys. Lett. {\bf88}, 52105 (2006).} is the third reported effect following extraordinary magnetoresistance (EMR) and extraordinary piezoconductance (EPC) that is based on the geometric enhancement of the conductivity of a metal semiconductor hybrid structure (MSH). Using a Van der Pauw plate setup, the voltage of the bare sample is compared directly to that of the MSH to determine the EOC, defined as $\{V_{MSH}-V_{bare}\}/V_{bare}$. In GaAs-In at 30K EOC $\approx$500\% has been observed. Bulk InSb-In structures have a room temperature EOC $\approx$50\%. Prior research\footnote{K. A. Wieland \textit{et al}., Phys. Rev. B., \textbf{73} 155305 (2006).} has shown that the one may increase the EOC by using a semiconductor with a large differential e-h mobility. Te doped $n$-type InSb thin films (n$= 2.11\times10^{22}$ m$^{-3}$ and $\mu_{e} =4.02$ m$^2/$Vs) are therefore prime candidates for room temperature EOC. Thin film Au shunts were chosen because of their coefficient of thermal expansion and non-magnetic properties. These MSHs were illuminated with Ar focused 488.0nm light and studied as a function of the position of the laser spot and temperature.\\   

200 nm deep ultraviolet photodetectors based on AlN

Aluminium nitride possesses the widest direct bandgap ($\sim $6.1 eV) among all semiconductors and appears to be promising material for the development of deep ultraviolet (DUV), vacuum UV and extreme UV (EUV) detectors. Detectors based on AlN would overcome many limitations imposed by Si technology. The 6.1 eV bandgap not only permits the intrinsic solar blindness but also allows the room temperature operation, which relives the requirements of optical filters and cooling system. We report the fabrication and characterization of Metal-Semiconductor-Metal (MSM) deep ultraviolet photodetectors based on high quality AlN epiyayers grown on sapphire substrate using metal organic chemical vapor deposition. The fabricated detectors have 80 $\mu $m x 80 $\mu $m as an active area with interdigital fingers for Schottky contact with 2 $\mu $m/2 $\mu $m to 4 $\mu $m/ 4 $\mu $m finger width/spacing. The photodetectors exhibit lowest cutoff wavelength at 207 nm with peak responsivity at 200 nm. The AlN MSM detectors possess outstanding features such as extremely low dark current, high breakdown voltage, high responsivity, and high UV to visible rejection ratio. These results demonstrate the high promise of AlN as an active material for DUV opto-electronic device applications.   

Microlens Fabrication by Fluid Deposition of Curable Optical Polymers

We utilize a Fluid Microplotter instrument to produce microlenses fabricated from UV-curable polymers with a variety of focal lengths and diameters. The diameters are as small as 10 microns and focal lengths range controllably from 25 microns to several hundred microns. Microlenses such as these are of interest to optics companies for use with diode laser products, for coupling light into waveguides, fiber optics, and other optical components. In addition to precise and repeatable control of the position, size, and focal length of the lenses, we are able to produce them in moderately large arrays consisting of hundreds of lenses, each with a user-defined location. We dispense fluid from a piezoelectrically driven glass micropipette attached to a robotic positioner. Through electronic and positional control of the pipette we are able to precisely vary the position, size, and volume of the lenses. By varying the temperature and surface coatings on the substrate we are able to alter the wetting properties and further control the size and focal length of the lenses. Fabrication and physical properties of the lenses will be discussed as well as preliminary results of the lenses in a prototype device scheduled for future production by a commercial optics company.   

Efficient calculations of the dielectric response in semiconductor nanostructures for optical metrology

The ability to predict the optical and dielectric properties of semiconductor nanostructures is highly desirable, in order to efficiently couple theory and experiment in the characterization and design of nanostructured materials. For example, in designing optimal procedures for optical metrology, the knowledge of the full dielectric matrix of nanostructures as a function of size and shape would be desirable. However, methods based on Density Functional Theory (DFT) are only computationally feasible for sizes below 2 nm and it is still difficult to extend them to the 5 - 25nm size regime of interest to many experiments. In order to make contact between theory and experiment for this important class of systems, we have developed computational techniques based on the empirical pseudopotential method (EPM). Here we compare EPM and DFT results for small nanostructures and we then use EPM results to discuss the properties of Si spheres and rods in the larger size regime.   

Session J42: Focus Session: Metal-Semiconductor Interfaces

Shape transition and migration of TiSi$_{2}$ nanostructures embedded in a Si matrix

While the embedding of epitaxial nanostructures, like SiGe, on Si surfaces does not affect their epitaxial position on the substrate, this study establishes that under conditions of epitaxial Si deposition, TiSi$_{2}$ nanostructures undergo a shape transition and ``migrate'' to the surface. They were grown on a Si(001) surface by depositing 0.5 nm of Ti at 750\r{ } C and annealing for 2 min. They were then buried under a Si capping layer at different temperatures and thicknesses. AFM and XREM have been used to study their shape, geometry and evolution. Many of the buried structures were found to display a near uniform hemispherical shape. Their density and size were observed to be temperature dependent. The buried islands induce inhomogeneous stress profiles on the capping layer surface. The AFM images of the islands showed square holes at the surface aligned along the [110] directions suggesting that the Si layer was terminated along {\{}111{\}} planes. Many islands displayed faceting observed in cross-sectional electron micrographs. The observed structural changes are rationalized in terms of the interplay between thermodynamics and kinetics, solid state capillarity, and the roughening transition.   

Growth and stability of dysprosium silicide nanostructures on Si(001)

The growth and coarsening dynamics of epitaxial dysprosium silicide nanostructures on Si(001) are observed using tunable ultra-violet free electron laser excitation for photo-electron emission microscopy (PEEM). A dense array of compact silicide nanostructures is observed to coarsen during annealing at 950-1050C. Some of the nanostructures grow into large flat-topped rectangular islands at the expense of smaller islands which disappear via Ostwald ripening. The coarsening rate of the island distribution increases with increasing temperature, and the formation of a flat top on the growing islands is related to strain relaxation. Additionally, the shape and growth rates of the islands may be influenced by the island crystal structure and/or local island distributions. A subsequent deposition of dysprosium onto the surface results in the nucleation of new island and nanowire structures. Immediately after the deposition is terminated the nanowires begin to decay from the ends while the larger island structures grow. The decay of the wires can be attributed to Ostwald ripening and is explained in terms of the Gibbs-Thompson relation, where the high adatom concentration at the nanowire ends leads to the diffusion of adatoms away from the wires towards the larger surrounding structures. In situ movies will be presented which detail the growth and coarsening processes.   

Spatial First-passage Statistics of Al$/$Si(111)-($\sqrt{3}\times\sqrt{3}$) Step Fluctuations: Implications for Nanoscale Structures

The step-edges on a multi-component surface of Al$/ $Si(111)-($\sqrt{3}\times\sqrt{3}$), observed via scanning tunneling microscopy, fluctuate in thermal equilibrium over a temperature range of 720K-1070K. For step lengths L = 65-160 nm, the measured first-passage spatial persistence and survival probabilities are found to be temperature independent and thus universally applicable. The power-law functional form for spatial persistence probabilities is confirmed, and the symmetric spatial persistence exponent is measured to be $\theta = 0.53 \pm$ 0.05, in agreement with the theoretical prediction $\theta=\frac{1}{2}$. The survival probability is found to scale with y$/$L, where y is the distance along the step edge. The functional form of the survival probabilities agrees quantitatively with the theoretical prediction, which decays exponentially as exp(-y/y$_{s}$) for small y$/$L. The experiment finds the decay constant to be y$_{s}/$L= 0.076 $\pm$ 0.033 for y$/$L $\le$ 0.2. The physical implications of these results for the predictability of nanoscale displacements and thus on device design and manufacturing will be discussed.   

Au-Induced Nanostructuring of Vicinal Si Surfaces

The deposition of extremely small amounts of metal onto vicinal semiconductor surfaces can cause dramatic changes in morphology on a nanometer scale. Recently this has been exploited to self-assemble arrays of atomic chains that exhibit bands with intriguing one--dimensional (1-d) metallic behavior. Depositing Au onto a vicinal Si(111) sample tilted either towards or away from the $[11\overline 2 ]$ can produce an array of 1--d chains running along the $[1\overline 1 0]$direction. To investigate the nanofaceting underlying chain formation, we have measured the surface morphology of several miscuts as a function of Au coverage using scanning tunneling microscopy. Samples oriented 3.8\r{ }, 8\r{ }, and 12.5\r{ } from [111] towards $[11\overline 2 ]$ have been measured with Au coverages ranging from less than 0.06~ML up to 0.5~ML. All surfaces exhibit nanofacets with orientations that depend critically on Au coverage. On the 8\r{ } sample, while the exact nature of the surface morphology depends on Au coverage, below 0.32~ML all surfaces incorporate (775)-Au nanofacets. Similarly, (775)-Au facets are also observed on the 3.8\r{ } sample. At 0.17~ML the surface consists of (111)7x7 and (775)--Au nanofacets. At 0.4 ML the (111) terraces transform from 7x7 to a 5x2, and the surface consists of Si(111)5x2--Au terraces separated by (775)-Au facets. The persistence of the (775)-Au facet reinforces the idea that it represents a low energy facet on these Au modified vicinal surfaces.   

Density Functional Study on Energetic Instability of the 5$\times$2 structure on Au/Si(111) and Au/Si(775) surfaces.

Atomic configurations of Au/Si(111) surface have been examined extensively. However, there are only few systematic theoretical analyses taking account of the variation of Au and Si coverages. Keeping this in minds, we have done such a systematic analysis on the energetic stability of various models proposed for Au/Si (111) structures so far using density functional calculations. As a result, we obtained good agreement with experimental results on the periodicity of energetically stable structures except that none of the models with 5$\times$2 periodicity are predicted to appear in our calculation. Then, we examined the possibility of stabilizing some of the 5$\times$2 models by the effects of steps, using Au/Si(775). We found that none of the 5 $\times$2 structures are stable even on Si(775). These results suggest that we have to explore a new model to explain the observed 5$\times$2 structure.   

One-dimensional plasmons in atom wires on Si(111)-5x2-Au

One-dimensional (1D) collective excitation in atom wires self-assembled on the Si(111)-5x2-Au surface is investigated. Electron scattering spectroscopy using highly collimated slow electron beam has detected a characteristic sound wave-like excitation that exhibits strong anisotropy along the wires. This excitation occurs in dipole scattering regime and its lifetime drops as a function of momentum. From these features, the observed loss is assigned to a unique plasmon mode confined in the atom wires which are revealed to be strongly metallic.   

First-principles calculations of low coverage growth of Ba on Si(001)

Ba is of interest to the semiconductor industry for it's possible use in replacement gate oxide materials and for possible use in a buffer layer between Si(001) and Ba containing dielectric materials. Thus it is of importance to understand the initial stages of growth. This paper reports state-of-the-art electronic structure calculations on the deposition of Barium on the technologically relevant, (001) orientated silicon surface. We identify the surface reconstructions from zero to one monolayer and relate them to previous theoretical studies of low coverage Ba growth and Sr growth.   

Controlled self-organization of atom vacancies in pseudomorphic adsorbate layers: Ga/Si(112)

Ga adsorption on the Si(112) surface results in the formation of pseudomorphic Ga chains. Compressive strain in these atom chains is effectively released via the creation of adatom vacancies and their self-organization into almost evenly spaced vacancy lines (VLs). Here, we present a detailed study of these line defects using scanning tunneling microscopy, low energy electron diffraction, and density functional theory calculations. The average spacing between line defects can be varied continuously, within limits, by adjusting a single control parameter: the chemical potential of the Ga adatoms. The present study not only establishes the driving force and control parameter for self-organization, but also allows for precise determination of the relevant thermodynamic quantities, even for disordered nx1 sequences that cannot be evaluated directly with DFT.   

Generalized Electron Counting in Determination of Metal-Induced Reconstruction of Compound Semiconductor Surfaces

Based on theoretical analysis, first-principles calculations, and experimental observations, we establish a generic guiding principle, embodied in generalized electron counting (GEC), that governs the surface reconstruction of compound semiconductors induced by different metal adsorbates. Within the GEC model, the adsorbates serve as an electron bath, donating or accepting the right number of electrons as the host surface chooses a specific reconstruction that obeys the classic electron counting model. The predictive power of the GEC model is illustrated for a wide range of elements from alkali to transition metals, and to noble metals.   

Transport properties of electrodeposited Ni films on GaAs(110)

Epitaxial ferromagnetic materials grown on semiconductors are potential candidates for new class of spintronic devices which can be used as contacts for spin injection. In this respect electrodeposition has proven to be a promising method as it avoids interface because it is a low-energy, room temperature process. Here we report the electrical transport properties of electrodeposited Ni thin films on GaAs(110) substrate. We have performed resistivity and Hall Effect measurements as a function of film thickness. The results show that the thickness dependence of the electrical resistivity follows the Fuchs$^{1}$ model. The bulk resistivity at room temperature is close to the reported bulk properties. The low temperature resistivity values are dominated by surface scattering with the bulk resistivity being essentially zero. The low temperature ordinary Hall coefficient $R_{o }$of the films is almost independent of the film thickness, whereas the spontaneous Hall coefficient R$_{s}$ decreases with increasing film thickness. The spontaneous Hall coefficient scales with the resistivity as R$_{s}\propto \rho ^{1.3}$, indicating a scattering mechanism which is mixture of both skew scattering and side jump scattering. 1. K. Fuchs, Proc. Camb. Phil. Soc. \textbf{34}, 100 (1938)   

Interfacial Structure of Fe-GaAs(001) with annealing and substrate surface morphology

The primary obstacle confronting `spintronics' is the inability to efficiently create spin resolved current within a semiconductor. There are two leading approaches, create a magnetic semiconductor with a Tc above room temperature, as yet elusive and the ability to inject a spin current from a transition metal film into a semiconductor. The issue at hand in the latter is the `spin scattering' interface between the overlayer and substrate. This has led to many studies to understand the magnetic evolution of very thin transition metal films on semiconductor substrates, most notably Fe on GaAs. Here we will report on the recent and ongoing high flux grazing X-Ray diffraction studies of in-situ grown Fe films on prepared GaAs(001) surfaces at the Advanced Photon Source. Extended specular diffraction data has been fit with kinimatic calculations to model the vertical structure of the film and interface. In plane grazing diffraction investigates the in-plane Fe relaxation process. This data has been coupled with pregrown Al capped Fe films also on GaAs(001) prepared at University of Arizona. The films have been annealed at incremental temperatures and have been studied with BLS in order to correlate the magnetic in--plane anisotropic nature with the in-plane film strain.   

Interaction of PH$_{3}$ with Si(111)-7x7 Surfaces: Adsorption, Desorption and P-segregation.

The reaction of PH$_{3}$ with Si(111) surfaces has been studied in the early 1990s by a number of analytical techniques including UPS, AES, EELS and ESD. We are interested in revisiting this system with an emphasis on P-delta layers formation for their potential technological applications. Here we present a STM study of PH$_{3}$ adsorption on Si(111)-7x7 at 300 K and 900 K. Reacted and unreacted adatom sites after room temperature exposures can be identified by different biases. Similar to the ammonia adsorption the center adatoms are more reactive than corner adatoms. A careful analysis of the surface coverage of PH$_{3}$, PH$_{2}$, and H, we conclude that most of PH$_{3}$ is dissociatively adsorbed on the surface at initial exposure generating H and PH$_{2}$ adsorption sites followed by molecular adsorption of PH$_{3}$. More interestingly, a quasi-regular $6\sqrt 3 $ surface structure forms by PH$_{3}$ exposure at 900 K. The dangling bonds of Si(111)-1x1 are completely terminated by a layer of P atoms. No epitaxial Si can be grown on this surface at low temperatures. Annealing the Si covered surface to 900 K recovers the $6\sqrt 3 $structure due to P segregating to the surface. Short heat pulses were used to find that P desorbs at 950 K but 7x7 domain was not observed until 1070 K.   

Session J43: Focus Session: Materials for Quantum Information Processing II

Solid-state materials and devices for single-photon generation and more

A single-photon device, which ideally emits exactly one photon on demand into a definite quantum state, can be constructed from a single atom or atom-like system excited by optical pulses and coupled to an optical micro-cavity. Solid-state single quantum systems are especially practical for this application because they do not require complicated trapping setups and can be integrated into monolithic micro-cavity structures. In the last several years single-photon generation has been demonstrated in a variety of solid-state systems including nitrogen-vacancy (NV) centers in diamond, epitaxial quantum dots in semiconductors such as InGaAs or AlGaN, and impurities in semiconductors. A variety of microcavity geometries have also been employed to improve photon extraction efficiency and to increase the spontaneous emission rate, including micro-pillars with distributed-Bragg-reflector mirrors, micro-disks and photonic crystal cavities. Results from various systems will be summarized and compared in terms of the suppression of the two-photon emission probability (compared with a Poisson distribution), efficiency, and quantum indistinguishability of the generated photon wave packets. A device that efficiently produces single photons with high spectral purity can also be used in other ways. For example, two photons incident onto such a device should in theory exhibit a strong optical nonlinearity. In addition, if the device uses a three-level Lambda-type system in which two lower long-lived levels are coupled by optical transitions to a common excited state, the possibility exists for efficient matter-photon quantum state inter-conversion, an important ingredient for quantum networks and other applications. It has recently been demonstrated that two solid-state systems, charged quantum dots and nitrogen-vacancy centers in diamond, have the required level structure for this scheme. Recent results demonstrating coherent population trapping in single NV centers will be described which are promising in terms of optical manipulation of single spins and eventually spin-photon inter-conversion.   

Dynamical nuclear spin polarization and the Zamboni effect in gated double quantum dots

The hyperfine interaction between electron spins confined in semiconductor quantum dots and the surrounding nuclear spins is one of the main sources for electron spin decoherence in low temperature GaAs quantum dots. We have investigated theoretically the dynamics of a system of two electrons and nuclear spin baths subject to the hyperfine interaction in a gated double dot system. It is shown that the hyperfine interaction can mediate a dynamical nuclear polarization by utilizing the degeneracy point between the two-electron singlet and polarized triplet states. Most importantly, we demonstrate that a small polarization ($~0.3\%$) is sufficient to enhance the singlet decay time by two orders of magnitude, in contrast with the single dot case, where nearly complete nuclear polarization is required to improve spin coherence time significantly. This enhancement is attributed to an equilibration process between the nuclear reservoirs in the two dots, mediated by the hyperfine interaction, an effect we have dubbed as the nuclear Zamboni effect. We explore other strategies to facilitate this effect and show that while equilibration of the two nuclear configurations is obtained, the singlet decay times are only modestly enhanced due to broadening of the nuclear spin distribution.   

Dephasing of exchange coupled spin qubits by electron-phonon coupling

Exchange coupled spin qubits in semiconductor nanostructures can be dephased by {\it charge fluctuations} in the semiconductor environment because of the fundamental Coulombic nature of the Heisenberg coupling. Even when charge fluctuations are suppressed through material improvement, such orbital-degree-of-freedom related fluctuations can still come from electron-phonon interaction in the semiconductor. Here we explore pure dephasing between the two-electron singlet and triplet states for two exchange-coupled spin qubits in a double quantum dot.   

Two- and three- energy level mixing effects in vertically coupled quantum dots

We investigate high bias single electron resonant tunneling through sub-micron gated AlGaAs/InGaAs/AlGaAs/InGaAs/AlGaAs triple barrier structures for which the tunnel coupling energy between the two quantum dots is very weak (less than 0.1meV). The two quantum dot ``disks'' in the vertical diatomic artificial molecule located in the circular device mesa can be almost circular or elliptically deformed. In a device where the constituent dots are elliptically deformed, the single particle states of each dot evolve almost ideally with magnetic field, except at several of the two- and three- energy level crossings. At these crossing points, we see pronounced two level anti- crossing behavior, with levels split by hundreds of micro-eV, and intriguing level crossing phenomena, like mixing of three resonances leading to resonance suppression. We analyze the observed quantum level mixing effects using a simple three level mixing model.   

Magnetic field induced resonance and hysteresis effects in the current flowing through coupled vertical quantum dots at high source-drain bias

We report on the basic properties, including the temperature, range of magnetic field, sweep rate and voltage dependence, of recently observed magnetic field induced resonance and hysteresis effects in the current flowing through two weakly coupled vertical quantum dots at high source-drain bias. Similar looking effects, attributed to electron spin - nuclear spin (hyperfine) coupling, have been seen in the low bias two- electron spin-blockade regime (K. Ono and S. Tarucha Phys. Rev. Lett. 2004), when the magnetic field is applied perpendicular to the flowing current, but the regime we study here is at much higher bias (up to a few 10's of mV) and for a magnetic field (0- 6T) applied parallel to the current. ``Slow'' current oscillations/fluctuations are also observed on the timescale of seconds to tens of seconds for certain conditions. Can nuclear spin related effects occur outside the N=2 spin-blockade region?   

Three-electron bonding and entanglement in single and molecular quantum dots

The study of three-electron quantum dots (QDs) is interesting in several ways. First, it was demonstrated\footnote{C. Ellenberger, T. Ihn, C. Yannouleas, U. Landman, K. Ensslin, D. Driscoll, and A.C. Gossard, Phys. Rev. Lett. {\bf 96}, 126806 (2006).} recently that detailed ground-state and excited spectra of few-electron elliptic QDs can be measured as a function of the externally applied magnetic field. Second, three-qubit electron spin devices are expected to exhibit enhanced efficiency for quantum computing purposes compared to single-qubit and two-qubit gates. We carry out exact diagonalization (EXD) studies for a three-electron single QD and for a wide range of anisotropies. We analyze the properties of the EXD many-body wave functions with respect to electron localization in a linear geometry, as well as to generation of model quantum entangled states that are often employed in the theory of quantum computing. We further examine three-electron bonding and entanglement in the case of a double quantum dot.   

Long range spin qubit interaction mediated by microcavity polaritons

Planar microcavities are semiconductor devices that confine the electromagnetic field by means of two parallel semiconductor mirrors. When a quantum well (QW) is placed inside a planar microcavity, the excitons in the QW couple to confined electromagnetic modes. In the strong-coupling regime, excitons and cavity photons give rise to new states, cavity polaritons, which appear in two branches separated by a vacuum Rabi splitting. We study theoretically the dynamics of localized spins in the QW interacting with cavity polaritons. Our calculations consider localized electron spins of shallow neutral donors in GaAs (e.g., Si), but the theory is valid for other impurities and host semiconductors, as well as to charged quantum dots. In the strong-coupling regime, the vacuum Rabi splitting introduces anisotropies in the spin coupling. Moreover, due to their photon-like mass, polaritons provide an extremely long spin coupling range. This suggests the realization of two-qubit all-optical quantum operations within tens of picoseconds with spins localized as far as hundreds of nanometers apart. [G. F. Quinteiro et al., Phys. Rev. Lett. 97 097401, (2006)].   

Optically detected quantum dynamics of hydrogenic donor qubits

Orbital states of electrons bound to shallow donors in GaAs provide many of the advantages of trapped atoms for quantum information studies, including optical readout and long lived excited levels. Shallow donors (e.g. S, Si) have a scaled hydrogenic potential with a bound electron 1S-2P transition at 1 THz (4 meV). In a 5 T magnetic field the 1S state and lowest 2P state (2P$^-$) serve as qubit levels. A cycling transition exists for detecting neutral donors in the ground state via the donor bound exciton resonance; excited bound states are dark. Using this optical quantum nondemolition measurement, the relaxation (T$_1$) of donors after THz excitation of the 1S- 2P$^-$ transition is observed to be $>1\mu$s. High resolution spectroscopy indicates dephasing (T$_2^*$) of an ensemble of neutral donors is limited by inhomogeneous broadening to 50 ps. In order to measure the decoherence time (T$_2$), which is expected to be much longer, a rephasing technique is required. For Hahn echo measurements of T$_2$ a 0-24ns, diffraction- compensating free space THz delay line has been constructed.   

Characterization of (In,Ga)As Quantum Posts for Terahertz Quantum Information Processing

Quantum posts (QPs) are a new kind of self-assembled semiconductor nanostructure which may be suitable for quantum information processing using terahertz frequencies.$^{1}$ A QP is a roughly cylindrical In-rich region embedded in a GaAs matrix whose height can be controlled with monolayer resolution. For a single electron trapped in a 40 nm high QP, the orbital transition between the ground and first excited state is predicted to occur near 1 THz. Since this is well below the optical phonon frequency (9 THz), decoherence is expected to arise primarily from very weak interactions with acoustic phonons. QPs grown in the insulating region of a metal-insulator-semiconductor structure allow voltage-controlled charging, which is measured by capacitance-voltage spectroscopy. Terahertz absorption spectra are also measured by Fourier-transform infrared spectroscopy. $^{1}$ M. S. Sherwin, A. Imamoglu and C. Montroy, PRA 60, 3508 (1999) Work supported by the NSF NIRT grant No. CCF 0507295   

Enhancement of electron spin coherence by optical preparation of nuclear spins

We study a large ensemble of nuclear spins interacting with a single electron spin in a quantum dot under optical excitation and photon detection. When a pair of applied laser fields satisfy two-photon resonance between the two ground electronic spin states, detection of light scattering from the intermediate exciton state acts as a weak quantum measurement of the effective magnetic (Overhauser) field due to the nuclear spins. If the spin were driven into a coherent population trapping state where no light scattering takes place, then the nuclear state would be projected into an eigenstate of the Overhauser field operator and electron decoherence due to nuclear spins would be suppressed: we show that this limit can be approached by adapting the laser frequencies when a photon is detected. We use a Lindblad equation to describe the time evolution of the driven system under photon emission and detection. Numerically, we find an increase of the electron coherence time from $5\,{\rm ns}$ to $500\,{\rm ns}$ after a preparation time of 10 microseconds.   

Exchange energy in vertically coupled double quantum dots

The exchange separation between spin singlet and triplet states was studied for vertically coupled double quantum dots in the Pauli spin blockade regime with the inter-dot level detuning as a parameter. Pauli blockade is established by the formation of an excited but long-lived triplet state in the double dot, and is lifted by a spin flip transition to the singlet state, generating a leakage current. The leakage current shows a step when the Zeeman energy equals the exchange energy thus turning on the flip-flop interaction with the nuclei. The threshold magnetic field increases on approaching the anti-crossing of the two triplets reflecting the increased exchange energy. We present a quantitative comparison of the exchange energy derived experimentally with exact theory.   

Electronic structure, entanglement and double occupancy in asymmetric dot molecule quantum gate

First, we describe the energy levels, degree of entanglement and double occupancy in fully symmetric (homopolar) quantum dot molecules (QDM) made of InGaAs dots in a GaAs barrier containing $\sim$ 3 x10$^6$ atoms. We describe the single-particle part by atomistic pseudopotential theory including strain and alloy effects, and the many body part via configuration interaction. Second, we note that in a realistic vertically coupled QDM the two dots often have different geometries, sizes, alloy compositions, (heteropolar QDM) and therefore, deviates from ideal homopolar QDM model used previously. We show that the electronic properties of such heteropolar QDMs are greatly modified by the asymmetry of the QDMs, showing larger two-electron double occupation rate, lower two-electron entanglement, and therefore reduced quantum gate quality. By symmetrizing the QDM via application of electric field, one can overcome these difficulties.   

Session J44: Focus Session: Optical Properties of Nanocrystals

Polarization-resolved fine structure and magneto-optics of single CdSe nanocrystal quantum dots

Low-temperature photoluminescence (PL) microscopy of single colloidal quantum dots has proven a very effective tool for probing the emission properties of the band-edge excitons in isolated CdSe nanocrystals (NCs). Past studies employing high spectral resolution have resolved the narrow `atomic-like' emission lines from single NCs, while separately, polarization- resolved measurements have shown that the $|+1>$ and $|-1>$ bright exciton states are nominally degenerate with transition dipoles oriented isotropically in the plane normal to the crystallographic {\it c}-axis of the NC. To date, however, these two powerful techniques have not been simultaneously employed. To this end we constructed a low-temperature (4 K) microscope to measure both polarization- and spectrally- resolved PL of individual nanocrystals. Both orthogonal polarizations (horizontal/vertical linear or right/left circular) are simultaneously recorded to minimize the effects of spectral diffusion and blinking. The data clearly show [1] that many NCs possess a clear bright exciton ``fine structure" consisting of two linearly- (and orthogonally-) polarized peaks split in energy by $\delta \sim 1-2$ meV. This splitting is attributed to a breaking of the nanocrystal's cylindrical symmetry, leading to an anisotropic electron-hole exchange that mixes the $| \pm 1>$ bright excitons. Inferred orientation of the NCs will be discussed. Finally, we study the interplay between the anisotropic exchange and magnetic Zeeman energy in single NCs by incorporating a 5 T magnet into the microscope. With increasing magnetic field, the fine structure states become elliptically polarized and eventually approach pure circular polarization in the limit where the Zeeman energy $1/2 g \mu_B B > \delta$. We extract the exciton {\it g}-factor of individual NCs from the variation of the observed energy splitting with field in this regime. \newline \newline [1] M. Furis, H. Htoon, T. Barrick, M. Petruska, V. I. Klimov, S. A. Crooker, Phys. Rev. B Rapid Comm. 73, 241313 (2006).   

State-to-state femtosecond relaxation dynamics of excitons in semiconductor quantum dots.

Size dependent exciton relaxation dynamics are measured in colloidal CdSe quantum dots using exciton selective femtosecond spectroscopy. Preparation of the initial excitonic state allows evaluation of state-to-state exciton dynamics. These methods reveal the electron and hole relaxation dynamics from a specified initial state to a specified final state, with a precision of 10 femtoseconds. These state selective, size dependent experiments confirm previously observed confinement induced femtosecond Auger channels for electrons with increased precision. This increased precision allows for unambiguous, quantitative evaluation of size dependent transition matrix elements. These experiments furthermore show that the hole relaxation rate increases for smaller quantum dots, contradicting expected relaxation mechanisms for holes. We propose a new confinement enhanced non-adiabatic pathway for hole relaxation in colloidal quantum dots, overcoming the predicted phonon bottleneck for holes. Finally, these experiments show exciton state specific biexcitonic interactions.   

Size Dependence of Fluorescence Blinking Statistics from CdSe Nanorods

We report fluorescence blinking statistics measured from single CdSe nanorods (NRs) of seven different sizes with aspect ratios ranging from 3 to 11. The off-times follow a power-law probability distribution; on-times follow a truncated power law distribution,$ P$(\textit{$\tau $}$_{on})\sim $\textit{$\tau $}$_{on}^{-\alpha }e^{-\tau on/\tau c}$. At fixed excitation intensity, the truncation rate 1/$_{\tau c}$ increases with increasing aspect ratio. For a particular sample, 1/$_{\tau c}$ increases gradually with increasing excitation intensity. Examining 1/$_{\tau c}$ vs. single-particle photon absorption rate for all samples indicates that the shape dependence of the absorption cross-section does not fully account for the observed variation in crossover time \textit{$\tau $}$_{c}$. Surprisingly, we observe no significant difference between core and core/shell nanorods or core rods with different surface ligands. Our results suggest that NR internal structural defects or degree of quantum confinement may contribute to the shape dependence of the crossover time.   

Size and shape dependence of CdSe nanocrystal band-edge exciton fine structure$^{\ast }$

Advances in growth methods of nanocrystals led to controlled synthesis over size and shape, influencing their optical properties. Ground exciton states of CdSe nanocrystals are shown to be sensitive to their geometries. We investigate the exciton fine structure of CdSe nanocrystals using empirical pseudopotential and configuration interaction methods$^{1,2}$. Systematic studies of the size and shape dependency are performed on the band edge states of CdSe spherical quantum dots, elongated nanorods, flattened nanodisks, nanowires and quantum wells. Large-scale electronic calculations consisting of 100--20,000 atoms with diameters from 2 to 8 nm and lengths from 2 to 11 nm were carried out. We explore size and shape dependence of exciton fine structure over the diameter-length space and explain it by the interplay of quantum confinement, crystal field splitting, and exchange interaction. We find the experimentally observed dark-bright exciton crossing$^{3}$ and discuss its size-shape dependency. [1] L. W. Wang and A. Zunger, \textit{Phys. Rev. B} \textbf{51}, 17398 (1995). [2] A. Franceschetti, \textit{et al.}, \textit{Phys. Rev. B} \textbf{60}, 1819 (1999). [3] N. Le Thomas, \textit{et al.}, \textit{Phys. Rev. Lett.} \textbf{94}, 016803 (2005). *This work was supported by US DOE-SC-BES and ASCR~TMSN Initiative.   

Ab initio Theory of Semiconductor Nanocrystals

With blooming experimental synthesis of various nanostructures out of many semiconductor materials, there is an urgent need to calculate the electronic structures and optical properties of these nanosystems based on reliable ab initio methods. Unfortunately, due to the O(N$^{3})$ scaling of the conventional ab initio calculation methods based on the density functional theory (DFT), and the $>$1000 atom sizes of the most experimental nanosystems, the direct applications of these conventional ab intio methods are often difficult. Here we will present the calculated results using our O(N) scaling charge patching method (CPM) [1,2] to nanosystems up to 10,000 atoms. The CPM yields the charge density of a nanosystem by patching the charge motifs generated from small prototype systems. The CPM electron/hole eigen energies differ from the directly calculated results by only $\sim $10-20 meV. We will present the optical band gaps of quantum dots and wires, quantum rods, quantum dot/quantum well, and quantum dots doped with impurities. Besides good agreements with experimental measurements, we will demonstrate why it is important to perform ab initio calculations, in contrast with the continuum model k.p calculations. We will show the effects of surface polarization potentials and the internal electric fields. Finally, a linear scaling 3 dimensional fragment (LS3DF) method will be discussed. The LS3DF method can be used to calculate the total energy and atomic forces of a large nanosystem, with the results practically the same as the direct DFT method. Our work demonstrates that, with the help of supercomputers, it is now feasible to calculate the electronic structures and optical properties of $>$10,000 atom nanocrystals with ab initio accuracy. \newline \newline [1] L.W. Wang, Phys. Rev. Lett. 88, 256402 (2002). \newline [2] J. Li, L.W. Wang, Phys. Rev. B 72, 125325 (2005).   

Size-dependent optical spectrum of CdSe nanocrystals

An empirical $sp^3d^5$ tight-binding model has been employed to describe the optical properties of CdSe nanocrystals over a wide range of sizes. The $sp^3d^5$ model explains successfully the single-particle electron levels and excitonic effects including the evolution of both the emission and absorption peaks with confinement. We provide an interpretation of the band-edge fine structure in agreement with both the one- and two- photon spectroscopies and the PLE resonant and non-resonant Stokes shifts. Previous effective mass, pseudopotential and $sp^3s^*$ tight-binding models were unable to explain such experiments. The wurtzite lattice structure splits the lowest $S$- and $P$- hole states into two doublets that overlap, in accordance to the indistinguishability observed between the one-photon and two-photon spectroscopies. A correct description of the spin-orbit coupling allows the non-resonant Stokes shift to be reproduced. Finally, for dot radius below 2.3 nm, an optically passive $P-$ level becomes the ground hole state giving rise to the large resonant Stokes shift observed experimentally.   

Carrier Multiplication in PbSe Quantum Dots

The efficiency of conventional solar cells is limited, because the energy of absorbed photons in excess of the band gap is converted to heat, instead of producing electron-hole pairs. Recently, efficient carrier multiplication has been observed in smiconductor quantum dots. In this process, a single, high-energy photon generates two or more electron-hole pairs, thus potentially increasing the efficiency of solar cells. Rather exotic mechanisms have been proposed to explain carrier multiplication in PbSe quantum dots. Using atomistic semi-emprical pseudopotential calculations, we show that the more conventional impact ionization mechanism - whereby a photogenerated electron-hole pair decays into a biexciton in a process driven by Coulomb interactions between the carriers - can explain both the rate ($<1$ ps) and the energy threshold ($\sim 2.2$ times the band gap) of carrier multiplication in Pbse quantum dots [1,2], without the need to invoke alternative mechanisms. The reason is that the density of biexciton states increases very rapidly with energy, thus making the rate of impact ionization faster than the rate of competing decay channels. [1] A. Franceschetti, J.M. An and A. Zunger, Nano Letters, 6, 2191 (2006). [2] J.M. An, A. Franceschetti and A. Zunger, Nano Letters, nl061684x (2006).   

Optical Properties of PbSe Nanocrystal Quantum Dots Under Pressure

The optical properties of PbSe nanocrystal quantum dots (NQDs) were studied as a function of applied hydrostatic pressure over the range from ambient to 4 GPa. PbSe NQDs exhibit an energy gap that is dominated by quantum confinement energy. Despite such strong confinement, we find that the energy gaps of 3, 5, and 7 nm PbSe NQDs change monotonically with pressure with a dependence that is almost entirely determined by the deformation potential. The sizable dependence of the NQD energy gap with pressure invites applications in the areas of high speed pressure sensing and tunable IR lasers. We will also present x-ray diffraction data, including the data indicating new phase transition not observed earlier.   

Theory of InP nanocrystals under pressure

An empirical tight-binding theory which includes the effects of the relaxation of the lattice is employed to investigate the role of an external hydrostatic pressure on the opto-electronic properties of InP nanocrystals. For the bulk, our model describes accurately the evolution of the lowest conduction band-edges with pressure and predicts the $\Gamma_{1c}$-$X_{1c}$ crossover at the same lattice contraction as measured in the experiment. For small InP nanocrystals, the bandgap dependence on pressure predicted with this model is, for the first time, in agreement with the experimental results. Previous atomistic models, which assumed a bulk-like arrangement for the atoms in the nanocrystal under pressure, led to negligible mixing of the $\Gamma_{1c}$- and $L_{1c}$- minima and did not account for the increasing localized character of the electron and hole states as a function of pressure. The lattice-relaxed tight-binding model suggests a mechanism for the experimental red-shift different from the $\Gamma_{1c}$-$X_{1c}$ crossover predicted by bond-distance scaling models. In the lattice-relaxed model, the experimental red-shift is explained as a transition from bound states localized inside the dot to surface-like states in the dot exterior. The evolution of the near-band-edge optical spectra as a function of pressure has been analyzed for different nanocrystal sizes, geometries and degrees of surface passivation with both the bond-length scaling and lattice-relaxed tight-binding approaches.   

Quantum Dots Confined in Nanoporous Alumina Membranes

Precise control over the dispersion and lateral distribution of quantum dots (QDs) within nanoscopic porous media provides a unique route to manipulate the optical and/or electronic properties of QDs in a very simple and controllable manner for applications related to light emitting, optoelectronic, and sensor devices. Here we filled nanoporous alumina membranes (PAMs) with CdSe/ZnS core/shell QDs by dip coating. The deposition of QDs induced changes in the refractive index of PAMs. The amount of absorbed QDs was quantified by fitting the reflection and transmission spectra observed experimentally with one side open and freestanding (i.e., with two sides open) PAMs employed, respectively. The fluorescence of the QDs was found to be retained within the cylindrical nanopores of PAMs.