Session K1: Poster Session II

Role of Physique on Probability of Injury to the Neck

The primary emphasis in governmental regulation for the prevention and mitigation of automotive trauma tends to be restricted to just a few human surrogates; for example, most Federal Motor Vehicle Safety Standards in the United States involve a 50th-percentile male. This practice is known to conflict with real life, and it can lead to the problem of having safety equipment that is designed to protect some people but can kill or seriously injure others. The influence of physique on neck injury potential is best determined by isolating it as the sole variable in an insult, a constraint satisfied only when there are isokinetic and isogeometric conditions. The latter require there to be pairs of same-vehicle occupants, which are much less common than those relating to the effect of other parameters for impact[1] and environment[2]. Among a possible 60 or so such pairs in the large patient cohort assembled for this study, various subsets were identified as being suitable for detailed research on neck injury and the results of this are reported here. \newline [1] Bull Am Phys Soc, 45, 1018 (2000);\newline [2] Bull Am Phys Soc, 51, 1524 (2006).   

Biosensors based on carbon nanotube resonators

We have developed biosensors based on carbon nanotubes (CNT) utilizing the resonance frequency measurements. The CNTs, which was attached to a resonant LC impedance-matching circuit whose resonance frequency was about 12GHz, have exhibited the resonance frequency of about 14GHz. However, when biotin molecules were attached on the CNT surface modified with linker and streptavidin, the resonance frequency was shifted to the lower frequency. For comparison, we have carried out similar measurements using as resonator without CNT, but found no resonance frequency shift. Possible origins of resonance frequency shift after attaching biotin on the CNT surface are discussed.   

Distinguishing cancerous from non-cancerous cells through analysis of electrical noise

For more than a decade, electric cell-substrate impedance sensing (ECIS) has been used to monitor cell behavior in tissue culture and has proven to be very sensitive to cell morphological changes and cell motility [1]. We have taken ECIS measurements on several cultures of human ovarian cancer (SKOV) cells and on human ovarian surface epithelial (HOSE) cells. Our goal is to develop a numerical technique that can distinguish these cells. From the noise of the impedance measurements we characterize the cells using the power spectrum, Hurst exponent, first zero crossing, and first 1/e crossing of the autocorrelation function. We then performed a principal-component analysis. Our results show that there is some separation of the two cell types in the multidimensional PCA space. Most of the maximized variance is from the contribution of the autocorrelation function. We present these results and show that this technique can be used to distinguish between the two cell types. [1] Lo, C.-M., Keese, C.R. and Giaever, I., Monitoring motion of confluent cells in tissue culture, Exp. Cell Res. 204:102-109 (1993).   

Quasispecies theory for multiple-peak fitness landscapes

We used a path integral representation to solve the Eigen and Crow-Kimura molecular evolution models for the case of multiple fitness peaks with arbitrary fitness and degradation functions. In the general case, we find that the solution to these molecular evolution models can be written as the optimum of a fitness function, with constraints enforced by Lagrange multipliers and with a term accounting for the entropy of the spreading population in sequence space. The results for the Eigen model are applied to consider virus or cancer proliferation under the control of drugs or the immune system.   

Phase transitions in deoxyadenosine at high pressures: A mid-infrared study.

Crystalline deoxyadenosine has been studied via infrared spectroscopy at room temperatures up to 10 GPa of pressure. Samples, typically 250 microns in diameter and roughly 25 microns in thickness, were loaded into a piston-cylinder type diamond anvil cell supplied by Diacell Ltd. and fitted with type IIa diamonds. To avoid saturation of strongly absorbing modes, the deoxyadenosine sample was diluted with KBr powder, which also served as an isotropic pressure-transmitting medium. Significant changes in the infrared spectra are noted near two different pressures: about 2 GPa and about 4 GPa, suggesting two separate phase transitions.   

Stability of the B conformation in wet-spun films of CaDNA.

Highly oriented, wet-spun films of CaDNA have studied via Raman spectroscopy as a function of both water content and excess CaCl$_{2}$ concentration. The secondary structure of the double-helical DNA can be determined by monitoring the conformationally-sensitive vibrational modes at 807 cm$^{-1}$ for A-DNA and 834 cm$^{-1}$ for B-DNA. We find that the DNA molecules are in the B conformation between relative humidities of 98$\%$ and 75$\%$ for all salt contents.   

Flexibility of the sugar moiety of nucleosides at high pressures

In this poster we review our recent high pressure experiments on deoxyadenosine, adenosine, deoxycytidine and cytidine via mid-infrared absorption. These experiments reveal the presence of phase transitions near 2 GPa in these four different nucleosides. The spectroscopic evidence indicates that the sugar pucker changes at the phase transition in all four nucleosides. Differences between the deoxyribose nucleosides and the ribose nucleosides are compared to the known differences in the conformational flexibility of DNA and RNA.   

Phase separation in model lipid membranes: physics and biophysics

Lipids are biological amphiphiles that, in aqueous solution, self-assemble into a variety of structures, including bilayer membranes that form hollow vesicles. In membranes with two or more constituents, lipids are well-mixed when the temperature is sufficiently high. As the temperature is lowered, systems undergo ordering transitions, membranes phase-separate laterally, and the resulting domains pattern vesicles. Here we demonstrate different aspects of this patterning that are important both for basic science and for their technological potentials: the packings of lipids in ordered-phase domains determine the domains' morphologies and inclusivities; adherent regions of membranes favour the growth of ordered-phase domains; rapid phase separation forms many small stripe domains that pattern vesicles with a `baseball' texture. These phenomena demonstrate basic physics and also have strong potential for exploitation to achieve vesicles with controllable patterns and properties. Lipid structures such as vesicles are attractive candidates for such technological development because they are intrinsicly biocompatible and because technologies using liposomes for controlled delivery and release are already widespread and under active development.   

Molecular Dynamics Simulation of the Fully Hydrated Dipalmitoylphosphatidylcholine (DPPC) Bilayer at Different Temperatures and Pressures

The structural properties of lipid bilayers in biological membranes are of great interest in biochemistry, biophysics, and medicine. The main goal of this study is to use molecular dynamic (MD) techniques to investigate physical properties of the hydrated dipalmitoylphosphatidylcholine (DPPC) bilayer. The bilayer model consists of 25 DPPC molecules per each monolayer and 44.8{\%} water by total weight. A modified version of AMBER MD suit of programs with CHARMM22 force field for phospholipids and the isothermal-isobaric or NPT ensemble with a fully flexible simulation box in ROAR program was used in this study. Simulations were performed under different pressure and temperature conditions. A liquid crystal phase ($L_{\alpha })$ is (experimentally) expected with the DPPC bilayer under 1 atm pressure and 323 K temperature conditions. However, area per lipid, bilayer thickness, chain tilt, and the order parameters resulting from the present simulation appeared to be more consistent with the known properties of the $L_{\beta \prime }$ phase. The results of the simulations will be discussed.   

AFM studies of homogeneous and mixed lipid mono- and bilayers

Phospholipid mono- and bilayers have potential research applications in various areas of biology and medicine, where they serve as substitutes for cell membranes. The use of atomic force microscopy (AFM) to characterize such materials allows for the measurement of the topographic features of the material on a subnanometric scale and of the forces arising due to the interaction between the AFM tip and the phospholipid surface; the addition of biological molecules commonly found in cells to the AFM tip, such as proteins, allows the interaction between these molecules and a cell membrane to be studied. For this study, mixed phospholipid monolayers consisting of 1,2-Distearoyl-\textit{sn}-Glycero-3-Phosphoethanolamine (DSPE) and 1,2-Dioleoyl-\textit{sn}-Glycero-3-Phosphoethanolamine (DOPE) as well as bilayers consisting of 1,2-Dipalmitoyl-sn-Glycero-3-Phosphocholine (DPPC) were synthesized and studied using AFM imaging and force measurements.   

Mechanism of Membrane Disruption by Antimicrobial Peptide Protegrin-1

Protegrin-1 (PG-1), a cationic antimicrobial peptide, kills bacteria by causing an increase in membrane permeability to ions or larger molecules. To understand the mechanism of antimicrobial peptide action, we investigated, via atomic force microscopy, topological changes in supported phospholipid bilayers induced by PG-1. We have observed PG-1 induces structural transformations progress from fingerlike instabilities at bilayer edges, to the formation of sievelike nanoporous structures and finally to a network of stripelike structures in a zwitterionic dimyristoylphosphatidylcholine (DMPC) model membrane in buffer, with increasing PG-1 concentration. In addition, to investigate the dependence of lipid-peptide interactions on electrostatics, studies involving charged lipids have been carried out. Similar progression of structural transformations has been observed in membranes containing anionic lipids, but with lower critical concentrations compared to the zwitterionic system. The visualization of structural transformation provides details of membrane disruption mechanism by PG-1.   

Enhanced protein folding by removal of kinetic traps

The presence of non-native kinetic traps along the free energy landscape of a protein may significantly lengthen the overall folding time so that the folding process becomes unreliable. We used a computational 3-D lattice model to investigate the free energy landscape of a model alpha helical hairpin peptide. We used two slightly different sequences and show that strategic substitutions of only a few amino acid residues greatly enhance the folding process. These strategic substitutions prevent the formation of long-lived misfolded configurations which not only lengthen the folding time but also may cause unwanted aggregation. Detailed kinetic and thermodynamic analysis was carried out for the folding of these two sequences and the results are consistent with the experimental and molecular dynamics simulations of small helical bundle proteins.   

An In-Depth Impedance Analysis of Rectangle-Shaped Cell Layers Cultured on Microelectrodes

Transcellular impedance of smooth muscle cell layers cultured on microelectrodes is measured by electric cell-substrate impedance sensing (ECIS). An extended cell-electrode model for impedance analysis of cell layers where cellular shape is long rectangular is developed. This is especially appropriate for normal fibroblasts and smooth muscle cells in culture. The model considers two different pathways for the current flowing from the electrode through the cell layer: (1) in through the basal and out through the apical membrane, and (2) in through spaces between the ventral cell surface and the substratum and out through the paracellular space. By comparing model calculation with experimental impedance data, several morphological and cellular parameters can be determined: (1) the junctional resistance between cells, (2) the average cell-substrate separation, and (3) the capacitance of apical and basal cell membranes. This model is used to analyze impedance changes upon addition of RGD peptide to confluent SMC layers at different concentrations. The method shows that RGD peptide causes junction resistance between cells to drop and the distance between the basal cell surface and substratum to increase.   

Manipulation and analysis of Xenopus laevis embryos by femtosecond near infrared lasers

Given the demand for new and more reliable methodologies for live cell manipulation, we have used a technique (demonstrated earlier by Tirlapur and K\"{o}nig, Nature, \textbf{418}$, $290, 2002) using near infrared laser pulses (NIR) to manipulate living cells, specifically, cell of Xenopus laevis embryos, without harming them. In addition nanoparticles such as silica-coated CdSe and CdTe quantum dots are injected into the cell through pores formed by the laser pulses. Due to the highly efficient and size-dependent fluorescence of QDs, they can be used in place of conventional dyes to perform live-cell imaging. In this work, we will discuss our current understanding of NIR lasers and QDs interactions with the Xenopus laevis embryos. The outcome of this project can help us to understand the fundamental phenomena and processes important in biological systems and cellular function.   

Detection of Telomerase Activity Using Capacitance Measurements

Telomerase activity has been found in about 85{\%} cancer cells, while no activity observed in normal cells, so that telomerase has been proposed as a marker for cancer detection. Here, we describe electrical detection of telomerase activity using capacitance measurements. We have investigated the length dependence of capacitance on DNA solutions and found that the capacitance of DNA solutions were dependent on the DNA length. In addition, upon adding telomerase into the solution of telomeric substrate primer, the capacitance was observed to change as a function of time due to the telomeric elongation. These results suggest that this novel nanosensor may be used for rapid detection of telomerase activity.   

Vertically Aligned Antimony Nanowires as pH Sensors

Antimony (Sb) has remarkable linear response to solution H$^{+}$ concentration and has been built into commercial pH electrode. Unlike conventional glass tube-based pH electrode, solid state Sb electrode represents a good candidate for integrated pH sensor with fabrication compatible to silicon-based complementary metal oxide semiconductor (CMOS) process. In addition, due to its high resistance to corrosion, Sb-based pH electrode has been used in a wide range of applications. Because of its versatile sensing applications, Sb probe in size scale compatible to cells is perceived to hold great potential to meet the demand of biomedical research.$^{ }$The \textit{in vivo }and \textit{in vitro} intracellular real time monitoring of pH,$^{ }$Na$^{+}$, K$^{+}$ -- a key subject in cell biology and physiology, may require vertically oriented nanoscale electrodes in close contact with cells. Such nanoelectrodes can be implemented by vertically grown nanowires and be applied to penetrate smoothly and gently into a cell without causing cell apoptosis. In this talk, we describe a method to fabricate vertical Sb nanowire electrode, and present their property characterizations and demonstrate their potential application in pH sensing.   

Utilizing Profile-to-Profile Alignments for Predicting Protein-Protein Interactions

One of the biggest challenges in homology-based modeling of protein complexes is the detection of remote similarities between query and template. Most prominent way in this direction embraces alignments of profiles for query and template sequences by means of dynamic programming algorithm. While this technique is well elaborated for single protein molecules, little was done in employing it for prediction of protein-protein interactions. Here we present our recent development of a profile-to-profile alignment algorithm for predicting protein complexes. The core of the algorithm is enhancement of profiles with information about existing (template) or putative (query) interfacial residues. This is further used in order to alter the standard dynamic programming algorithm by putting an extra weight in matching interfacial residues and increasing gap penalties at the interface. We aligned all sequences in our PROTCOM40 database against each sequence in the dataset, except sequence with itself. For further examination we chose only alignments that are statistically significant and have template sequences belonging to different chains of a complex in our dataset. We have clearly demonstrated that profile-to-profile alignment technique outperforms considerably the standard BLAST homology modeling with respect to both amount and quality of the produced models of protein complexes.   

Optimization of the electrostatic interactions in protein-protein complexes

Electrostatic energy is one of the driving forces of protein-protein association. Understanding the role of the energy components on the energetics of protein-protein association will help us in engineering protein-protein interactions and could lead to development of scoring functions that can rank alternative models and decoys. Here we investigate whether the components of the electrostatic energy of protein-protein complexes is optimized in respect to random distribution of the charged residues. We report a clear tendency that coulombic electrostatic interactions are optimized, while the reaction field energy is inversely optimized. It was found that the maximum of the coulombic energy Z-score is shifted 3 units away from the origin and the maximum of the reaction field energy by 2 units. Such a large shift of the Z-score of both coulombic and reaction field energies indicates that wild-type protein-protein interactions are in most cases optimized in terms of coulombic interactions while compromising reaction field energy. Based on these finding a scoring function was developed as a linear combination of the Z-score of the coulombic interactions minus Z-score of the reaction field energy. The scoring function was tested against the decoy sets and it was shown that in majority of the cases we can identify the wild-type complex among hundreds of decoys.   

Implementation of the Surface Charge Method for Calculation of Biomolecular Electrostatic Forces

Due to the presence of ions, a high dielectric constant solvent (water with $\varepsilon=80$), and significant charges and polarizabilities associated with many biomolecules, electrostatic forces play a crucial role in biomolecular interactions. It is particularly important to account for the effects of the solvent. The surface charge method (SCM), developed in PRE {\bf 73}, 061902 (2006) can be applied straightforwardly to a system of an arbitrary number of charged dielectric spheres embedded in an infinite dielectric solvent. Although in principle the electrostatic energy and force can be calculated to any desired accuracy using the SCM by simply including more multipole moments in the expansions, in practice this process leads to large matrix inversions that are inefficient to carry out. Therefore it was necessary to derive a scheme so that higher order multipole moments could be incorporated into the description of the system without leading to unacceptably large matrix inversions. We have implemented such a scheme and applied it to a system of such spheres in a solvent and explored the effect in this system of asymmetric dielectric screening, which was revealed during the development of the SCM.   

A coarse-grained computational model of DNA from all-atom molecular dynamics simulations.

DNA folds into a highly compact chromatin structure in the eukariotic cells. Counterions and the aqueous solvent provide a stabilizing medium for the maintanence of the highly compact and organized DNA structures. Thus, detailed understanding of counterion condensation around DNA is required to build a coarse-grained computational model of a chromatin fiber. We carried out large-scale all-atom Molecular Dynamics simulations of a 16-mer DNA in explicit water with Na+ and K+ counterions to gain insight into generic aspects of monovalent counterion condensation around the whole DNA molecule, focusing on the discrete nature of water and ions. We found that the Na+ ions penetrate the DNA interior and condense around the DNA exterior to a significantly larger degree compared with the K+ ions. We have provided a microscopic explanation for the larger Na+ affinity towards DNA, that is based on a combination of steric, electrostatic, and hydration effects. Our simulations are consistent with the prior DNA compaction and electrophoretic mobility experiments. We developed a coarse-grained DNA model based on the results of these all-atom simulations.   

Simulation of the rotational mobility and hydration of laurdan in a DPPC bilayer

Laurdan (2-dimethylamino-6-lauroylnaphthalene) is a fluorescent dye commonly used in biophysical experiments to detect ordered regions in lipid bilayers through changes in the polarization and wavelength of emitted light. It is supposed that the fluidity of the bilayer affects changes in polarization by allowing or hindering free rotation of the dye molecule. We have used molecular dynamics (MD) to investigate the rotational diffusion of Laurdan in liquid and gel dipalmitoylphosphatidylcholine (DPPC) bilayers at temperatures above and below the phase transition. Results will be examined for correlation between relaxation times and experimental observations of the decay of anisotropy of the emitted light and between hydration and shift in fluorescence wavelengths. We will also show similar data for Prodan (2-dimethylamino-6-propionylnaphthalene) for comparison.   

Phase Separation and Membrane Protrusions Driven by Actin Polymerization and Adhesion

Formation of protrusions and protein segregation on the membrane is of a great importance for the functioning of the living cell. This is most evident in recent experiments that show the effects of the mechanical properties of the surrounding substrate on cell morphology. We model the cell membrane as having a mobile but constant population of protein with a convex spontaneous curvature. Our basic assumption is that these membrane proteins represent small clusters that may include both adhesion proteins (integrins) and proteins that activate actin polymerization (WASP). We propose a continuum model based on the Helfrich's Hamiltonian for the membrane elastic energy, including the adhesion, with the protrusive actin force added to the equations of motion. Linear stability analysis shows that sufficiently strong adhesion energy and actin polymerization force, can bring about phase separation of the membrane protein and the appearance of protrusions. Specifically this occurs when the spontaneous curvature alone does not. Different instability characteristics are calculated for the various regimes, and are compared to various types of observed protrusions.   

RAId{\_}DbS: Method for Peptide ID using Database Search with Accurate Statistics

The key to proteomics studies, essential in systems biology, is peptide identification. Under tandem mass spectrometry, each spectrum generated consists of a list of mass/charge peaks along with their intensities. Software analysis is then required to identify from the spectrum peptide candidates that best interpret the spectrum. The library search, which compares the spectral peaks against theoretical peaks generated by each peptide in a library, is among the most popular methods. This method, although robust, lacks good quantitative statistical underpinning. As we show, many library search algorithms suffer from statistical instability. The need for a better statistical basis prompted us to develop RAId{\_}DbS. Taking into account the skewness in the peak intensity distribution while scoring peptides, RAId{\_}DbS provides an accurate statistical significance assignment to each peptide candidate. RAId{\_}DbS will be a valuable tool especially when one intends to identify proteins through peptide identifications.   

Computational modeling of the quorum-sensing network in bacteria

Certain species of bacteria are able produce and sense the concentration of small molecules called autodinducers in order to coordinate gene regulation in response to population density, a process known as ``quorum-sensing''. The resulting regulation of gene expression involves both transcriptional and post-transcriptional regulators. In particular, the species of bacteria in the \textit{Vibrio} genus use small RNAs to regulate the master protein controlling the quorum-sensing response (luminescence, biofilm formation, virulence...). We model the network of interactions using a modular approach which provides a quantitative understanding of how signal transduction occurs. The parameters of the input-module are fit to current experimental results allowing for testable predictions to be made for future experiments. The results of our analysis offer a revised perspective on quorum-sensing based regulation.   

Sculpting the Immunological Response against Viral Disease: Statistical Mechanics and Network Theory

The twin challenges of immunodominance and heterologous immunity have hampered discovery of an effective vaccine against all four dengue viruses. Here we develop a generalized NK, or spin glass, theory of T cell original antigenic sin and immunodominance. The theory we develop predicts dengue vaccine clinical trial data well. From the insights that we gain by this theory, we propose two new ideas for design of epitope-based T cell vaccines against dengue. The H5N1 strain of avian influenza first appeared in Hong Kong in 1997. Since then, it has spread to at least eight other Asian countries, Romania, and Russia, and it is widely expected to enter the rest of Europe through migratory birds. Various countries around the world have started to create stockpiles of avian influenza vaccines. However, since the avian influenza is mutating, how many and which strains should be stockpiled? Here we use a combination of statistical physics and network theory to simulate the bird flu transmission and evolution. From the insights that we gain by the theory, we propose new strategies to improve the vaccine efficacy.   

A safe Taser dose: Evaluation of Taser-related in-custody deaths, with implications for law enforcement policy and training

The Taser, an electroconductive skeletal-muscle-incapacitating device originally designed by Taser International Inc. as a {\it non-lethal weapon}, is used by increasing numbers of law enforcement agencies (LEAs) in the USA and Canada. Since 1999, over 200 people ``Tasered'' by law enforcement personnel (LEP) have collapsed and died, prompting public calls for a moratorium on LEA Taser use except when deadly force is justified. If a sufficiently long Taser shock can kill, as seems likely [metabolic acidosis climbs, impairing respiration and elevating the risk of ventricular fibrillation], the data on Taser-related in-custody human deaths collectively support a {\it single-shock policy} for LEAs (ideally, Taser use on people exhibiting physical exhaustion or any type of delirium, or who are taking drugs for mental health reasons, or are pregnant, is {\bf prohibited unless deadly force is justified}), with a {\it second shock} permitted in emergencies {\bf only} for people {\bf not} in the foregoing ``prohibited'' category. If all Taser-using LEAs in North America were to adopt a policy of this type, a {\it 10- to 20-fold reduction} in the rate of Taser-related in-custody deaths is projected! To protect the public, LEP training should distinguish between lethal and non-lethal Taser deployment using a ``safe Taser dose'' concept.   

Mechanical properties of healthy and tumor tissue

Many biological processes depend on cellular organization and the mechanical properties of multicellular aggregates. In general, the cells forming a tissue are in contact with an extracellular matrix, which, along with adhesion between cells, helps them to bind into tissues. Experiments on embryonic tissue [M. S. Steinberg, et al., J. Cell. Physiol., 173, (1997)] reveal viscoelastic behavior. During embryonic development, infections and healing processes, cells move and reorganize themselves intensively. Analogously, tumors are characterized by a continuous spatial reorganization which minimizes intracellular stresses. In this work we study the mechanical properties of cellular aggregates using computer simulations. The cells are represented by semi-rigid bodies with position and shape exhibiting thermal fluctuations. The corresponding Langevin equations are solved numerically. The interaction between cells involves an adhesion force, simulated by a harmonic potential, and a random force. We adjust the parameters that control the random force intensity and the interaction potential, to reproduce real biological tissue behavior. Then we introduce a tumor cell in the center of the tissue and compare the adhesion properties in healthy and tumor tissue.   

The Dependency in the Elasticity of the \textit{Saccharomyces cerevisiae} Cell Wall upon Cell Viability and Membrane Integrity

In this study the elastic spring constant of the yeast cell wall is probed with the atomic force microscope (AFM) under variable conditions. Cells were sequentially analyzed in rich growth medium (YPD), a 0.8 M NaCl rich growth medium solution and an injection of 0.01{\%} sodium azide solution. Cells in late log phase, which have variable diameters within three to five microns, were immobilized on a patterned silicon substrate with holes approximately 3.8um in diameter and 1.5um deep that was functionalized with polyethylenimine prior to cell application. Force curves were taken moving laterally across the cell in one dimension after exposure to each medium. Spring constants of the cells, calculated from force curves, displayed a positional dependency and marked differences in high osmolarity medium and after the injection of sodium azide. This study demonstrates the ability of the AFM to investigate changes in cell morphology and correlate those findings to underlying physiological processes.   

A Two-State Analysis of ERP Activity Measures and fMRI Activations Relevant to the Detection of Deception

A novel analysis approach for high-density event related scalp potential data (ERP) gathered druing various scenarios is presented. We construct energy-density functional clusters using the empirical voltage and power values and extract extrema of these cognitive activity mesaures to assess the temporal dynamics in areas of physiological significance for the detection of deception. These studies indicate that for questions relating to autobiographical knowledge neocortical interaction times are greater for deceptive responses. This finding is reproduced when workload requirements are increased and suggests that a ``neocortical circuit'' involving activity in short-term memory, visual processing, and executive control regions of the cortex is present. Individual and group analyses are given and continuing experiments involving questions where misinformation is used illustrate that early, up-front control may also be present during deceptive repsonses. A comparison of dipole source models with fMRI data collected in our lab confirms that BOLD activation in the ROIs is consistent with our model of deception.   

Electrical Monitoring Cytotoxic Effect of Cigarette Smoke Condensate on Transendothelial Invasion of Ovarian Cancer Cells

We investigated the effects of cigarette smoke condensate (CSC) on barrier function and cellular migration of human umbilical vein endothelial cells (HUVEC), and on the invasive activities of ovarian carcinoma cells through HUVEC monolayers as well. Central to this work was the use of electric cell-substrate impedance sensing (ECIS), a cell-based biosensor that monitors motility and other morphology changes of cells adherent on small gold electrodes. Upon addition of different concentrations of CSC, the junctional resistance and the wound healing rate of the HUVEC layers decrease as CSC concentration increases from 0.01 to 0.25 mg/ml, whereas the average cell-substrate separation increases with CSC concentration. Following the addition of OVCA429 ovarian cancer cells to HUVEC layers with the presence of different CSC concentrations, dose-dependent changes of the transcellular resistance drop were observed. Our results suggest that CSC is detrimental to normal endothelial cell function in maintaining vascular integrity. In addition, the chemicals present in CSC may increase transendothelial invasion of ovarian cancer cells.   

Silver and Gold:Palladium nanoparticles produced by Inert gas condensation

We report the synthesis of (AuPd and Ag) metallic nanoparticles (NPs) deposited on silicon and sapphire wafers and TEM grids. The NPs are formed by an inert gas condensation technique, based on dc-magnetron sputtering followed by condensation in high pressure zone. The size of the NPs was controlled through the variation of gas flow (Ar and He) inside the condensation zone, magnetron power, and condensation zone length. The NPs are negatively charged and may therefore be mass selected by a quadrupole mass filter, obtaining the size-distribution of NPs. We performed morphological, structural and composition studies of the NPs by mass spectroscopy, AES, XPS, AFM, UV-Visible spectroscopy, TEM, and HRTEM. Our procedure allows both a remarkable control over average size of the nanoparticles on the sample, and deviations below 5{\%} around this average size.   

A New Route for the Synthesis of Silver Nanocubes in PSS under Microwave irradiations

Shape control synthesis of inorganic nanostructures has received considerable attention in recent years because of applications in catalysis, optics, microelectronics and magnetics and medical diagnostics. Faceted nanostructures, such as cubes can provide properties distinct from spherical nanoparticle, for example, pinning of the magnetic domains to attain ferromagnetic properties at nanoscale, catalysis and SERS-based sensing that are lost in spherical shape due to thermal fluctuation. Noble metal nanoparticles (NPs) such as silver with cubic shape used as a template formation of gold nanoboxes and iron nanocubes used as the building block of magnetic superlattices. Here, we demonstrate a new approach for the rapid synthesis of silver nanocubes by a simple microwave irradiation approach. The present approach we described here for connecting nanomaterials into desired shapes and thereby tuning their optoelectronic properties may find wide application in nanotechnology particularly in nanoelectronics and plasmonics.   

Surface states in quantum confined periodic systems

Using recently published analytical expresions for eigenvalues and eigenfunctions of 1-D finite periodic systems, we calculate surface energies and wave fuctions of 1-D quantum confined systems. We analyse the surface repulsion effect and calculate exactly the surface energy levels for different potential profiles and different potential discontinuities at the surfaces. We study also the localized surface states and their relevance in the optical response of laser devices, which active region is a semiconductor superlattice. We discuss the advantages of this approach compared with the approximate calculations based on the Bloch functions and the so-called band- edge states.   

Thermoelectric properties of $p$-type Bi$_2$Te$_3$/Sb$_2$Te$_3$ and $n$-type Bi$_2$Te$_3$/Bi$_2$Te$_{3-x}$Se$_x$ superlattices

Thermoelectric superlattices are good candidates for obtaining high figure of merit (ZT) values. Indeed, the highest ZT of 2.4 at room temperature in $p$-type Bi$_2$Te$_3$/Sb$_2$Te$_3$ superlattices and the high ZT of 1.4 in $n$-type Bi$_2$Te$_3$/Bi$_2$Te$_{2.83} $Se$_{0.17}$ superlattices are found. \footnote{R.Venkatasubramanian, E.Siivola, T.Colpitts, and B. O'Quinn, Nature {\bf 413}, 597 (2001).} While it is well known that phonon-blocking and electron- transmission is a possible mechanism for the highest ZT in superlattices, the electron-transmission near the interface has not been studied much at the microscopic level. By first-principles calculations with the highly precise full- potential linearized augmented plane wave (FLAPW) method, \footnote{Wimmer, Krakauer, Weinert, Freeman, Phys.Rev.B, {\bf 24}, 864 (1981).} the electronic structures and thermoelectric properties of bulk Bi$_2$Te$_3$, Sb$_2$Te$_3$ and Bi$_2$Te$_{3-x}$Se$_x$ and of their superlattices Bi$_2$Te$_3$/Sb$_2$Te$_3$ and Bi$_2$Te$_3$/Bi$_2$Te$_{3-x}$Se$_x$ are investigated and will be reported.   

Transport properties in magnetic field of Pb1-xSnxTe alloys doped with Indium

The galvanomagnetic and thermomagnetic transport properties of single-crystal In-doped Pb1-xSnxTe are presented as a function of Sn (10 to 30{\%}) and In (0 to 10{\%}) concentrations. The concept is that the In level might pin the Fermi energy in a position with and enhanced density of states, which might increase the thermoelectric figure of merit. The transport properties were measured in a transverse magnetic field and at temperatures varying from 80 to 380K. Depending on the Sn concentrations, the prepared samples are p and n type semiconductors. The measurements of the electrical conductivity, Hall, Seebeck and transverse Nernst-Ettingshausen effects yield the carrier density and mobility, the density of states effective mass, and the scattering exponent, following the method of the four coefficients. The transport properties are interpreted in terms of hybridization of the In levels and density of state of the host alloy and observations are discussed in terms of Mahan-Sofo theory. The model provides an explanation for unexpected variation in thermoelectric and thermomagnetic properties of these alloys.   

DFT modeling of filled (Fe,Co)P$_{3}$ skutterudites for thermoelectricity

Skutterudites on the form $M_{x}$Co$_{4-y}$Fe$_{y}$P$_{12}$, with $M $being La, Y, and Sc, $x $= {\{}0, 0.125, 0.25, 0.50, 1{\}}, and $y $= {\{}1, 2, 3, 4{\}}, have been studied using band-structure density-functional theory (DFT) calculations. The stability of these hypothetical phases (only two have so far been reported in the literature) has been assessed by comparing to other relevant phases. The Sc-filled compounds, which would have had very large rattling amplitudes due to the small size of the Sc atom, were all shown to be thermodynamically unstable, but several of the La- and Y-filled compounds should be possible to synthesize. The maximum stability was found for the compositions with $y$ = 3$x$. Such compounds also seem to be best suited to thermoelectric purposes from their electronic structure.   

Energy Band Calculations for Maximally Even Superlattices

Superlattices are multiple-well, semiconductor heterostructures that can be described by one-dimensional potential wells separated by potential barriers. We refer to a distribution of wells and barriers based on the theory of maximally even sets as a maximally even superlattice. The prototypical example of a maximally even set is the distribution of white and black keys on a piano keyboard. Black keys may represent wells and the white keys represent barriers. As the number of wells and barriers increase, efficient and stable methods of calculation are necessary to study these structures. We have implemented a finite-element method using the discrete variable representation (FE-DVR) to calculate E versus k for these superlattices. Use of the FE-DVR method greatly reduces the amount of calculation necessary for the eigenvalue problem.   

Electronic Spectrum of Superlattice Wires

We consider superlattice nanowires, and find an exact solution to the model-independent quantum Hamiltonian. We obtained a closed-form solution to this problem. The energy levels for general interatomic interactions are calculated in the context of the Hubbard model. We obtain an explicit formula for the function whose roots render the energy states. The corresponding energy bands can be tuned by the usual superlattices concept of pattern control but also, in the case of nanowires, by controlling the interatomic separation of the structure. We apply our results to nanowire tunneling diodes, angle-resolved photoemission spectroscopy, and Si-Ge superlattice nanowires.   

STM study of some strain induced Electronic Patterns on Graphite Surface

Abstract: We report on the STM observation of spatially varying super-lattice structures on (0001) graphite surface in a region confined by two straight carbon fibers. It varies from a hexagonal lattice of 6nm periodicity to nearly a square lattice of 13nm periodicity. It then changes into a one-dimensional fringe-like pattern before relaxing into a pattern-free region. Scanning on the surface gradually removes one fiber along its length and this shrinks the area of the super-lattice region. The boundary between 2-D lattice and 1-D fringes is found to be pinned to one end of this fiber and it moves as the fiber end recedes. We attribute this pattern to a spatially varying moir\`{e} rotation of one of the top layers giving rise to a spatially varying stacking pattern. This also affects the local density of electronic states near the surface as observed in the local tunneling spectra and the conductance imaging that we also report here. We have also successfully modeled this pattern using moir\`{e} rotation hypothesis with successful modeling of the 1-D fringes. In fact this new model gives much better insight into the large scale structure of such patterns.   

Electron transport of a driven three-level system in an asymmetric double quantum dot

Electron tunneling through a three-level system in an asymmetric double quantum dot irradiated by an external field is investigated. For a resonant external field, two symmetric peaks occur in the current spectrum. If the field frequency is detuned, unequal contributions from two channels lead to two asymmetric peaks with population inversion, which can be also observed with the increasing of Rabi frequency. On the other hand, as the ground states in two dots are equal, a suppression of current occurs around the resonant frequency. In contrast, an enhanced behavior is found for the case of unequal ground states.   

Jahn-Teller distortion of the Wigner molecule in a three-electron quantum dot and a magnetic field: pair function approach

We considered a two dimensional three electron quantum dot in a magnetic field in the Wigner limit. Using a unitary coordinate transformation of the Hamiltonian (with Coulomb interaction between the electrons included) into a sum of pair Hamiltonians, and the Pauli principle for the resulting product of pair functions, we have shown that the three electrons in the ground state of the Wigner molecule form an equilateral triangle (as expected from naive reasoning) only, if the state is a quartet (S=3/2) and the orbital angular momentum is a magic quantum number L=3 m, (m=0,1,2,...). Otherwise the triangle is isosceles. For L=3 m+1 one of the sides is longer and for L=3 m-1 one of the sides is shorter than the other two.   

Polarization in Nano-Spherical Shell Arrays

With the improvement in fabrication techniques on the micro- and nanometer scale, experimentalists are now able to produce atom-like electronic devices with their own unique spectra and shell~structures. In particular, we may now examine periodic arrays of nanostructures without atomic analogues in which exchange-interaction, polarization, and separation may all be varied. Through the use of Monte Carlo simulation, in the classical case, and Schroedinger variation, in the quantum mechanical case, we search for novel properties in an array of singly-charged nano-spherical shells,~sometimes called ``quantum well quantum dots.'' A phase transition has already been shown to exist in a similar system consisting of nano-rings, making the same in nano-spherical shells seem a distinct possibility. Such control of the electron on the nano-scale may~prove useful in optical and spin- or charge- based quantum information schemes.   

Exciton complexes in self-assembled quantum dots: Magnetic field direction dependence

Energy levels of up to four excitons in unstrained GaAs/AlGaAs and strained InAs/GaAs quantum dots (QD) are studied in the presence of an external magnetic field. Influence of the direction of the applied magnetic field on the exciton fine structure is shown, as well as the way to control the fine structure splitting. The single particle states are extracted from the nonsymmetrized eight-band k.p model including strain, piezoelectricity and Zeeman effect, whereas the employed discretization scheme preserves the gauge invariance. The singles and doubles configuration interaction method is used for extracting the few-particle states.   

Two dimensionally patterned GaN$_{x}$As$_{1-x}$ Quantum Dots Fabricated using Ion Implantation and Pulsed Laser Melting characterized by Ballistic Electron Emission Microscopy.

We will present Ballistic Electron Emission Microscopy (BEEM) measurements on 2D patterned GaN$_{x}$As$_{1-x}$ nanostructures fabricated in a GaAs matrix using nitrogen ion implantation followed by pulsed laser melting and rapid thermal annealing (RTA). As a three terminal scanning tunneling microscopy technique, BEEM can image both the surface topography and the local hot electron transport. Using ion implantation through a lithographically patterned mask and varying subsequent processing conditions, we have made locally confined GaNxAs1-x dots with different activated nitrogen concentrations. By analyzing BEEM images of the quantum dots, we study giant bandgap bowing effects on the Schottky barrier height. We will also discuss the effects of different implanted nitrogen concentrations, laser fluences and RTA conditions on the conduction band structures of these quantum dots.   

A Modeling of Photonic Crystal Fiber with a Boundary Integral Equations

A boundary integral equation (BIE) for the photonic crystal fiber is formulated using the free space Green's function and Huygen's principle. The BIE reduces the number of unknowns significantly and is flexible to handle the geometry of the fiber owing to its nature of the formulation. The real and imaginary parts of the propagating constant, which is related to the dispersion and the confinement loss of the fiber, are calculated as a function of wavelength for both the air-silica fiber and the photonic bandgap fiber by the root searching method. The numerical simulations show that the air-silica fiber guides the light according to the total internal reflection and that the photonic bandgap fiber guides the light based on the scattering from the Fabry-Perot-like high-index inclusion. As a consequence, the spectrum of photonic bandgap fiber shows the discontinuities, and the locations of discontinuities obtained with BIE are compared with the simple analytical model based on the AntiResonant Reflecting Optical Waveguide (ARROW) model suggested by Natalie et al.   

Experiment and Calculations on the Defect-Induced Broadening in One-Dimensional Photonic Bandgaps

Impurity bands are generated by the coupling of defect modes in one-dimensional photonic crystals, which move the bandgap edges towards the shorter and the longer wavelengths. Combining one structure with impurity band and another without it, a greatly broadened photonic bandgap is created when the two structures are well designed. The samples are fabricated by the electron-beam evaporation, which are composed of SiO$_2$ and TiO$_2$. A good agreement between the experimental and the theoretical results is obtained. The further calculations, based on the 4$\times$4 transfer-matrix method, strongly reveal the validity of this approach, so-called the defect-induced broadening. More importantly, the widened omni-directional photonic bandgaps are also realized by this means.   

Giant band gaps in photonic heterostructures obtained by calculation of the group velocity

In this work we show that is possible to design photonic heteroestructures with giant frequency ranges of low transmission by calculation of the group velocity. A heterostructure is the union of two lattices characterized with distinct relation dispersions. We calculate the band structure of the complete heterostructure implementing the super cell technique in the wave plane method. Even if the heterostructure energy bands present a complicate structure, we discriminate the energy the pass (stop) bands region by the calculation of the group velocity. Our predicted pass (stop) bands are verified by the direct calculation of light transmission using the transfer matrix method.   

The influence of the disorder in the omnidirectional photonic band gap for one-dimensional photonic crystals

In this work we study the influence of the disorder in the omnidirectional photonic band gaps. Even if recently has been reported different results on the influence of disorder in the optical properties of disorder in one-dimensional photonic crystals. Most of these studies have been focused on the influence of disorder to the light transmission in the ballistic direction using transfer matrix methods. In difference, we focus in the influence of disorder in the omnidirectional photonic band gap. We present calculations of projected band structure by using the Plane Wave Method with a supercell technique. We perform numerical calculations of the supercell Fourier coefficients in order to study different disorder configurations. The intricate information of the supercell projected band gaps is discriminate by the consideration of the group velocity.   

Fullerene nanostructures on defect-rich graphite investigated with STM

In this work we examine the feasibility of creating tailored fullerene nanostructures on Highly Oriented Pyrolitic Graphite (HOPG). Based on recent findings [M. Buettner et al., Surf. Sci., 2006, in press] we illustrate a new preparation method employing Focused Ion Beam (FIB) techniques in combination with fullerene deposition and subsequent sample annealing. The pristine HOPG surface is treated with FIB, creating a predefined structure of defects that serves as a template. Typical length scales available with FIB and relevant for our purpose are several nanometers to micrometers. Fullerene deposition performed in UHV followed by an annealing step results in fullerene-decorated defects, thus forming the final C$_{60}$ nanostructure. All preparation steps are accompanied by UHV Scanning Tunneling Microscopy (UHV-STM) imaging. We will discuss the formation of the C$_{60}$ nanostructures in detail, examine the achievable accuracy of the process, and finally comment on the viability of the proposed method for preparing functionalized molecular nanostructures.   

A Simple Route for Preparation of Nanoporous Templates

Spin-coating method of block copolymers represents a very simple means of producing thin films for the fabrication of nanostructured materials. However, the orientation and long-range ordering of the block copolymer microdomains must be controlled to maximize areal density. Here, spin coated films of poly(styrene-b-4-vinylpyridine) (PS-b-P4VP) diblock copolymers in THF/toluene solvent mixtures were investigated and it was found that highly oriented cylindrical microdomains with long-range lateral order were obtained on a variety of substrates, such as silicon oxide, polystyrene, polyimide, poly(butylene terephthalate), and germanium. The preferential solvation of P4VP block with an alcohol enabled a surface reconstruction to produce a structure of the block copolymer film that leads to the generation of a nanoporous template upon drying. The gold evaporation on the reconstructed films produces thermally stable and plasma resistant films.   

Observation of a Small Number of Molecules at Metal Nano-dimers arrayed on Solid Surface via Surface-Enhanced Raman Scattering

Novel approach for the detection of small number of molecules was investigated using metal coupled dot structures showing strong surface-enhanced Raman scattering (SERS). The SERS activities of Ag and Au nano-dot array were controlled via choosing appropriate gap distance between metal nano-dots and their structural anisotropy. Intense signals due to the strong Raman scattering of target molecules were observed when the optical absorption of the dots was tuned to be at the excitation wavelength in near-infrared region. Characteristics behavior of spectral blinking was observed at the system of the Au-Ag hetero-dimer array in solution.   

Polarization Studies of PL from QD's Multi-Excitons

We study the multi-exciton emission from single CdSe/ZnS quantum dots. Fluorescence decay dynamics as well as bunching behaviors at various excitation fluence indicates the fast PL dynamics due to the relaxation from multi-exciton. The results show threshold energy level for multi-exciton generated from exciton states. At low excitation fluence, anti-bunching behavior, and nearly single exponential relaxation dynamics are observed. Above a certain threshold, additional fast relaxation and bunching behavior from the same QD indicates that the multi-step radiative cascade relaxation processes. In addition, polarization dependence of the fast relaxation dynamics will be reported.   

Anti-bunching Behavior of the Red-Emission.from Single Quantum Dots

Ultra-small quantum dots exhibit band-to-band transition emission, as well as strong broadband red-emission. It is believed that the red-emission originates from the surface mid-gap state transitions. We study the correlation of the red-emission properties of CdSe/ZnS quantum dots by single object fluorescence. It is found that the red-emission portion is very different from one QD to another QD. The collected fluorescence was dichroic split, and sent into two photon counting detectors for monitoring band-to-band transition, and red-emission, respectively. The red-emission species exhibits different decay dynamics from the band emission. However, anti-bunching correlation indicates that quantum emitter behavior of single quantum dot, and the red-emission is not directly photo-excited by the excitation laser, but energy transfered from the exciton in the quantum dot.   

Direct Comparison of one-photon and two-photon cross-sections of chromophores by low power ps laser.

Two photon absorption is generally interested for its basic photophysics and nonlinear optical application. In particular, fluorescence excited by two-photon processes attracts much attention for the sensitive fluorescence detection. However, due to the strong power dependence of the two-photon processes, many two-photon fluorescence experiments require high peak power laser as an excitation source. We present a study of direct fluorescence intensity comparison of 532 nm /1064 nm low power pulse laser excitation. The sample was inserted on an inverted optical microscope equipped with a high numerical aperture oil-immersed objective. Switching between two optical paths allows us to directly compare one-photon and two photon excitation processes within good accuracy. Low power/ high rep. rate scheme provides low damage/ high stability capability. Direct comparison between one-and two- photon excitation processes allow us to take advantage of the easily determination of one-photon excitation coefficients. The comparison can be further extended into tunable light source, such like fs Ti:S laser for the wavelength dependence of two photon cross-section studies.   

Raman Spectroscopy from Optical Trapped Nano Coated Microsphere Complexes

Raman measurements from optically trapped gold nanowires and from magnetic nanoparticles coated on microspheres, under various visible laser excitation wavelengths, are being studied. Changes in the Raman spectra for these trapped microsphere complexes when they were positioned at different distances from an immersed photonic crystal (PC) are also being investigated.   

Surface states of multilayerd conducting heterostructures.

We discuss the existence of surface states of multilayerd conducting heterostructures, doing an analogy among the equations wich describe the physics of quantum-well structures and those wich describe the physiscs of multilayerd conducting heterostructures. The physics of infinte quantum-well structures has been studied extensibly as well as semi-infinite quantum-well structures. The electromagnetic propagation of multilayerd conducting heterostructures has been studied using different aproximations. Within the hidrodynamic model of electron dynamics, we found an expression for the bulk and for the surface states of multilayerd conducting heterostructures similar to those found in quantum-well structures.   

Spin Current from Quantum Dots embedded in a Microcavity

We examine the spin current generated by quantum dots embedded in an optical microcavity. The dots are connected to leads with zero bias voltage across the dot with one of the Zeeman states lying below the Fermi level of the leads and the other above. The spin current is generated by spin flip transition from the lower to upper Zeeman states induced by Raman transition involving a classical pump field and a quantized cavity mode. In the case when the spin flip transition is accompanied by emission of a photon into the cavity mode, we calculate the spin current and the photon current associated with the photons leaking out of the cavity as well as the shot noise. We show that the photon current is equal to the spin current and that the spin current can be significantly larger than for the case of a classical driving field. The frequency dependent spin (photon) current shot noise show dips (peaks) that are a result of the discrete nature of photons. In the case of absorption of the photon from the cavity mode, we find that the cavity mode and the spin current exhibit bistability as a function of the laser amplitude, which is driving the cavity mode. Even for a single dot, the spin current and the cavity field have a bimodal structure.   

Effect of Modulation on Coupled 2D-Surface Plasmons

We present a calculation for a two-dimensional (2D) electron gas layer interacting with a slab of conductive material. We treat the plasmons in the slab in the local limit and obtain the frequency of the coupled mode corresponding to the extended 2D plasmon interacting with the background plasmons in the presence of a conducting surface. The dispersion equation of a double quantum well is obtained and we show how the split symmetric and antisymmetric modes are formed and modified by the localized surface plasmon. For a single layer, we show that when a one-dimensional (1D) periodic electrostatic potential is applied to the surface, each of the symmetric and antisymmetric modes will be further split by the interaction with the 1D modulation, leading to folding of plasmon dispersion curves for different modes. For double layers, we show that the coupled 2D and surface plasmons may result in radiated energy. Our analysis is based on a calculation of the surface response function obtained using a transfer matrix method.   

Essential role of strain in Photoluminescence of band gap engineered ZnO nanocrystals synthesized by low

The band gap of wide band gap semiconductor ZnO can be engineered by substituting (alloying) bivalent metals like Cd and Mg in place of Zn. We report synthesis of high quality nanocrystals of Mg and Cd substituted Zinc Oxide nano- crystals (8-10nm) and nanorods (length 25nm) by low temperature solution route. The substitution maintains the Wurzite structure. A continuous compaction of the lattice occurs when the Mg substitution takes place while there is a continuous expansion of the lattice on Cd substitution. This lattice compaction (expansion) blue shifts (red shifts) the band gap as well as the near band edge photo luminescence (PL) at room temperature. An analysis of the time resolved photoluminescence (TRPL) as well as the microstrain dependence of the PL line width suggests that in these nanoparticles there is excitonic localization by the random strain produced by the substitution. The investigation of optical and its correlation with structural properties indicate that in the band gap gets engineered mainly by strain.   

Large cooling differentials and high heat flux capability with p-type Bi$_{2}$Te$_{3}$/Sb$_{2}$Te$_{3}$ and n-type Bi$_{2}$Te$_{3}$/Bi$_{2}$Se$_{x}$Te$_{3-x}$ Superlattice Thermoelectric Devices

Thin film superlattice (SL) based thermoelectric (TE) devices offer the potential for improved efficiency and high heat flux cooling over conventional bulk materials. Recently, we have demonstrated external cooling of 55K and heat pumping capacity of 128 W/cm$^{2}$.\footnote{$^{.}$ G.E. Bulman, E. Siivola, B. Shen and R. Venkatasubramanian, Appl. Phys. Lett. 89, 122117 (2006).} These high heat fluxes in thin film devices, while attractive for cooling hot-spots in electronics, also make the device performance sensitive to various thermal resistances in the device structure. We will discuss advances in the cooling performance of Bi$_{2}$Te$_{3}$-based SL TE devices and describe a method to extract device material parameters, including thermal resistance, from measurements of their $\Delta $T-I-V characteristics. These parameters will be compared to values obtained through Hall and Seebeck coefficient measurement on epitaxial materials. Results will be presented for both single couple and multi-couple modules, as well as multi-stage cascaded devices made with these materials. Single stage cooling couples with $\Delta $T$_{max}$ of 57.8K (T$_{c} \quad \sim $242K) and multi-stage modules with $\Delta $T$_{max}\sim $92.2K (T$_{c} \quad \sim $209K) have been measured.   

A First-principles Molecular Dynamics Investigation of Superionic Conductivity

Superionic materials---solids with liquid-like transport properties---have found widespread use in a variety of applications in fuel cells, switches, sensors, and batteries. However, reasons for fast-ion conduction in such materials, as well as the specific atomistic mechanisms involved, remain ill understood. Our work uses first-principles molecular dynamics to illuminate the mechanisms, pathways, and motivations for superionic conductivity in two materials representing different classes of ion conductors: $\alpha$-AgI, an archetypal Type-I superionic; and CsHSO$_{4}$, an anhydrous solid-state electrolyte candidate for hydrogen fuel cells. For $\alpha$-AgI, we trace common pathways for silver ion conduction and discuss how a chemical signature in the electronic structure relates to enhanced silver ion mobility. We also characterize the dynamical lattice structure in the superionic phase and present the likely motivations for its existence. For CsHSO$_{4}$, we isolate the dominant atomistic mechanisms involved in superprotonic conduction and discuss the effect of correlated diffusive events in enhancing proton transport. We also offer a detailed description of the dynamics of the hydrogen bond network topology in the course of proton diffusion and discuss the relevance of atomistic processes with competing timescales in facilitating proton transport.   

Multiscale computer simulation of failure in silica aerogels

Silica aerogels are low-density materials whose low thermal conductivity makes them of interest for applications requiring lightweight thermal insulation. However, the fragility of these gels is problematic; they exhibit brittle failure at low stress levels. In order to better understand the mechanical behavior of these materials, we have performed multiscale computer simulations of failure in aerogels. Gel structure is modeled using Diffusion Limited Cluster Aggregation (DLCA) simulation, resulting in clusters having the stranded ``pearl necklace'' morphology characteristic of the gels. Such strands consist of so-called ``secondary particles'' connected by bridges of low-density amorphous silica. We model the mechanical behavior of our model gel clusters by assuming that failure occurs through the breaking of bridges. The energetics of bridge strain are computed by atomistic simulation using a Rappe-Goddard potential for amorphous silica, and the results used to determine an effective secondary particle interaction potential that includes a Morse pair contribution and an angular component. We present strain energetics of a number of model clusters, and discuss the factors which determine the ductility of the cluster failure.   

Ferromagnetism and electron pairing in tetrahedral Hubbard clusters

The canonical and grand canonical calculations in exactly solvable three dimensional tetrahedrons elucidate the origin of Mott-Hubbard-like transition, electron pairing and ferromagnetism in frustrated Hubbard clusters. The thermal properties of planar and tetrahedral clusters in magnetic field with one hole off half-filling provide a rigorous proof for the existence of Mott-Hubbard type insulators with spontaneous magnetization in the ground state and finite temperatures. Rigorous conditions for electron pairing instability and phase separation in frustrated tetrahedral cluster for all $U>0$ is also provided. We show that Nagaoka-type spin flip instability with ``unsaturated ferromagnetism'' is equivalent to electron charge and spin pairing instabilities with minimal, zero {\it spin}. The theory gives strong evidence for existence of charge- spin separation in three dimensional clusters and calculated phase diagram for condensation crossover temperatures at various instabilities encompass a number of phases recently discovered in clusters, small nanoparticles, transition metal oxides and high T$_c$ cuprates.   

Nanowires nucleation by impact of metallic nanoparticles

We performed molecular dynamics simulations of impacts between gold spherical and icosahedral nanoparticles, at different relative velocities, temperatures and orientations. We found that the coalescence process depends not only on the energy supply available for the reconstruction of the structures, but the coalescence times and final equilibrium structures are strongly determined by the relative orientations of the particles at the moment of the impact. For most of the conditions, the resulting particle show a tendency to take a quasi-icosahedral shape with triangular (111) faces but, when two icosahedra are impacted in a face-to-face orientation at 300m/s and T=350K, the resulting structure is elongated on the impact direction. A quiral elongated structure results from the impact of two spherical particles, and this geometry survives even after the impact of a third particle. The stability of these structures may be of importance for the nucleation of metal nanowires. Finally, we made a comparison of the simulation results with TEM observations.   

Density functional theory and DFT+U study of transition metal porphines adsorbed on Au(111) surfaces

We apply Density Functional Theory (DFT) and the DFT+U technique to study Pd(II) and Mn(II) phosphines adsorbed on atomistically flat Au(111) surfaces. PdP is found to adsorb preferentially on gold in a flat geometry, not in an edgewise geometry, in qualitative agreement with experiments on substituted porphyrins. The DFT+U technique is found to be crucial for reproducing the correct magnetic moment and geometry of the isolated manganese porphine (MnP) molecule. Adsorption of Mn(II)P on Au(111) substantially alters the Mn ion spin state and electronic structure. Its strong binding to the gold surface can be partially reversed by applying an electric potential, which leads to significant changes in the electronic, magnetic, and structural properties of the adsorbed MnP.   

Synthesis of Multiwalled Carbon Nanotubes using Natural Iron-Containing Minerals

We introduce a novel approach for synthesizing multiwalled carbon nanotubes (MWNTs) via chemical vapor deposition. Natural iron-containing minerals are treated with hydrofluoric acid, calcinated in argon, and reduced in hydrogen, resulting in uniform iron-containing silicate nanoparticles, which catalyze MWNT growth in acetylene (C$_{2}$H$_{2})$. As-grown MWNTs are characterized using electron microscopy and small angle neutron scattering. Through systematically varying C$_{2}$H$_{2}$ flow rate, growth temperatures and time, optimal conditions have been determined for synthesizing large quantities of uniform and clean MWNTs.   

Electrochemistry of 2-dimensional networks of carbon nanotubes

Electrochemical studies of 2-dimensional networks of carbon nanotubes (CNTs) will be presented. A new method of creating electrically continuous arrays of CNTs has been used to investigate electrodeposition of nanostructures at highly oriented nano-scale templates. Unidirectional air flow was used to order CNTs in aqueous suspension and deposit them on a hydrophobic SAM-modified surface (i.e. 3-aminopropyl-triethoxysilane on Si/SiOx). These 2-dimensional networks of CNTs show potential as a method of circumventing the difficulties associates with lack of control over the electrochemical properties of individual CNTs. For a random distribution of CNTs, density control is the major factor controlling device properties, as fluctuations in characteristics of individual CNTs are averaged. These ordered arrays of CNTs exhibited anisotropic electrical conductivity over macroscopic lengths (up to 3$''$), and have shown promise in a wide variety of electrochemical applications. Electrochemical reduction of water-soluble diazonium salts will be discussed.   

Nanotube Epitaxy

A review is presented of the organization of carbon nanotubes by substrate-directed growth on crystal surfaces. The production of ordered carbon nanotube arrays on surfaces is a critical prerequisite for their large-scale integration into nanocircuits. We have recently elaborated a series of surface-directed mechanisms of carbon nanotube growth, which can be classified as different modes of ``nanotube epitaxy''. These epitaxial modes of carbon nanotube growth include ``lattice-directed epitaxy'' (by atomic rows), ``ledge-directed epitaxy'' (by atomic steps) [1], and ``graphoepitaxy'' (by nanofacets) [2]. Some of these epitaxial modes can be simultaneously combined with electric-field directed growth [3] for the orthogonal self-assembly of carbon nanotube crossbar architectures [4]. Nanotube epitaxy with different crystal surfaces yields unprecedented carbon nanotube array morphologies, including highly straight, kinked, wavy, crossbar, serpentine [5], and more. [1] Ismach, et al., \textit{Angew. Chem. Int. Ed.} \textbf{2004}, $43$, 6140. [2] Ismach at al. \textit{J. Am. Chem. Soc.} \textbf{2005}, $127$, 11554. [3] Joselevich et al., \textit{Nano Lett.} \textbf{2002}, $2$, 1137. [4] Ismach et al. \textit{Nano. Lett. }\textbf{2006}, $6$, 1706. [5] Ismach et al., in preparation.   

Carbon nanotube twist-spun yarns cold cathodes: properties and application

Multiwall carbon nanotube (MWCNT) twist-spun yarns are the novel type of material with unique mechanical and electrical properties [1]. In this study we present the study of electron field emission of these yarns. It was found that FE from MWCNT yarns are very uniform with threshold voltage as low as 1 V/um. After conditioning process we have found that FE properties considerably self-improved. The origin of this self improving process is discussed in terms of influence of electrostatic force which tousles the MWCNT on the yarn. The combination of advanced mechanical and field electron emission properties of MWCNT make them ideal candidate to use as a cold cathode for vacuum devices. The lateral electron emission from MWCNT twist-spun yarns may be suitable for application in flat panel displays and lighting elements. The prototype of the indicator based on MWCNT yarn was successfully demonstrated. [1]. M. Zhang, K. R. Atkinson, R. H. Baughman: Science 306, 1358 (2004).   

Electronic structure of carbon nanotube under extreme shear strains

Deformation such as stretching, compression, torsion, or bending is known to change the electronic properties of carbon nanotubes in a controlled manner. Previous studies [1,2] provided details of such behavior based on understanding at linear response regimes. We have studied the electronic and optical properties of zigzag single-walled carbon nanotubes (SWNTs) at extreme shears. It is found that a certain type of small-radii semiconducting SWNTs can have an indirect band gap while other types exhibit armchair-like metallic characteristics. These behaviors can be understood in terms of electronic states in the twisted Brillouin zone of a single graphene layer with an appropriate folding constraint. We briefly discuss a possible application of these phenomena. [1] C. L. Kane and E. J. Mele, Phys. Rev. Lett. 78, 00193(1997). [2] Liu Yang and Jie Han, Phys. Rev. Lett. 85, 154, (2000).   

The structure dependence of the electrical conductance in the single carbon fullerene

In this study, we observed simultaneously the conductance and the structure of the single carbon fullerene (SCF) which was composed with a single shell or multi shells. A STM system fitted with a TEM operated at 200kV system was employed to the conductance measurement at room temperature. As a result, it was found that a SCF showed a linear I-V character at zero bias voltage like a metallic conductance. The order of the conductance value was not affected by the structure of SSF whether the single shell or multi shells, which implied that the electrical conductance of SCF did not depend on the internal structure.   

Photoluminescence microscopy of carbon nanotubes grown by chemical vapor deposition: influence of external dielectric screening on optical transition energies

Photoluminescence (PL) laser microscopy was applied to determine optical transition energies E$_{11}$ and E$_{22}$ of individual semiconducting single-walled carbon nanotubes (SWNTs) suspended on top of carbon nanotube `forests', grown by chemical vapor deposition (CVD) on silicon substrates. A uniform increase of E$_{11}$ and E$_{22}$ energies by 40--55 and 24--48 meV, respectively, was found for 19 different (n,m) nanotube species suspended in air/vacuum -- relative to SWNTs in a reference water-surfactant dispersion. We did not find any systematic correlation between nanotube (n,m) structure and energy shifts. CVD-grown SWNTs embedded in paraffin oil and 1-methylnaphthalene show nearly the same PL peak positions as SWNTs in aqueous dispersion, indicating similar dielectric screening of excitons in SWNTs in these media.   

Dielectric Properties of Individual Carbon Nanotubes Studied by Electric Force Microscopy

Dielectric properties of Carbon nantoubes (CNTs) have far-reaching implications on their nanoelectronic and optoelectronic applications, and are also critically important in the separation and solution processing of CNTs. Theoretical calculations have yielded some insights on the subject but no experimental investigation has been reported. Here we report direct measurements of the transverse polarization of individual CNTs using electric force microscopy (EFM). Individual CNTs on Si substrates are prepared with the chemical vapor deposition method When an AC bias (frequency $\omega )$ is applied between the sample and the conductive probe in EFM experiments, the interaction force between the charges on the probe and the induced dipole moment in CNT can be detected at the 2$\omega $ frequency. Our experiment gives the first dielectric property measurement of individual CNT and verifies the tube size dependence of the transverse polarizability predicted by first-principle calculations.   

The temperature dependence of excitonic decay in single-wall carbon nanotubes

Recent theoretical calculations indicate that the existence of a multiplet of excitonic states may affect recombination kinetics in single-walled carbon nanotubes. The possibility of a multiplet of coupled excitonic bands has strong implications for the temperature dependence of the effective radiative lifetime. We have performed steady-state photoluminescence, time-correlated single photon counting, and Raman spectroscopy measurements on single-wall carbon nanotubes from 4 to 293 K. We observe novel photoluminescence spectra that cannot be attributed to vibronic transitions and verify the existence and energy levels of weakly emissive excitonic states. We determine how nonradiative and radiative excitonic decay rates change as a function of temperature and contrast this with theoretical predictions. The results suggest that recombination kinetics are influenced by multiple excitonic bands, including a dark lower state. The long lifetimes of the low-energy peaks measured here suggests that other bands may have different excitonic decay and transport properties that may be potentially useful in photoconversion devices.   

Single wall carbon nanotubes transparent electronics

This work introduces a new junction method for single wall carbon nanotube-based transparent devices and report on their gate-dependent electro-magnetic behaviors. In order to realize the transparent CNT-based electronics we designed a diluted magnetic impurity doped transparent catalyst electrode. The suspended single wall nanotube channel showed ambipolar operation and almost the same low barrier height for the holes and electrons, which was determined by temperature dependent current measurements as functions of bias and gate voltage. Though at present the junction is not selective with respect to which types of carriers can be controlled in electric, we anticipate being able to provide a promising unit for spintronics.   

Numerical and experimental studies of enhanced electron emission from functionalized carbon nanotube emitters

Vertically aligned carbon nanotubes (CNTs) were grown using plasma enhanced chemical vapor deposition (PECVD) method. The CNTs were further functionalized by coating their surface with a thin layer of low work function oxide emissive materials. The electron emission capability of the coated CNT emitters was greatly improved with the low work function emissive layer, particularly at high temperature. Thermionic emission current three orders magnitude higher was observed. The emission properties of the oxide coated CNTs were measured and characterized over a wide temperature and field ranges. It was found that neither the Fowler-Nordheim theory for field emission nor the Richardson theory for thermionic emission were adequate to describe the electron emission characteristics of these emitters in certain range of temperature and field. However, by adopting a general electron emission formulism developed by Murphy and Good, we were able to simulate the electron emission from the coated CNTs over the whole temperature and field range and fit the experimental data.   

Kinetics of Gas Adsorption on External Surfaces of Carbon Nanotube Bundles

We present a study of adsorption kinetics on the exterior of a carbon nanotube bundle by means of computer simulation. The surface is modeled as a group of one-dimensional chains of sites with different binding energies. The simulation is performed by using a Kinetic Monte Carlo scheme that follows the time evolution of the gas uptake for different values of external pressure and temperature. The results are analyzed in terms of the difference between the binding energies and the amount of particles that are exchanged between the lines as adsorption is taking place. We show that preliminary experimental results for CF$_{4}$, Ar, and CH$_{4 }$on nanotube bundles with closed ends are consistent with our results.   

Catalytic role of defective carbon in the methane decomposition for CO and CO$_{2}$-free hydrogen generation

Decomposition of methane is an environmentally attractive approach to CO and CO$_{2}$-free hydrogen production. The decomposition of methane over defective carbon was studied by using state of the art first principles modeling techniques. Our studies demonstrate that the defective carbon itself can be used as catalyst for methane decomposition, without the need for other catalysts like transition metals or oxides, and the catalytic sites in defective carbon can be regenerated by the deposition of carbon decomposed from methane, to make the hydrogen production a continuous process. Additionally; since no other gases are produced in the process, the cost of CO$_{2}$ sequestration and hydrogen purification from CO contamination will be dramatically reduced.   

Porous diamond-like-carbon films deposited using pulsed plasma sputtering and pulsed laser deposition techniques

The ordering porous diamond-like-carbon (PDLC) thin films are prepared on silicon substrate using pulsed plasma sputtering deposition and pulsed laser deposition techniques. Scanning electron micoscope, X-ray diffraction and Raman scattering are employed to characterize the morphology and bonding structures of the porous carbon networks. Different structures and properties of the PDLC thin films prepared using two deposition techniques have been identified. The effect of high d.c. bias voltage on the properties of the PDLC films is studied.   

Modeling of Magnetoelectric Effect in Ferromagnetic-Piezoelectric Porous Composites

One of the important factors that influence the magnetoelectric (ME) properties is the porosity in bulk ferromagnetic-piezoelectric composites. Bulk composites have the advantage of superior mechanical strength over layered samples. Here we provide a systematic experimental investigations and modeling of porous bulk composites. Samples of ferrite and piezoelectric composites have been synthesized with porosity ranging from 5 to 40{\%}. Studies reveal a 60-90{\%} reduction in the ME voltage coefficient \textit{$\alpha $}$_{E}$ as the porosity $p$ is increased from 5 to 40{\%}. We also discuss a model that considers the influence of porosity on ME interactions in a bulk composite. Expressions for ME voltage coefficients have been obtained for low frequencies and at electromechanical resonance. The calculated ME coefficients are in very good agreement with the data. The research was supported by a grant from the NSF (DMR-0606153).   

Resonance enhancement of magnetoelectric coupling in ferrite-piezoelectric bilayer at bending and shear vibrations

Magnetoelectric (ME) couplings in bilayers of magnetostrictive and piezoelectric phases are mediated by mechanical deformation. We discuss here the theory and experiments on ME interactions in a ferrite-lead zirconate titanate (PZT) bilayer at frequencies corresponding to bending and shear modes of electromechanical resonance (EMR). Excitation conditions for bending and shear vibrations of a bilayer are considered. Estimated ME voltage coefficients versus frequency profiles for nickel ferrite or cobalt ferrite and PZT show a giant ME effect at EMR with the highest coupling expected for cobalt ferrite-PZT. Measurements of resonance ME coupling have been carried out on layered composites of nickel ferrite-PZT. We observe the expected increase in ME voltage coefficient at EMR. Theoretical ME voltage coefficients versus frequency profiles are in excellent agreement with data. The research was supported by grants from the NSF (DMR-0606153; NIRT-0609377; ECCS-0621907).   

Ultra Low Density Materials Synthesized via an Aerosol Gelation Process

We synthesized carbon and silica ultra low dense and high specific surface area solids via aggregation and gelation of nano-sized particles in the aerosol phase. We named such solids as Aerosol Gels. The process of formation of an Aerosol Gel starts with a collection of individual nano-sized monomers which undergo Brownian aggregation and finally gel quickly relatively to the gravitational settling when the monomers are small enough ($\sim $ 10 nm radius) and the monomer volume fraction is high enough ($\ge $ 10$^{-4})$. The Carbon Aerosol Gel was found to have high specific surface area (200 -- 350 m$^{2}$/g), an extremely low density (2.5 -- 5 mg/cc) and a high electrical conductivity, properties similar to conventional aerogels. The Silica Aerosol Gel was also extremely porous with high specific surface area ($\sim $ 450 m$^{2}$/g) and a very low density ($\sim $ 4mg/cc).   

Carbon Nanotube -- Supported Metal Nanoparticles: Properties of Novel Heterogeneous Catalysts Predicted by Molecular Simulations

We conducted all-atom molecular dynamics simulations for Pt nanoparticles composed of 150-500 atoms on either graphite or bundles of CNTs. The CNTs considered are (4,4), (10,10), and alternating (10,10) and (4,4) CNTs. The diameter of the simulated Pt nanoparticle is comparable to those typically employed experimentally. Our results suggest that the substrate strongly influences the morphology of the Pt nanoparticles, in particular the number of Pt atoms with low-coordination number (characterized by higher chemical reactivity). Results for diffusion coefficients of the Pt nanoparticles on CNTs are one order of magnitude lower than those on graphite, suggesting that Pt syntering is less likely on CNTs than on graphite. At low temperature the Pt nanoparticle resides within the trench formed by two adjacent CNTs, but at high temperature it is located on top of one CNT. If supported by experimental data, the results presented here could lead to the production of catalysts that are stable at very large operating temperature.   

Theoretical Design of Nanostructured Materials for CO$_{2}$ Adsorption, Activation and Reduction

Using first principles simulations based on Density Functional Theory, we have investigated the adsorption and activation properties of CO2 on a variety of materials both in bulk and in nanostructured form. In particular, we will discuss the interaction of CO2 with elemental transition metal surfaces, oxide-supported nanoparticles, artificial molecular systems and nanoporous materials. Particular emphasis is given to the construction of complex systems where interfaces between heterogeneous materials are the active site for catalytic reactions. Our investigation is focused on the prediction of possible techniques to tune the properties of the interfaces in order to enhance the desired chemical activity. A preliminary study on bulk systems already showed promising results for possible applications to catalytic processes.   

Synthesis and Growth Polarity of ZnO Nanostructures

The semiconductor ZnO nanostructures are attracting increased attention in the nano-community due to their wide range potential applications. We present in this paper the synthesis of ZnO nanostructures including nanowires, nanobelts, and nanopillars using chemical vapor deposition method via vapor-liquid-solid growth mechanism. The as-grown ZnO nanostructures were examined in transmission electron microscope (TEM). Electron energy loss spectroscopy (EELS) and chemical mapping were used to verify their chemical composition. Furthermore, our interest particularly focused on ZnO nanopillars due to their properties are strongly polarity dependence. It was studied with convergent beam electron diffraction (CBED) and channeling-enhanced EELS methods. It indicates that the nanopillars are narrow hexagonal columns, grown along the c-direction, with their $\{1\bar {1}00\}$-type side-walls in the width range of 65-70 nm. Their tips had the form of a truncated hexagonal pyramid, with top surface terminated at the (0001) plane and the side-walls at the $\{1\bar {1}01\}$-type planes. Nanoplliars were found to grow along the (0001), Zn-polarity. Further studies on ZnO nanopillars mechanical properties and bio-senor applications are undergoing.   

Quick growth of nanostructured SiC films by using short-pulse laser ablation deposition technique

Nanoscale silicon carbide (SiC) particles, wire and nanostructred films have been synthesized on different substrates by using short-pulse KrF excimer laser plasma deposition (SP-LPD) technique. Setting laser repetition of 2Hz, 30 minutes of SP-LPD yielded large area of SiC films with thickness up to 30um. Higher repetition of laser plasma deposition produces higher growth rate, resulting in thicker SiC films. Nanoscale particles and wire are observed. The advantage of SP-LPD technique for quick synthesis of SiC is to avoid film thermal ablation or to have low heat generated during deposition. A novel laser line-focusing lens technique combined with special configurations of SiC target are used to synthesize preferred nanostructures of particles, wire, and films. Structures, composites, and properties of the SiC samples are characterized by using scanning electron microscopy (SEM), energy loss spectroscopy (ELS), X-ray photoelectron spectroscopy (XPS), and Raman scattering techniques.   

Temperature-Dependent Electrical Conductivity Measurements on Hydrated and Alkali-metal Intercalated Zeolite LTA and FAU

Zeolite LTA and FAU films were made from zeolite powders using a hydrothermal method. Electrical conductivity measurement were performed on the zeolite films in temperature range between 180 K and 430 K, using an LCR meter with the sweeping frequency varied from 20 to 1 MHz and drawing the Cole-Cole plots. The resistivities of both hydrated LTA and FAU zeolites increased with increasing the sample temperature from RT to 430 K, which might be caused by loss of water molecules from the pores of zeolite crystals. Also, the resistivities increased with decreasing the sample temperature from RT to 180 K, probably caused by freezing of water molecules in the zeolite. When the dehydrated zeolite samples were intercalated with alkali metals (Rb and K), the resistivities of the samples did not vary much at RT. However, the resistivities of the intercalated zeolite films decreased drastically by four orders of magnitude when the sample temperature was varied from RT to 180 K. We speculate that the dynamics of alkali atoms in the zeolite pores (electron-phonon scattering) may be responsible for the drastic change in the electrical conductivity.   

Ab initio calculation of vibrational frequencies of clusters in As $_{x }$S$_{1-x }$ glass and Raman spectra

We have used the density functional theory (DFT) to calculate the vibrational frequencies from first principles by making clusters of atoms on a high speed computer. We have made the clusters, AsS$_{4}$, As$_{2}$S$_{3}$, As$_{3}$S$_{2}$, As$_{4}$S, AsS$_{7}$, As$_{2}$S$_{6}$,As$_{3}$S$_{5}$,As$_{4}$S$_{4}$,As$_{5}$S$_{3}$,As$_{6}$S$_{2}$, As$_{7}$S, L-As$_{4}$S$_{3}$, CS-As$_{4}$S,CS-AsS$_{4}$,ES-As$_{2}$S$_{6}$(L=linear zig-zag, CS=corner sharing, ES=edge sharing) and optimized the bond lengths and angles for the minimum energy. The number of computed values of the vibrational frequencies are consistent with 3N-3, where N is the number of atoms in a cluster. All of the computed values are tabulated and compared with those found in the Raman spectra of As$_{x}$S$_{1-x}$ (x=0.35-0.45) glasses. The experimentally found modes at 183, 221, 355, 371 cm$^{-1}$ are consistent with those calculated for chain mode-As$_{4}$S$_{3}$, As$_{3}$S$_{2}$, As$_{2}$S$_{6}$, AsS$_{7}$. In the net work glass the frequencies found are, 195, 206, 227, 351, 369, 371, 388 cm$^{-1}$ which are consistent with those calculated for, As$_{7}$S, As$_{5}$S$_{3}$, As$_{7}$S, As$_{6}$S$_{2}$, As$_{5}$S$_{3}$, AsS$_{7}$ and As$_{4}$S. We have previously interpreted the Raman spectra of GeSI galass by this method ,V.R. Devi, et al, J. Non Cryst. Solids 351,489-494(2005).   

Electronic and magnetic properties of endohedrally doped fullerene Mn@C$_{60}$: An all electron theoretical study

In this work we calculated the total energy of a Mn atom encapsulated inside C$_{60}$ cage by using GGA density functional theory at the PBE/PAW level. We found that the magnetic properties of free Mn are preserved when the atom is located at the central region and spin multiplicity M = 6 has the lowest energy among M = 6, 4 and 2. When we started to shift Mn away from the center, the energy and spin multiplicity initially remained the same, then gradually dropped when Mn reached about the half way between the center and edge of cage. The spin multiplicity M changed to 4 at a local minimum with energy lower than that at the center by about 0.2 eV. As we kept on moving Mn towards to the edge, the total energy increased monotonically and the spin multiplicity M reduced to 2. Hence the energy curve has two identical minima situated symmetrically with respect to the center of the cage.   

Electronic, vibrational dispersions and alkali-halides encapsulation in carbon nanotubes

Density functional calculations of electronic and vibrational dispersion energies for prestine graphite and single-walled carbon nanotubes (SWCNT) are presented. Optimized parameters for nonlocal norm-perserving pseudopotentials which replace the potential field due to core electrons are given and the valence electrons are treated with linear combination of localized atomic orbitals. The effect of encapsulation of carbon nanotubes with alkali-halide matrices is numerically investigated. The electronic band structure of encapsulated SWCNT is noticeably modified and hence its optical properties.   

Electronic structure control of carbon nanotubes by using electrophilic molecules

In the applications of SWCNTs, the uniformity of the electronic properties of individual nanotubes can be crucial, especially in case of electronic devices. Since there is no synthetic method to produce nanotubes with uniform chirality, several techniques such as dielectrophoresis, chemical treatment, or DNA wrapping were reported to separate semiconducting nanotube from metallic ones or vice versa. Here, we present the electrical transport properties of SWCNT-FETs treated with four different electrophilic molecules, bromobenzenediazonium-, nitronium-, benzodithiolylium-, and triphenylpyrylium tetrafluoroborate. All four molecules show similar effect on SWCNT-FETs, about 1/3 of the devices that were originally containing both metallic and semiconducting nanotubes showed complete depletion at positive gate voltages.   

Microwave shielding of transparent and conducting SWNT films

Single-walled carbon nanotubes (SWNTs) are emerging as building blocks of electronics for a variety of applications. In particular, films of nanotubes have found potential applications for electronics and optoelectronics. The transport properties of SWNT films in the microwave frequency range from 10 MHz to 30 GHz was measured by using the Corbino reflection technique from temperatures 20 K to 400 K. Based on the real and imaginary parts of the microwave conductivity, we calculated the shielding effectiveness for various film thickness. Shielding effectiveness of 43 dB at 10 MHz and 28 dB at 10 GHz is found for films with $90\%$ optical transmittance, which suggests that SWNT films are promising as a new type of transparent microwave shielding material. By combining our data with those from the literature, the conductivity of SWNT films was established in a broad frequency range from DC to visible.   

Transport in Carbon Nanotubes with Ferromagnetic Contacts in Low and High Magnetic Fields

Spin transport in carbon nanotubes has been attracting considerable attention due to its novel device structure. A single-wall nanotube (SWNT) contacted by ferromagnetic materials can be considered as either a spin-valve device with a nano-size channel or a ferromagnetic contacted quantum dot system. We have made Co- and Ni-contacted SWNT devices and studied transport properties in both low and high magnetic fields. At low magnetic field we observed spin-valve effect, which showed hysteresis of magnetoresistance (MR) in opposite sweeping directions of field. In addition to low field measurements, MR was also observed in high magnetic field. However, differing from low-field MR, high-field MR didn't have hysteresis and rather symmetrical. Details of the transport results will be discussed in this presentation.   

Low-Frequency Noise of Individual Single-Walled Carbon Nanotube Field Effect Transistors

Low-frequency current fluctuations in individual single-walled carbon nanotube field effect transistors (SWNT FETs) were studied by several research groups recently. It was reported that such devices exhibit significant 1/f-type noise. Its noise coefficient (A), in Hooge's empirical rule, is several orders of magnitude higher than that observed in more conventional conductors. The question is: what is the origin of the excess noise in SWNT? One group stated that such significant noise coefficient is due to the small number of carriers (N) with the Hooge constant ($\alpha _{H})$, which is comparable to most bulk materials. From this N-dependent property, they developed a novel technique to characterize the carrier numbers of nanotubes or even any other nanostructures. Another group reported that the noise coefficient is inversely proportional to gate voltage. They concluded that the noise is due to mobility rather than number fluctuations. However, the mechanisms of many behaviors of the noise are still unclear. We will present new sets of results of 1/f noise in individual SWNT FETs.   

Jahn-Teller distortion of C(5,0) carbon nanotubes inside AlPO$_4$-5 zeolite channels

The electronic structure of C(5,0) carbon nanotubes inside zeolite channels is studied by means of {\it ab initio} simulations. The band structure of the tubes is analyzed in detail as a function of the environment and the breaking of the LUMO state degeneracy and subsequent band-gap opening is interpreted as a Jahn-Teller distortion, previously refereed as Peierls instability, ocurrying at room temperature. However, we show that the zeolite matrix considerably screens this transition, reducing the opening of the gap observed in isolated tubes by more than 50\%. A strong lateral interaction between the nanotube and the zeolite is revealed, but this is not enough to reconciliate the resulting picture of semiconducting tubes at T=300 K with the superconducting transition of CNTs inside AlPO$_4$-5 zeolite channels, observed at Tc=15 K.   

Electronic structure calculations of tapered silicon nanowires

Nanowires are observed experimentally to be tapered rather than straight-edged, with diameters shrinking by as much as 1 angstrom per every 10 angstroms of vertical growth, depending on the synthesis technique and conditions. Yet, most theoretical work to understand the electronic, optical, and structural properties of nanowires have assumed a straight-edge geometry. In this work, we focus on the impact of tapering on nanowire properties. Using ab initio pseudopotential calculations, we show that tapered silicon nanowires have axial-dependent electronic structure properties due to quantum confinement effects. Other electronic and structural properties related to the tapering will also be presented. Further, we illustrate how these properties may be advantageous for nanowire- based photovoltaic applications, where the highest absorption efficiency and lowest thermalization loss for a photon occurs when it has an energy close to the gap of the solar cell.   

Dielectric response of carbon and boron nitride nanotubes from first-principles calculations

We present a complete characterization of the dielectric response of isolated single- and multi-wall carbon (CNT) and boron-nitride nanotubes (BNNT) using first-principles calculations and density-functional theory. The longitudinal polarizability of a nanotube is sensitive to the band gap and its radius, and in multi-wall nanotubes and bundles it is trivially given by the sum of the polarizabilities of the constituent tubes. The transverse polarizability of both types of nanotubes is insensitive to band gap and chirality and depends only on the radius. However, the transverse response and screening properties of BNNTs are qualitatively different from those of metallic and semiconducting CNTs. The fundamental differences in electronic properties of the two materials are inherited from the corresponding two-dimensional sheets - graphene and boron-nitride. The screening of the external field in CNTs is stronger than in BNNTs and has a different radius dependence. The transverse response in BNNTs is found to be that of an insulator, while in CNTs it is intermediate between metallic and semiconducting. Our results have practical implications for selective growth of different types of nanotubes using aligning electric fields and for Raman characterization of nanotubes.   

Multisubband Boltzmann Carrier Transport in Carbon Nanotube Transistors

Theoretical predictions of multisubband Boltzmann carrier transport are compared with recent experimental characterization [1] of single-walled carbon nanotube field-effect transistors. Theory includes both intrasubband and intersubband deformation potential carrier-phonon scattering. Results compare well with measured device characteristics, accurately predicting performance as a function of temperature, gate voltage, and nanotube diameter. [1] X. Zhou, J. Y. Park, S. Huang, J. Liu, and P. L. McEuen, Phys. Rev. Lett. 95, 146805 (2005)   

Electrical Measurements of CVD grown Double-Walled Nanotubes

The CVD method of carbon nanotube growth pioneered by Dai, et al. often results in nearly perfectly transparent electrical contacts, and is thus ideally suited for transport measurements in single-walled carbon nanotubes SWNTs. However, we have observed through HRTEM that growth by the ``standard recipes'' used to produce individual single-walled carbon nanotubes often result in a fraction of double-walled carbon nanotubes (DWNTs). The existence of these samples not only reduces the yield of devices based on single-walled nanotubes, but also affords the opportunity of a new structure to study the interactions between neighboring shells in a multi-walled sample. Because we expect the differences in transport behavior between SWNTs and DWNTs to be subtle, we would like to combine transport measurements with HRTEM on the same DWNT. We have developed a substrate based on silicon nitride membranes compatible with both HRTTEM and electron beam lithography, allowing us to combine structural information from TEM with standard transport measurements. We will present preliminary transport results on DWNTS and discuss future experiments to understand the coupling between shells in multi-walled nanotubes.   

Template-Assisted Large Scale Vertical Implantation of Crystalline Titania Nanotube Arrays

This work presents an AAO template-assisted method to fabricate the TiO2 nanotube arrays implanted on a transparent electro- conducting glass substrate. The titania nanotubes well aligned within the template were adhered orthogonally to the substrate with the assistance of a thin particulate titania layer. By controlling the fabrication procedure such as pre-etching, sintering, thickness of the AAO template and etching conditions, and by manipulating the filter layer of the AAO membrane, diverse morphology of the final titania nanostructure layer was attained. As observed from SEM images, the assisting nanoparticulate titania layer provided implantation of the high- aspect-ratio titania nanotube arrays perpendicularly to the substrate. It is likely that the thickness of the assisting titania layer could be further reduced to a minimum. Exploration for an optimal design of the nanostructured titania film to achieve a high performance of the electrode in DSSC is the challenge of our on-going work.   

Quick synthesis of nanoscale tungsten oxide particles and rods

Novel Hot Filament Chemical Vapor Deposition (HFCVD) set has been installed and used for quick synthesis of nanoscale tungsten oxide particles and nanorod over large area. The present work addresses two issues: to simplify the deposition processing and to quickly deposit well-aligned nanoscale tungsten oxide rods over large area. An effect of temperature on the synthesis of the nanoscale tungsten oxide rods and particles is studied. Raman spectroscopy (RS), scanning electron microscopy (SEM), energy dispersive X-Ray microanalysis (EDS), X-ray diffraction (XRD), and X ray photoelectron spectroscopy (XPS) have been used to characterize the samples. Experimental results reveal that low-temperature HFCVD yields nanoscale tungsten oxide particles, whereas high temperature of HFCVD results in large crystalline particles with size up to 3mm. Only in the case of temperature around 2200$^{o}$C, large area, well-aligned nanoscale tungsten oxide rods can be obtained.   

Optical Characteristics of Bismuth Nanowires

Bismuth, in bulk form, is a semimetal with a rhombohedral structure. It has a small band overlap between the conduction and valence bands and a highly anisotropic electron effective-mass tensor. Bismuth nanowires with small enough diameters ($<$50 nm) undergo a transition from a semimetal with a small band overlap to a semiconductor with a small indirect bad gap; hence significant quantum confinement can occur. These quantum confinement effects can be potentially useful in optical and electrooptical devices. Here, we investigate the optical properties of bismuth nanowires (average diameter 10-12 nm) using Micro-Raman, UV/Visible, and Infrared Spectroscopy techniques. It is known from the literature, that bismuth nanowires exhibit a strong absorption peak ($\sim $1000 cm$^{-1})$ in the mid-IR that is not present in bulk bismuth. Here we show experimentally that bismuth nanowires exhibit a blue-shift in the mid-IR.   

Nanomechanics of prismatic noble-metal nanowires

In the nanometer diameter regimes, nanowires have a large number of surface atoms as compared to the bulk atoms, which determines a change in the cross sectional shape of the wires with respect to predictions based on the Wulff construction. The mechanical response at this scale also change, and we present here a systematic study of mechanical properties of thin noble-metal nanowires as a function of wire diameter and wires axis orientation. Implications for the wire of metal wires in nanoelectromechanical devices are also described.   

Development of micro engine oil condition sensor using multi-wall carbon nanotube films

A new interdigit-type micro oil condition sensor was designed and fabricated for monitoring the deterioration of lubricating and insulating oils. The designed sensor operates based on the change of the dielectric constant and electrical conductivity. In order to improve sensor performance, an oil condition sensor was fabricated using MEMS technology and multi-wall carbon nanotube film. The experiment was performed with automobile engine oils with the same brand and quality so as to ensure measurement reliability. Capacitance changes were measured according to increasing mileage and the sensors' performance was improved. These results show that the proposed sensor could measure the degree of oil deterioration with a high sensitivity and it is applicable to other lubricating systems as well as insulating systems.   

Photoluminescence Characterizations of Ultra-fine ZnO nanowires

ZnO nanowires as 1D materials display novel and unique physical properties. For the optical properties, the origin is attributed to (i) large surface to volume ratio which results in increased surface states, (ii) waveguiding property which is typical for the 1D structure, and (iii) reduced dimensionality which results in a quantum confinement. Due to the rather small exciton bohr radius in ZnO the latter effect is expected only for wires with a diameter smaller than 5 nm. Here we present luminescence studies of ZnO nanowires with diameters in this range. The nanowires were synthesized using two different methods: pulsed laser deposition and carbon thermal. TEM studies show their excellent crystalline quality, with diameters ranging between 2 and 40 nm. Temperature dependence photoluminescence (PL) measurements were carried out on the as-grown samples. At low temperatures, the spectra are dominated by a feature at 3.366 eV, which is attributed to a surface bound exciton. With rising temperature, four phonon replica of the free excitonic transition are observed, showing quantum confinement effect in the ultra-fine nanowires. These results will be discussed together with $\mu $-PL measurement on single nanowires.   

Anti-Stokes Photoluminescence and Enhanced Raman Scattering from InSb Nanodots and Nanowires

InSb nanodots, nanowires and bulk crystals were examined by conventional micro-Raman and scanning near-field optical microscope (SNOM) systems. InSb nanodots and nanowires examined were fabricated on Si substrates by a vapor-transport method using a focused-ion beam(FIB) system[1]. Surprisingly strong anti-Stokes photoluminescence was found from the samples when a high power laser density (up to $\sim $10$^{6}$ W/cm$^{2})$ was used. Drastic decrease of the anti-Stokes luminescence with decreasing the sample temperature suggested that phonon-carrier scattering played an important role. Moreover, enhanced second-order Raman scattering was observed, which depended on both the laser power density and samples. We speculate that the second-order Raman enhancement may be caused by the excitons created by photoirradiation. [1] M. Ishizuka, T. Horiuchi and N. Wada, \textit{Narrow Gap Semiconductors}, edited by J. Kono and J. L\'{e}otin, Taylor {\&} Francis, 2006, p. 125-130.   

Fabrication of Single-Wall Carbon Nanotubes in Catalyst-Dissolved Liquid Alcohols and Temperature-Dependent Raman Scattering

Uniform, well-aligned Single-Wall Carbon Nanotubes (SWNTs) were grown on Si substrates in catalyst-dissolved liquid alcohols. To fabricate SWNTs, cobalt acetate tetrahydrate was first dissolved in distilled water and mixed with alcohols. Si substrates were then ohmic-headed in the liquid up to $\sim $1000 K for a short time (up to 3 minutes). Raman spectra taken from the samples showed some radial breathing mode (RBM) of SWNTs, indicating that the samples consisted of SWNTs. In addition, TEM pictures showed bundles of SWNTs whose diameters centered around 1.7 nm. Raman spectra of our liquid-phase grown SWNTs and HiPco SWNTs were taken varying the sample temperature between RT and 6 K. Temperature dependence of the G, D and RBM modes in the samples will be discussed in terms of alcohol molecules confined in the nanotubes and phonon dynamics.   

Carbon Evaporation from Carbon Nanotube Field Emitters Studied by Conductivity Change of CNT Anode.

The study of degradation of carbon nanotube (CNT) electron field emitters under high current conditions allowed to reveal interesting details of carbon evaporation from the CNT cathodes. Single-wall and multi-wall CNT papers were investigated as electron field emissive cathodes. Due to high field emission currents going through small number of protruded nanotubes, only those nanotubes overheat and melt at the ends, evaporating C atoms and small clusters. Evaporated carbon atoms deposit on the anode forming spherical and elliptical patterns, which are similar in shape to the patterns, induced on phosphorescent screens by field emitted electrons. To clarify the details of carbon deposition we used transparent CNT thin films and CNT aerogel sheets [1] as anodes and found that deposited carbon layers significantly decrease the resistance of CNT films with only a small decrease in transparency. Thus this method allows to decrease the sheet resistance of T-CNT from 700 ohm/sq to 100 ohm/sq level, required for many optoelectronic applications. The structure of deposited C and origin of deposition patterns are analyzed and discussed in terms of correlated ionic and electronic flows. [1] M. Zhang et al., Science, 309,(2005) 1215.   

Thermal expansion in carbon nanotubes

We present a density-functional study of the thermal expansion properties of carbon nanotubes. The thermal expansion coefficients are calculated from the minimization of the vibrational free energy in the quasi-harmonic approximation. We show that carbon nanotubes contract at low and room temperature and expand at higher temperatures, and that the expansion coefficients strongly depend on the diameter, while chirality has a small effect. The role of different phonon modes in the thermal contraction is discussed togheter with the relation with the thermal expansion properties of graphene.   

Grand Canonical Monte Carlo simulations of the uptake of gases inside Single Wall Carbon Nanotubes

Gases adsorbed in the interior of open-end carbon nanotubes have different characteristics depending on the size of the molecules, diameter of the nanotubes interaction potentials, temperature and chemical potential. For example, at low temperature and low coverage H$_{2}$ is deposited on a shell against the internal carbon wall, and populates the center of the tube only at a higher chemical potential. The porpuse of the present work is to estimate the conditions necessary for a signigicant uptake and for capillary condensation of different species in different-size of tubes. For example, at a temperature of 120 K, the uptake of Ar starts at a pressure P =10$^{-4}$ atm, while for Helium a pressure of 1 atm is needed to start the adsorption. The conditions for adsorption and configurations of the adsorbate are dramatically different for a different set of parameters. In this work we calculated the uptake for Ar, Kr, Ne, CH$_{4}$, Xe, He and H$_{2}$ in a broad range of pressures, temperatures and diameter of the nanotubes. We also studied the phases, configurations and energies.   

Chemical vapor deposition of SiGe alloy nanowires

We present chemical vapor deposition synthesis of Si$_{1-x}$Ge$_{x}$ nanowire alloys on Si substrates via vapor-liquid-solid (VLS) mechanism. Silane and germane were employed as source materials and self-assembled gold nanoparticles were used as the catalyst. We discuss the compositional dependence on precursor partial pressures and temperature. We investigated effects of the growth temperature and the process gas flow rate on surface morphology, microstructure, and compositional uniformity of the grown nanowires. Initial nucleation time and the nanowire growth rate were monitored \textit{in-situ} optical reflectivity. We discuss the compositional dependence on precursor partial pressures and temperature. Primary goal of this study is to achieve high crystalline quality Si$_{1-x}$Ge$_{x}$ nanowire alloys with large range of compositions ($x)$ and to establish abrupt interface between Si and Si$_{1-x}$Ge$_{x}$ layers in nanowire axial heterostructures, so that we can engineer the bandgap energies for versatile nanowire-based advanced electronic, photonic, and thermoelectric devices.   

Bulk Mechanical Properties of Single Walled Carbon Nanotube Electrodes

The unique properties of single walled carbon nanotubes (SWNTs) make them especially well suited for use as electrodes in power devices such as lithium ion batteries, hydrogen fuel cells, solar cells, and supercapacitors. The performances of such devices are expected to be influenced, at least in part, by the mechanical properties of the SWNTs used in composites or in stand alone ``papers.'' Therefore, the elastic moduli and ultimate tensile strengths of SWNT papers were measured as functions of temperature, SWNT purity, SWNT length, and SWNT bundling. The SWNTs used to produce the papers were synthesized in an alexandrite laser vaporization reactor at 1100$^{o}$C and purified using conventional acid-reflux conditions. Characterization of the SWNTs was performed using SEM, BET, TGA, and optical and Raman spectroscopy. The purified material was filtered and dried to yield papers of bundled SWNTs which were analyzed using dynamic mechanical analysis (DMA). It was observed that the mechanical properties of acid-refluxed SWNT papers were significantly improved by controlled thermal oxidation and strain-hardening. Elastic moduli of SWNT papers were measured between 3 and 6 GPa. Ultimate (breaking) tensile stresses were measured between 45 and 90 MPa at 1-3{\%} strain. These results and their implications in regard to potential applications in power devices will be discussed.   

Improving step bunch ordering by tuning the interplay between thermodynamic and kinetic factors

Strain-induced self-organized step-flow growth provides an effective method for fabricating quantum wire arrays. Here we report development of a modified step-flow growth model accounting both thermodynamic step-step interaction due to misfit strain and kinetic step-edge barriers. We show that improved step bunch ordering can be achieved by tuning the interplay of thermodynamic and kinetic factors. Computer simulations show that the best step bunch ordering occurs when the two factors drive the average bunch size to correspond to an integer number of steps in the bunch.   

Large polaron in an anharmonic crystal lattice

Extending the large Frohlich polaron problem to an anharmonic lattice the author studies a polaronic state with a large radius of the wave function. The appropriate anharmonic part of the e-ph interaction Hamiltinian is derived, based on methods of quantun field theory. Then, with the help of perturbation theory, the anharmonic contribution to the ground-state energy of a polaron is calculated. To estimate shows that the anharmonicity can considerably increase the large polaron binding energy in a situation of a moderately strong e-ph coupling. A comparison of a developed theory with other anharmonic models of a polaronic state is drawn.   

Optical excitation and electron relaxation at the Si (001) 2x1 surface: A combined approach of density functional and density matrix theory

We present a microscopic theory to describe optical excitation and the subsequent phonon-induced relaxation dynamics of non-equilibrium electrons at a Si(001)(2x1) surface. Density matrix formalism is used to derive dynamical equations for the electronic occupations and polarizations in a surface/bulk system, where the electron-optical and electron-phonon interaction up to second order are considered. The matrix elements that govern the dynamics in the equations and the band structure of the surface system are determined by density functional theory calculations within the local-density approximation. These are performed on a slab geometry in order to take the buckled dimer surface reconstruction into account in the dynamics. The interplay of the population dynamics of the $D_{\mathrm{down}}$ state with the bulk conduction bands after an optical excitation is discussed and the typical deexcitation timescales for non-equilibrium surface electrons are deduced and compared to two-photon photoemission experiments.   

The electronic band alignment on nanoscopically patterned substrates.

Understanding band alignment on the molecular scale is vital for the future of molecular electronics, but it is also important in today's devices as their contact interfaces can have defects on the nanoscale. The band alignment of the organic semiconductor sexiphenyl on a nanoscopically patterned substrate, was investigated with UV-photoemission and STM. We show that for increasing coverage on inhomogeneous surfaces shifts in electronic level alignment occur, which are due to the change from local to average band alignment. The Cu-(2x1)O stripe phase, used as a model substrate consists of alternating stripes of bare and oxygen passivated copper, with stripe widths comparable to the molecular length. In the first molecular layer the electronic bands are aligned to the local surface potential of the specific stripe, resulting in a superposition of two photoemission spectra offset by 1eV. Beyond two monolayers the valence band spectra clearly indicate a single electronic level alignment, which is determined by the average interface dipole.   

Atomic and Molecular Collisions with Surfaces: Comparisons of Ar and N$_2$ Scattering from Ru(0001)

Recently reported molecular beams studies of Ar and N$_2$ scattering from Ru(0001) at thermal and hyperthermal energies exhibited a number of characteristics that are unusual in comparison to other systems for which molecular beams experiments have been carried out under similar conditions. For both systems the measured energy losses were unusually small and for Ar scattering in some cases quantum mechanical diffraction was observed under conditions for which it was not expected. These measurements are analyzed and compared to calculations with a mixed quantum-classical scattering theory. Many of the unusual features observed in the measurements are explained by the theory but only upon using an effective surface mass of 2.3 Ru atomic masses, which implies collective effects in the Ru crystal. The large effective mass, because it leads to substantially larger Debye-Waller factors, explains and confirms the observations of diffraction features. It also leads to the interesting conclusion that Ru is a metal for which molecular beams scattering measurements in the purely quantum mechanical regime, where diffraction and single-phonon creation are dominant, should be possible not only with He atoms, but with other atomic and molecular species with masses up to that of Ar atoms.   

Studies on the controlled morphology and wettability of PS surfaces by electrospinning or electrospraying.

Electrospinning/electrospraying is a simple and effective way to fabricate various polymer surfaces such as beads, fibers and other shapes in the range of micro- to nanometer. Various surface morphologies have been produced by electrospinning or electrospraying: beads with different sizes and shapes, bead-on-string structure with different aspect ratios and fibers with different diameters and shapes. Physical properties of the PS solutions such as viscosity, surface tension and conductivity greatly influence the electrospun or electrosprayed PS morphology. The wettability of a solid surface is greatly influenced by its surface morphology: A spin-coated PS membrane has a water contact angle of 97$^{\circ}$, while electrospun PS membranes have water contact angles around 150$^{\circ}$. The most hydrophobic membrane has a water CA of 159.5$^{\circ}$.   

Optical Properties of Pd and PdO films exposed to hydrogen

Pd and PdO are of great interest in industry due to their chemical and electrical properties. For long time, Pd have been studied as a solid state storage medium of hydrogen, due to the huge absorption capacity of this gas. PdO could be used as a photocathode for the production of hydrogen in photoelectrolysis cells. In this work transmittance spectroscopy measurements in the optical/NIR range of thin films of Pd/quartz and PdO/quartz are shown. This property is monitored as a function of time upon hydrogenation. Semiconductive PdO films gradually reduce into metallic Pd when exposed to hydrogen, as confirmed by XRD, showing no appreciable shift of the bandgap during the process. The transmittance of Pd films increases by $\sim$30\% in this spectral range for pressures of $\sim$40 torr of hydrogen, the increase peaking at about 730nm. The transmittance measurements seem to be a very simple and fast method for monitoring this kind of systems.   

Observation of the high sensitivity of Positron Annihilation induced Auger electron spectroscopy to thermally induced changes in the oxidation state of Cu atoms at the surface of previously oxidized Cu(100)

Changes in the surface of an oxidized Cu(100) single crystal resulting from vacuum annealing over a temperature range from 20 $^{0}$C to 800 $^{0}$C has been investigated using Positron annihilation induced Auger electron spectroscopy (PAES). The PAES measurements show a large monotonic increase in the intensity of the annihilation induced Cu (M$_{2,3 }$VV) Auger peak as the sample is subjected to a series of isochronal anneals in vacuum up to annealing temperature 300 $^{0}$C. The intensity then decreases monotonically as the annealing temperature is increase to $\sim $600 $^{0}$C. The Ps fraction, f$_{PS}$ of these surfaces was found to have the opposite trend going from f$_{PS}$ = 0.79 for the surface before any annealing to f$_{PS}$ =0.51 after annealing at 300 $^{0}$C. These results provide a clear demonstration of the thermal reduction of the copper oxide surface after annealing at 300 $^{0}$C followed by re-oxidation of the copper surface at the higher annealing temperatures presumably due to the diffusion of subsurface oxygen to the surface.   

Angular dependence of linear and nonlinear magneto-optical properties of magnetic films

The magnetic properties of Ni and Fe films were studied by measuring the linear and the nonlinear magneto-optical properties. The linear and the nonlinear magneto-optical Kerr-effect (MOKE) measurements were employed to understand the bulk magnetism and the surface and interfacial magnetism, respectively. The linear MOKE is an important probe for studying magnetic thin films and the nonlinear MOKE is a very sensitive tool for investigating the surface and the interface magnetisms of magnetic thin films, which enable us to elucidate the skin-depth effect. For analyses of these magnetic films, the optical and the magneto-optical properties were simulated. The linear-MOKE simulation was performed in the polar and the longitudinal modes to obtain the Kerr rotation and ellipticity of the linear and the nonlinear MOKE. The linear and the nonlinear MOKE measurements were carried out in the longitudinal mode. To determine the skin-depth effect, each sample was made with different thickness and the angular dependence of MOKE signals was obtained and compared with that of the simulated one.   

Stark shift of transmission resonance in scanning tunneling spectroscopy

It is known that the free electron scattered by the quantum well in the metal film may manifest the transmission resonance and it can be probed by scanning tunneling spectroscopy (STS). We use STS to observe the transmission resonance on Ag films grown on the Si(111)7x7 surface. In addition to reveal the signal of the transmission resonance in the tunneling spectrum, there also appears that its energy level can be shifted by tuning the tunneling current, i.e. the electric field in the tunneling gap. Our results demonstrate that the transmission resonance is shifted to higher energies with increasing electric field, but beyond a critical field, it will drop to a lower energy discontinuously. This field-dependent behavior can be qualitatively explained by a field-induced phase variation in the quantization rule.   

Long-range Ordered Surface Phase in Liquid AuSi$_{x}$Ge$_{1-x}$ Eutectic Alloys

Synchrotron Xray studies of the free surface of the liquid phase of the Au$_{82}$Si$_{18}$ eutectic show that it is coated with a 2D AuSi$_{2}$ crystalline monolayer[1]. In contrast the surfaces of comparable liquid phases of Au$_{73}$Ge$_{27}$ and Au$_{77}$Ge$_{14}$Si$_{9}$ resemble normal liquids without surface crystalline order. The differences are presumably caused by the fact that Ge has a lower surface tension than both Si and Au and as a result Si is displaced from the surface. The amount of Ge in a lower concentration alloy(Au$_{82}$Si$_{18-x}$Ge$_{x}$, x$\le $ 1 atm{\%}) is not sufficient to completely replace the surface Si and in this alloy and the 2D lattice structure order is identical to that of Au$_{82}$Si$_{18,}$ except for the $\sim $ 20K higher melting temperature of the 2D lattice and the fact that it is a bilayer, rather than a monolayer[1]. \newline [1] Shpyrko, O.G., et al., \textit{Surface crystallization in a liquid AuSi alloy}. Science, \textbf{313}.77(2006)   

Marangoni Effect and ``chasing drops''

The spontaneous motion of a liquid droplet due to the placement of another drop of a different liquid has been studied. The motion is recorded when both the drops are placed on one smooth surface as well as when the two drops are kept on different surfaces still retaining the same distance between them. It is traditionally believed that Marangoni Effect is caused when vapors of one liquid deposit on the closer region of the other drop thus creating surface tension gradient within the drop. Through our experiments, we observed that despite keeping the same distance between the drops, thereby allowing for vapor deposition, upon surface separation the pursued drop did not run away spontaneously the way it did for continuous smooth surface. This indicates that the surface is an important factor in the spontaneous drop flow.   

Potential energy landscape of monolayer-surface systems governed by repulsive lateral interactions: the case of $(3 \times 3)$-I-Pt(111)

Combined Density Functional Theory (DFT) and Monte Carlo (MC) approach is applied to study the potential energy landscape of four iodine atoms adsorbed on the Pt(111) surface in $(3 \times 3)$ unit cell. Three critical points were identified: $(3 \times 3)$-sym and $(3 \times 3)$-asym, corresponding to structures well-known from experimental studies, while the third one $(3 \times 3)$-zigzag is a new structure not reported before. An interaction model fitted to DFT calculations allows us to explain the difference between arrangements of iodine monolayer in vacuum, air and solution environments as a result of different repulsion regimes.   

Atomic structure of Holmium (Ho) silicides on Si(111) by low energy electron diffraction (LEED) Patterson inversion at multiple incident angles

The surface structure of Ho silicide grown on Si(111) has been determined by LEED Patterson inversion at multiple incident angles. The experimental LEED I-V data is inverted as Patterson function. The Patterson inversion gives the inter-atomic distances between all atomic pairs in the structure. Signal from single scattering is enhanced and the multiple scattering is averaged to background so artifact-free Patterson function map can be obtained. It is an accurate and direct method to discriminate the optimum structure from several atomic structural models. For low coverage, about 1 ML, a 1x1 LEED pattern was observed. The preliminary results find that a hexagonal Ho plane is located below reverse buckled Si bilayer. Ho atoms situate on T4 sites above the bulk surface of Si. For higher converages, several monolayers, an r3xr3 LEED pattern was formed. The preliminary result favors that alternating flat layers of Ho and Si are located in between the buckled Si bilayer and bulk Si surface.   

A New Method for Computing the Anisotropy in Crystal-melt Interfacial Free Energy.

The anisotropy in the crystal-melt interfacial free energy is very important for understanding the crystal growth morphology. In this work, we have performed molecular dynamics simulations to calculate the interfacial free energy and its anisotropy. By inserting crystal nuclei with different geometric shapes into a supercooled liquid, we determined the corresponding critical temperatures. The spherical, cubic, and bipyramidal nuclei were used in this work. Using the classical nucleation theory but with different geometric factors, we have calculated the interfacial free energies in different orientations.   

Relation between electrical properties and surface morphology of indium tin oxide thin films deposited by RF magnetron sputtering

Conductive and transparent indium tin oxide (ITO) thin films are widely used for the electrode of optoelectronic devices. The surface morphology of the ITO thin films is an important feature because the transparent TFTs or OLED devices are fabricated on ITO thin films. We investigated the surface morphology and local current conduction properties along the direction normal to the surface of ITO thin films using a conductive-AFM (CAFM). ITO thin films were deposited on Si, SiO$_{2}$ and Pt substrate by RF magnetron sputtering technique. The resistivity of the films decreased with the increasing deposition temperature or working pressure. The lowest resistivity of 9.6$\times $10$^{-4}\Omega $cm was obtained at 250${^\circ}$ and 50mTorr. ITO thin films with a thickness of 200$\sim $300 showed low surface roughness with an RMS roughness value $<$ 10. CAFM of ITO thin films showed that the local high current conduction occurs where the surface protrusion was formed. Detailed investigation results on the correlation between the local current conduction and surface morphology and the film growth behavior will be presented.   

Annealing Behaviour of Hydrogenated and Oxidized Nanocrystalline Diamond

Hydrogenated and oxidized nanocrystalline diamond (NCD) is a very promising material for future electronic, especially bioelectronic applications. In the past it has been shown that hydrogen, oxygen, and gases from the ambient environment as well as water can be responsible for causing drastic changes in surface conductivity and wettability (hydrophobicity, hydrophilicity), friction, wear, etc. In this contribution we have investigated differently prepared NCDs as function of the annealing temperature under ultrahigh vacuum conditions (UHV) with various electron spectroscopies like UPS and XPS as well as High Resolution Electron Energy Loss Spectroscopy (HREELS). We were able to identify the thermal stability of a number of different species at the interface, which are related to different characteristics of C-H, O-H, C-O, and C-C bonding. Finally, a carbonization of the interface appeared at higher annealing temperatures.   

Atomic structure of the CoS$_{2}$(100)-(1$\times$1) Surface

Atomic structure and top-layer relaxation of the CoS$_{2 }$(100) surface are studied by using the quantitative low energy electron diffraction (LEED). From the LEED images, we observed a clear 1$\times $1 pattern with 4-fold symmetry. Over 18 beams of intensities versus energy curves are acquired from the LEED spots. With a series of trial model structures, theoretical intensities computed from full dynamical method are used to compare with experimental data by means of the Pendry R-factor. The clearly favored structural model from the LEED analysis is the 1S-terminated surface, in which the S-S dimmers remain intact and keep a complete S-Co-S sandwich structure. We also find that sub-surface Co and the third layer S atoms relax downward and upward by 0.10 {\AA}, respectively. These results could be compared with other theoretical and experimental studies. *This work was supported by RGC grant No. 8730017 of Hong Kong, Nebraska Research Initiative, the UNL NSF ``QSPINS'' MRSEC (DMR 0213808) and the UMN NSF MRSEC (DMR-0212302).   

Atomic structure of the carbon-induced Si(001)-c(4x4) surface

First-principles methods are employed to identify the reaction pathways for Si dimer rotations on the carbon-induced Si(001)-c(4x4) surface. The nudged elastic band calculations show that the recently proposed rotated dimer model can be obtained from the refined missing dimer model by dimer rotations with small energy barriers. The energy barriers are found to be sensitive to the rotation directions of Si dimers. The energy barrier along the minimum energy path is 0.82 eV. A new low-energy structure with a single rotated dimer is identified along the minimum energy path. This new structure is energetically more favorable than the rotated dimer model, the previously most stable structure, by 0.25 eV per c(4x4) cell.   

Molecular Dynamics computer simulations of hexane on graphite at submonolayer coverages

Results of Molecular dynamics computer simulations of hexane on graphite at submonolayer densities are presented. Two models are utilized; the first is a united atom (UA) representation which suppresses hydrogen atoms and the second is NAMD Scalable Molecular Dynamics in parallel computing environments with explicit hydrogens. Large UA systems having N = 1008 molecules and smaller (N = 112) explicit - hydrogen systems both show three distinct regimes: vacancy dominated at higher densities, a connected network at intermediate densities and individual islands at low densities. Various structural and thermodynamic quantities are utilized to understand how the system’s behavior correlated to the topology it exhibits.   

Thermotropic Liquid Crystalline Phases in $\alpha,\alpha^{'}$-dioctyl Pentathienoacene

Solution cast thin films of $\alpha,\alpha^{'}$-dioctyl pentathienoacene were studied using differential scanning calorimetry (DSC), hot-stage optical microscopy and hot-stage x-ray diffraction. DSC experiments indicate four thermotropic phase transitions in the temperature range of 100 $^{\circ}$C--180 $^{\circ}$C. Hot-stage optical microscopy experiments show these films to be birefringent and fluid at temperatures corresponding to two of these phases. At temperatures above the last transition, the films lose all birefringence. Hot-stage x-ray diffraction of these films indicates the presence of periodic packing of these molecules into planes parallel to the substrate surface. The evidence for regular packing disappears as the films are heated into the second of these phases. These data indicate that these films progress from a crystalline phase, to a smectic phase, to a nematic phase, and finally to an isotropic liquid phase as the films are heated.   

Comparison of Epitaxial Graphene grown on SiC(0001) and SiC(000$\bar{1}$)

Epitaxial graphene (EG) has been grown on both the Si-terminated (0001) and C-terminated (000$\bar{1}$) faces of 4H and 6H SiC. It has been shown that EG on the C-terminated face has substantially higher carrier mobility ($\sim 25,000 \textrm{cm}^2/\textrm{V}\cdot\textrm{s}$) , although EG on either face shows similar intrinsic carrier density and magnetoresistance (MR) characteristic of graphene [1,2]. In this work, we use STM, LEED and Auger spectroscopy to compare the different growth methods and resulting morphologies of EG grown on the silicon- and carbon-terminated faces of SiC. UHV sublimation of silicon from SiC(0001) results in controllable growth of 1-5 ML of EG, with the thickness determined predominantly by the growth temperature. EG growth on SiC(000$\bar{1}$) via Si-sublimation is done in a low vacuum induction furnace, resulting in 5-30 ML thick films. Since MR results indicate that transport is dominated by an EG layer near the SiC interface [2], we will discuss methods to access the EG/SiC(000$\bar{1}$) interface by STM. [1] C. Berger, et al., J. Phys. Chem. B 108,19912 (2004). [2] C. Berger, et al., Science 312, 1191 (2006)   

Exploration of new methods for growing Ag films on Au(111) studied by ARPES

Ag/Au(111) thin films have attracted lots of interests as a model system in the past decades. Ag and Au are lattice-matched, and thin Ag films of very high quality are expected to grow on Au(111). However, the intermixing between Ag and Au at elevated temperatures has been a major concern during the growth of Ag films on the Au(111) surface. In many previous studies, Ag was deposited on the Au(111) surface at near room temperature to avoid the intermixing problem. Investigating the results from these studies, the Ag films on Au(111) grown by this recipe still show clear thickness variation. This thickness variation may result from Ag-Au intermixing or film roughening during the process of room temperature deposition. We are revisiting this classical model system with new growth methods. Our goal is to find growth methods that will stop the intermixing between Ag and Au and reduce the variation in the thickness of Ag films. Preliminary results from our study will be presented in this poster.   

Growth of uniform metal nanowires on stepped Si surface

We investigated Indium (In) nanowires on the stepped Si surface by scanning tunneling microscopy (STM). The stepped surfaces have attracted much attention because their structure can be a useful template for the growth of nanowires. We used the stepped Si(557) surface; its clean structure is composed of a half-unit cell of the Si(111)7x7 surface and a triple bunched step. The In nanowires are found to form on the triple bunched steps uniformly along the step edges with a width of 1 nm at low coverage. At higher In coverage, another nanowire is noticed to grow proximate to the nanowire grown at lower In coverage. We will present the detailed STM images of the In nanowires and will describe their geometric structures.   

Exploration of Controlled Nitrogen Doping of TiO$_{2}$ Single Crystalline Thin Films

TiO$_{2}$ is a promising material for many photochemical applications, notably the direct conversion of solar energy into chemical (H$_{2})$ via photocatalytic splitting of water. One of the biggest challenges to incorporating TiO$_{2}$ into a practical device is developing accessible routes to reliably dope TiO$_{2}$ with impurities like C, N, or B that will extend the photoactivity of TiO$_{2}$ from the UV into the visible part of the solar spectrum. Fundamental studies of the doping and the resulting changes in photocatalytic properties require well-defined model systems, such as bulk-doped single crystalline TiO$_{2}$. We present results on in situ N-doping of single crystalline TiO$_{2}$ films during homoepitaxy on rutile TiO$_{2}$(110) using reactive magnetron sputtering. NO$_{2}$ is injected into the plasma to achieve bulk doping of the TiO$_{2}$ films. The doped film morphology and near-surface electronic structure is studied in situ by scanning tunneling microscopy, without any treatment that might affect the dopant distribution. Analytical transmission electron microscopy is used to map film structure, defects, and interfaces, and determine the doping profile.   

Characterization of Vapor Deposited Antimony Nanoparticles on HOPG and MoS$_{2}$ by Scanning Force Microscopy

Atoms or clusters deposited from the gas phase onto a substrate surface provide a suitable model system to study the early stages of material growth. In particular, diffusion, aggregation, and coalescence have been recognized to determine the morphology of the emerging nanoparticles. We present an analysis of antimony nanoparticles spontaneously formed from thermally evaporated monodisperse Sb$_{4}$ clusters on the (0001) basal planes of highly ordered pyrolytic graphite (HOPG) and molybdenum disulfide (MoS$_{2})$. The spontaneous formation of the antimony nanoparticles on the substrate surfaces is controlled by the deposition conditions (i.e., coverage and flux) at room temperature under ultrahigh vacuum conditions. After deposition the samples were transferred to air for further analysis. Scanning electron microscopy (SEM) was used as an alternate means for the qualitative evaluation of the surface morphology, whereas scanning force microscopy (SFM) has been employed for disclosing detailed information on the three-dimensional shape of the antimony nanoparticles.   

Getter Sputtering System for High Throughput Fabrication of Composition Spreads

We describe a sputtering system that can deposit composition spreads in an effectively UHV environment but which does not require the high-throughput paradigm to be compromised by a long pumpdown each time a target is changed. The system deploys four magnetron sputter guns in a cryoshroud (getter sputtering) which allows elements such as Ti and Zr to be deposited with minimal contamination by oxygen or other reactive background gasses. The system also depends on custom substrate heaters to give rapid heating and cooldown. The effectiveness of the gettering technique is evaluated, and example results obtained for catalytic activity of a pseudoternary composition spread are presented.   

Appearance Potential Study of Ti-Ni Alloys

The electronic structure of Ti-Ni alloys have been investigated by the technique of appearance potential spectroscopy. This technique is sensitive to the density of unoccupied states above the Fermi level and therefore becomes suitable to the investigation of these materials. The Ti 2p and Ni 2p regions have been investigated. The spectral features have been compared with the elemental ones. The density of states for TiNi has also been calculated using the DFT approach implemented in CRYSTAL98. The Becke exchange and the LYP correlation have been utilized. The atomic basis sets have been optimized for this purpose. The projected density of states for Ti and Ni in TiNi have been determined. The comparison between the experimental and theoretical data will be presented.   

Transmission Phase Holography: Spatial-Mode Filter Design for Quantum Information Applications

Photon spatial modes offer access to promising new applications in quantum information because they provide a higher-dimensional basis set than the usual two-dimensional one associated with polarization. Downconversion experiments have demonstrated spatial-mode entanglement [1], and even hyperentanglement in polarization and spatial mode [2]. However optical elements currently lack the refinement necessary to perform efficient, high-fidelity operations using spatial modes. Holographic filters for Laguerre-Gaussian and Hermite-Gaussian laser modes can act as modes converters, and have long been studied (under the terms ``modans'' and ``kinoforms'') for use in electrical engineering applications [3,4]. Her we present analytical refinements and optimizations of these techniques, with predicted mode fidelities over 95{\%} and diffraction efficiencies up to 98{\%}. Results of our experimental implementions of these solutions are presented. \newline [1] Walborn, S.P, et al, ``Entanglement and conservation of orbital angular momentum in spontaneous parametric down-conversion,'' Phys. Rev. A \textbf{69}, 023811 (2004); [2] Barreiro, J.T. et al, ``Generation of Hyperentangled Photon Pairs,'' Phys. Rev. Lett. \textbf{95}, 260501 (2005); [3] Soifer, V.A., ``Methods of Computer Design of Diffractive Optical Elements,'' John Wiley \& Sons, Inc., 2002; [4] Golub, M. and Soifer, V., ``Laser Beam Mode Selection by Computer Generated Holograms,'' CRC Press, Inc., 1994.   

An Eigenvalue Problem for Quantum Computing

A unitary operator on a quantum spin system of the form, $U = e^{-iH_1}e^{-iH_2}$, is introduced. Here, $H_1$ and $H_2$ are Hermitian and easily diagonalized; however, because the diagonalizing bases for $H_1$ and $H_2$ are quite different, the operator $U$ is strongly interacting. The eigenvalues of $U$ can be used to help factor products prime numbers in a manner similar, but not identical to the Shor algorithm. Indeed even approximate eigenvalues could be useful. Since $U$ is strongly interacting, the practical usefulness of this approach hinges of finding tractable approximations. Toward this end, results of exact diagonalization of $U$ for small systems are compared with the solution of several different approximate schemes.   

Quantitative treatment of decoherence

We outline different approaches to define and quantify decoherence. We argue that a measure based on a properly defined norm of deviation of the density matrix is appropriate for quantifying decoherence in quantum registers. For a semiconductor double quantum dot qubit, evaluation of this measure is reviewed. For a general class of decoherence processes, including those occurring in semiconductor qubits, we argue that this measure is additive: It scales linearly with the number of qubits.   

A Quantum Signaling Device

One may construct a message (i.e., binary information) and send it from one location to another where there is no relevant discernible measurable difference in the specified physical situation in the intervening space between where the message is constructed and where the message is decoded that allows for decoding the message. Also, attempting to intercept the message in the intervening space between where the message is constructed and where the message is decoded would likely result in the transmission of the message being disrupted. The ability to send a message in the manner noted is an extension of the idea in quantum mechanics that between the initial state and final state of a quantum system one does not really know what is happening ``in the middle.'' The quantum wave function allows predictions of what will occur if a measurement is made. In the absence of a measurement, there are only quantum mechanical predictions. In the device presented, these predictions for detecting a photon are the same in the two possible pathways in the intervening space between where the message is constructed and where the message is decoded irrespective of the nature of the two possible types of photon source/s. Yet because of the two possible types of photon source/s that can be used to create the message, the predictions regarding photon detection after the photons leave ``the middle'' of the device and reach the detectors are different, allowing the sent message to be decoded at the detectors.   

Relaxation in a System of Two Harmonic Oscillators

In quantum information, one is often interested in a physical system C, composed of two quantum subsystems A and B, interacting through some external interaction Hamiltonian. We are interested in how the interaction Hamiltonian forces the two subsystems to relax when in contact with each other. Entanglement between two initially uncoupled subsystems leads to the exchange of quantities such as purity or polarization, or for thermodynamical systems, temperature. We would like to understand the process by which this exchange occurs. Starting from the initial density matrices of the two subsystems, it is possible to calculate the corresponding time-evolved density matrices of the subsystems at some future time, by taking partial trace of the density matrix describing the complete interacting system. In this poster, we present the mechanisms which generate relaxation of a bipartite system composed of two harmonic oscillators A and B. Both the systems A and B are initially at equilibrium at temperatures T1 and T2 respectively, and are assumed to be uncoupled. We apply a general interaction Hamiltonian for some time interval t, and study how the two subsystems evolve under this operation. After successive application of the interaction Hamiltonian, we expect that, for each oscillator, the initial Boltzmann distribution will be replaced by another Boltzmann distribution at a new equilibrium temperature. We calculate this new temperature by numerical methods.   

Thermal and dissipative effects in Casimir physics

We have developed an apparatus to assess the thermal effects in Casimir force measurement of a cylinder-plane geometry. Preliminary electrostatic calibrations imply sensitivity sufficient to observe the Casimir force with submicron separation between reflecting surfaces. Work is in progress to improve the sensitivity in order to distinguish the thermal contributions up to 3 microns separation. Another project currently underway at Dartmouth addresses an experimental strategy to verify the dynamical Casimir effect, a dissipative feature of motion in quantum vacuum. In this scheme, Casimir photons generated inside a high-Q cavity with one of the walls driven at GHz frequency [2] would stimulate superradiant emission from ultracold sodium atoms injected into the cavity. We are modeling this system in order to identify the signal features distinguishing Casimir induced superradiance from sodium superflourescence. [1] M. Brown-Hayes, D. A. R Dalvit, F. D. Mazzitelli, W. J. Kim, and R. Onofrio, Phys. Rev. A 72, 051102 (2005). [2] W. J. Kim, J. H. Brownell, and R. Onofrio, Phys. Rev. Lett. 96, 200402 (2006).   

Quantum Teleportation in One-Dimensional Quantum Dots System

We present a model of quantum teleportation protocol based on one-dimensional quantum dots system. Three quantum dots with three electrons are used to perform teleportation, the unknown qubit is encoded using one electron spin on quantum dot $A$, the other two dots $B$ and $C$ are coupled to form a mixed space-spin entangled state. By choosing the Hamiltonian for the mixed space-spin entangled system, we can filter the space (spin) entanglement to obtain pure spin (space) entanglement and after a Bell measurement, the unknown qubit is transfered to quantum dot $B$. Selecting an appropriate Hamiltonian for the quantum gate allows the spin-based information to be transformed into a charge-based information. The possibility of generalizing this model to N-electrons is discussed.   

Entanglement in Anderson Nanoclusters

We investigate the two-particle spin entanglement in magnetic nanoclusters described by the periodic Anderson model. An entanglement phase diagram is obtained, providing a novel perspective on a central property of magnetic nanoclusters, namely the temperature dependent competition between local Kondo screening and nonlocal Ruderman-Kittel-Kasuya-Yoshida spin ordering. We find that multiparticle entangled states are present for finite magnetic field as well as in the mixed valence regime and away from half filling. Our results emphasize the role of charge fluctuations.   

Entanglement Verification from Partial Knowledge in Arbitrary Systems

Entanglement plays an essential role in quantum information theory. However, many of the known entanglement criteria are difficult to evaluate in practice. We present a method to verify entanglement which can easily be applied to a variety of experimental situations, especially when the measurements only give little information about the density matrix. The method allows to detect entanglement in finite and infinite dimensional settings, as well as hybrid systems. We demonstrate the formalism by applying it to a particular example which occurs naturally in the context of continuous variable quantum key distribution. This examples sheds light onto the question whether a possible eavesdropper could take advantage of the phase reference, which is typically sent with the signal over the channel.   

An Optical Quantum Information Transfer Device.

Binary information can be sent between locations remote from one another without the velocity limitation of the velocity of light in vacuum. The OQITD relies on ``hidden'' events for idler photons traveling through an interferometer where these ``hidden'' events point to which-way information for these photons. Through either: 1) keeping the ``hidden'' events ``hidden'' until which-way information is lost, or 2) instead making these events public before which-way information is lost, one can influence the spatial distribution of paired signal photons that were created in the same process and at the same location as the idler photons. Which-path information concerning the signal photons themselves becomes unavailable shortly after their creation. Two possible distributions for the signal photons can be developed in different sets of runs of the OQITD. One distribution is indicative of which-way information, and the other distribution is indicative of interference. These two different distributions can be used to create binary bits which themselves can be assembled into a message. The motion of the paired signal and idler photons does not violate the velocity limitation of the special theory of relativity. Nonetheless, the effect of manipulating the circumstances concerning the idler photons on the distribution of the distant paired signal photons is not limited by the velocity limitation of the special theory. This effect is used to transmit a binary message.   

Entanglement Swapping via Collective Decay

Quantum entanglement has achieved a prime position in current research due to its central role in quantum information science. Recently, attention has been focused on reservoir-induced entanglement with the purpose of shedding light on the generation of entangled qubits at remote separation [1]. In addition to the entanglement generation, a teleportation scheme for atomic and solid state qubits, which is based on the Dicke effect, is also proposed [1]. In contrast to usual schemes, it's a ``one-pass'' teleportation by a joint measurement Based on these, we go one-step further to propose a scheme for entanglement swapping. As shown in Fig. 1, the singlet entangled state is generated between atom 1 (2) and the left (right) cavity photon as atom 1 (2) has passed through the left (right) cavity. Atoms 1 and 2 then decay collectively. If the measurement outcome is a single photon with sub-radiant decay rate, entanglement swapping to the two cavity photons is achieved automatically. As for the result of one photon with super-radiant decay rate, all one has to do is to perform a phase-gate operation on the cavity photon state to complete the entanglement. [1] Y. N. Chen, D. S. Chuu, and T. Brandes, Phys. Rev. Lett. 90, 166802 (2003); New Journal of Physics 7, 172 (2005).   

Quantum key distribution with ``dual detectors"

To improve the performance of a quantum key distribution (QKD) system, high speed, low dark count single photon detectors (or low noise homodyne detectors) are required. However, in practice, a fast detector usually is noisy. Here, we propose a ``dual detectors" method to improve the performance of a practical QKD system with realistic detectors: the legitimate receiver randomly uses either a fast (but noisy) detector or a quiet (but slow) detector to measure the incoming quantum signals. The measurement results from the quiet detector can be used to bound eavesdropper's information, while the measurement results from the fast detector are used to generate secure key. We apply this idea into various QKD protocols. Simulation results demonstrate significant improvements in both BB84 protocol with ideal single photon source and Gaussian-modulated coherent states (GMCS) protocol, while in decoy state BB84 protocol with weak coherent source, the improvement is moderate.   

A Derivation of a Weaker, Local Bell inequality

We argue that Bell made a third assumption in his Theorem, and that by assuming that incompatible measurements (as opposed to hidden variables predicting the measurements) could occur simultaneously, Bell's inequality only applies to local hidden variable theories that violate Heisenberg's Uncertainty Principle. If the hidden variable(s) reproduce quantum mechanics, and we assume they prevent us from considering A-prime to be measured (or thought of theoretically) at the same time as A, our rederivation of Bell's inequality has extra terms that weaken the constraint. Since the same locality and reality assumptions hold for this derivation as for Bell's, we conclude that only time independent static, i.e., measurement-order independent, local hidden variable theories are constrained by Bell's inequality; time dependent, non-classical local theories ({\it i.e.} theories respecting Heisenberg's Uncertainty Principle, such as local de Broglie-Bohm models) can satisfy this new bound while exceeding Bell's limit. We note that quantum-mechanically, the square of the Bell operator has a similar extra term involving commutators of {\em local} measurement operators. Unconditional nonlocality is only expected to occur with Bell parameters between $2\sqrt{2}$ and $4$.   

Novel Relationships between Superoscillations, Weak Values, and Modular Variables

We present several novel, unexpected relationships between superoscillations, weak values and modular variables. For example, we show how an uncertain phase, which characterizes the process of projecting a particle onto a superoscillatory region, can create the high-momentum. If an uncertain phase can localize the particle, then a definite phase can also localize it. This introduction of a relative phase corresponds to a non-local exchange of modular variables. We also present a new way to measure the nonlocality in the equation of motion for modular variables by using weak measurements.   

Study of thin film and substrate losses affecting qubit coherence times

For superconducting qubits, losses in superconducting films as well as substrates and dielectrics can play important roles in determining the coherence time. We used co-planar waveguide resonators to study the losses in Nb films and substrates. Resonators with quality factors as high as $6 \times 10^5$ were fabricated and measured. We found that both the fabrication process and resistivity of the substrates contribute to the quality of the resonators and hence to the losses. With a proper fabrication process, very high quality Nb films can be made. The etching process, which is necessary for the fabrication of qubits, lowers the quality of the resonators. We are optimizing the fabrication process to reduce the losses and to increase coherence time for our qubits.   

Dynamics of dense converging plasmas studied with soft x-ray laser interferometry

Electron density maps of dense converging plasmas created by laser irradiation of semi-cylindrical targets at I = 1 x 10$^{12}$ W/cm$^{2 }$were obtained with soft x-ray laser interferometry. The plasma expands off the target surface converging in a focal region, creating a concentrated plasma where the electron density build-up exceeds 1 x 10$^{20}$ cm$^{-3}$. The measurements were conducted using a 46.9 nm wavelength Ne-like Ar capillary discharge laser probe and a soft x-ray Mach-Zehnder interferometer based on diffraction gratings. The short wavelength of the probe beam enables the study of dense plasmas beyond the limitations of optical lasers. The measurements were compared with simulations obtained using the code HYDRA. Work sponsored by the NNSA-SSAA program through DOE Grant {\#} DE-FG52-060NA26152 and the U.S. DOE Lawrence Livermore National Laboratory through ILSA, under contract No. W-7405-Eng-48.   

Molecular beam sorting by $\alpha $/m: from fullerenes to carbon nanotubes

We show that a matter-wave interferometer can be used to sort gas phase molecules according to their polarizability-to-mass ratio $\alpha /$m. We present a proof-of-principle experiment for the separation of C$_{60}$ and C$_{70}$. We propose to exploit the high molecular throughput and high spatial resolution of our setup for the enrichment of different biomolecular conformers or mixtures of single-walled carbon nanotubes with strongly varying $\alpha /$m-ratios inside a grating-based Stark deflectometer.   

Coherent control of wavepacket motion in Cesium dimers

We present an experimental investigation of wave packet dynamics in Cesium dimers. Using femtosecond pump-probe techniques (both degenerate and non-degenerate), we observe transmission oscillations which correspond to motion of the coherent wavepackets in the ground state and in excited states. The use of a second pump beam, the control, allows us to selectively excite certain oscillations and suppress others by adjusting the relative delay between them. Additionally, some preliminary results on Coherent Anti-Stokes Raman Scattering (CARS) in Cesium dimers will be presented.   

Perfect GHZ States from Imperfect Cluster States in Optical Lattices

Cluster states form a class of non-separable multipartite graph states, the entanglement of which is exceptionally persistent against the effects of single-qubit measurements. One of the most promising experimental approaches to the formation of cluster states employs a gas of ultracold atoms confined in an optical lattice. Starting with a Mott insulator state of pseudospin-1/2 bosons at unit filling, cluster states can be generated efficiently by preparing each spin state in its own sublattice, and inducing collisional phase shifts by varying the laser polarizations. In practice, systematic phase errors are expected to arise during this entangling process, resulting in the formation of imperfect cluster states. In this poster, we present a technique for using imperfect cluster states to distill perfect GHZ states. Applications include fault-tolerant quantum computing, open-destination quantum teleportation, quantum cryptography, Heisenberg-limited spectroscopy, and atomic clocks.   

``Trickle Meter Gravimetry'': Precision Interferometry from Residual Berry Phase Edge Effects Involving Atoms Exiting an Accelerating Optical Lattice

From a generalization of conventional band theory\footnote{S.R.Chubb, Proc Roy Soc A, submitted (2006).}, derived from a many-body form of multiple scattering theory, I rigorously showed that the semi-classical theory of cold atom transport in optical lattices could be related to changes in the zero of momentum of the ground state. The new formulation includes finite size effects. When the effects of excitation, associated with the loss of atoms at the boundaries of the lattice are included, in the adiabatic limit, in which the perturbing potential acts sufficiently slowly and weakly, topological changes in phase (which are equivalent to Berry phase effects in the conventional semi-classical theory) take place that introduce discontinuous changes in wave function phase (and flux). In a situation involving an accelerating optical lattice, containing ultra cold atoms in a Bose Einstein Condensate, these changes in wave function phase can be monitored and used to systematically alter the acceleration of the lattice (by altering the chirp frequency of one of one of the counter-propogating lasers), in such a way that a form of edge-effect interferometry can be performed, which, in principle, can be used to make precision measurements of gravity, with unprecedented accuracy.   

Generalized Mean Field Theory of Resonant Bose-Fermi Mixtures.

In the wake of successful experiments in Fermi condensates, experimental attention is broadening to include resonant interactions in degenerate Bose-Fermi mixtures. In order to study the equilibrium properties of the fermionic molecules that can be created in such a mixture, we develop formally, and solve numerically, a mean field theory approach generalized to properly reproduce the two body physics in the low density regime.   

Limits to $T/T_{\mathrm F }$ and progress towards ultracold $^{6}$Li-$^{87}$Rb mixtures

Ultracold dilute atomic gases are providing a new window into quantum physics, with particular regard to the first-principle study of superfluid phenomena. Fermionic degeneracy (measured by $T/T_{\mathrm F }$) achieved with current apparata using sympathetic cooling techniques seems to be limited by the heat capacity matching between the Fermi and Bose species and by Fermi-hole heating. We will discuss these factors and report on the development of an apparatus for simultaneous trapping and cooling of $^{6}$Li and $^{87}$Rb, using a light-assisted magnetic trap.\footnote{M. Brown-Hayes and R. Onofrio, Phys. Rev. A \textbf{70}, 063614 (2004).} The $^{6}$Li and $^{87}$Rb species combination, in conjunction with the improvement in heat capacity matching, should allow for an improved lower limit to attainable temperatures in Fermi-Bose mixtures.\footnote{R. C\^{o}t\'{e}, R. Onofrio, E. Timmermans, Phys. Rev. A \textbf{72}, 041605(R) (2005).} We also report on seemingly opposite approaches to reaching a lower $T/T_{F}$ and suggest the differences are due to two distinct measures of cooling efficiency.   

Quantum phase transitions, symmetry breaking, and the Goldstone mode in metastable Bose-Einstein condensates

It is commonly believed that in a superfluid with repulsive interactions the circulation is quantized and there is a discontinuous jump in states between different values of the circulation. In fact, this rule applies only to the ground state: continous transitions are possible for metastable states. We explicitly show this by considering a dilute Bose-Einstein condensate on a quasi one-dimensional torus with tunable atom-atom interactions and/or external rotation. The key to such transitions is the appearance of a dark or grey soliton train degenerate with an excited angular momentum eigenstate. These occurrences are characterized by second order quantum phase transitions between metastable states. In the mean field theory, they are associated with bifurcations; in the quantum field theory, they are connected with the appearance of quasidegenerate states, which construct the broken-symmetry state and Goldstone mode in the presence of an infinitesimal symmetry-breaking perturbation.   

Thermodynamic properties of bosons inside nanotubes

We describe the statistical behavior of an ideal boson gas among periodic channels which are simulated by two perpendicular external Kronig-Penney potential while bosons are allowed to be free in the residuary direction. The critical temperature goes from the 3D ideal boson gas critical temperature to that of a quasi-1D boson gas inside an impenetrable nanotube of square transversal section of wide $a$, as the wall penetrability ($P = mV_0 ab/\hbar^2$) goes from zero to infinity. We also calculate and discuss other thermodynamic properties such as the specific heat.   

Quantum fluids of self-assembled chains of polar molecules

We study polar molecules in a stack of strongly confined pancake traps. When dipolar moments point perpendicular to the planes of the traps and are sufficiently strong, the system is stable against collapse but attractive interaction between molecules in different layers leads to the formation of extended chains of molecules, analogously to the chaining phenomenon in classical rheological electro- and magnetofluids. We analyze properties of the resulting quantum liquid of dipolar chains and show that only the longest chains undergo Bose-Einstein condensation with a strongly reduced condensation temperature. We discuss several experimental methods for studying chains of dipolar molecules.   

Step-up versus ``Step-down'' scattering asymmetry in the charge transfer of H$^-$ on free-electron vicinal metallic surfaces

We present numerical results based on a wave-packet propagation study of the one-electron charge transfer between hydrogen anions and free-electron vicinal metallic surfaces [1]. An effective potential for the motion of the active electron is derived from Thomas-Fermi-von Weizs\"{a}cker theory, extended to include the image charge effects. The ion-survival probability near the surface is evaluated from a rate equation for projectiles that are incident with a kinetic energy of 50 eV. We find an enhancement of the electron loss near the steps of the surface, caused by the Smoluchowski effect. As a consequence, depending on the orientation of the surface steps, the ion- survival is more likely if the projectiles approach steps from above [2]. \newline \newline [1] B.~Obreshkov and U.~Thumm, Phys. Rev. A {\bf 74}, 012901 (2006). \newline [2] B.~Obreshkov and U.~Thumm, Surf. Sci., in press.   

Electron Charge Transfer between Hydrogen Anions and Si Surfaces

We show numerical results for the charge transfer between hydrogen anions and Si surfaces. The electronic structure of the substrate is derived from density functional theory with an application of the Thomas-Fermi-von Weizs\"{a}cker model extended to include linear and higher order non-linear dielectric response corrections to the kinetic energy density functional of non-interacting and non-uniform electron gas. An empirical local pseudopotential for Si [3] is used to represent the external field of the ionic cores of the substrate. The H$^-$ neutralization probability near the surface is evaluated within a wave-packet propagation method for anions at grazing incidence with kinetic energy of 1 keV. We compare our theoretical negative-ion survival probabilities [1] with the experimental data in [2]. \newline \newline [1] B.~Obreshkov, U.~Thumm, Phys.~Rev.~A 74, 012901 (2006); and in preparation. \newline [2] M.~Maazouz {\emph{et al.}}, Surf. Sci. 398, 49 (1998). \newline [3] L.~Wang and M.~Teter, Phys.~Rev.~B 45, 13196 (1992).   

Collisional mechanism and the temporal evolution of neon optogalvanic signals

We report on the temporal evolution of the optogalvanic signal in neon gas using a commercial hollow cathode lamp in conjunction with a Nd:YAG pumped dye laser system. We have recorded at least 3 transitions excited from each of the neon 1s levels. These transitions are recorded for a range of discharge current to study the decay rates of these states. We have found a linear relationship between the decay rates and the discharge current.   

Diffusion Dominated RF-Response of a Coated Rb-Vapor Cell in an Inhomogenous Magnetic Field

The magnetic resonance lineshape of Rb-vapor in a coated cylindrical cell [1] is modeled quantitatively. It is diffusion-dominated when l$^{3}$\textit{$\omega $}'$>$D. Here l is the cell thickness, D is the diffusion constant and \textit{$\omega $}' is the local gradient of the Lamor frequency (directed perpendicular to the cell's faces). We obtain the spectrum by averaging the path-dependent transition probability over all diffusion paths in a binomial ``hopping'' model. The resulting line shape also depends on the characteristics of the average interactions of a Rb atom with the coated cell surface. It agrees very well with the experimentally observed lineshape. The two most prominent peaks in the diffusion-dominated regime are due to modes concentrated at either face of the cell with a gradient-dependent contribution to the half-width c(\textit{$\omega $}'$^{2}$ D)$^{1/3}$, where c depends on the surface interactions but is $\sim $0.5 in most cases studied. This dependence of the line width on \textit{$\omega $}' allows one to measure the local gradient of the magnetic field without moving the magnetometer and should be particularly useful for measuring time-dependent field gradients. \newline [1] K. Zhao and Z. Wu, to be published.   

Feshbach Resonances in the Inelastic Collision B($^{2}$P$_{1/2})$ + H$_{2}(j) \quad \leftrightarrow $ B($^{2}$P$_{3/2})$ + H$_{2}(j$')

Feshbach resonances can occur during an inelastic collision when wave packet amplitude propagating on a lower potential energy surface is non-adiabatically coupled to an upper potential energy surface (1). If the upper potential energy surface exhibits a well, then components of the incoming wave packet with energies near the quasi-bound eigenvalues of the upper well can become trapped. Eventually, the trapped wave packet amplitude will exit the interaction region and give rise to sharply peaked resonant structure in the scattering matrix elements that characterize the collision. Using the wave packet based Channel Packet Method, we calculate scattering matrix elements for the inelastic B($^{2}$P$_{1/2})$ + H$_{2}(j)$ $\leftrightarrow $ B($^{2}$P$_{3/2})$ + H$_{2}(j$') collision (2). Sharp resonance features in the scattering matrix elements are observed to occur at energies that are in close agreement with numerical calculations of adiabatic vibrational eigenvalues of the weakly bound B{\ldots}H$_{2}$ van der Waals complex. (1) M.S. Child, Molecular Collision Theory, Dover Publications, Mineola NY, (1996). (2) D.E. Weeks, T.A. Niday, and S.H. Yang, J. Chem. Phys, 125, 164301 (2006).   

Nonlinear Scattering of Bose-Einstein Condensates on a Finite Barrier

We consider the scattering of a Bose-Einstein condensate (BEC), or atom laser, on a finite barrier. The nonlinear Schrodinger equation is used to model the mean field of the BEC. Although we cannot treat incident plus reflected waves, since the governing equation is nonlinear, nevertheless we can obtain steady state behavior of a BEC in this physical situation. We find the full solution to this problem in closed analytic form, including classes of solutions which have no analog in the standard problem from linear quantum mechanics of the finite square well, and describe the transmission resonances. Our study is useful in the construction of atom laser devices.   

Reduced Density Matrix Descriptions for Electromagnetically Induced Transparency in Atomic Systems

Reduced density matrix descriptions are developed for electromagnetically induced transparency and related pump-probe optical phenomena in moving atomic systems, taking into account atomic collisions and external magnetic fields. Time-domain (equation-of-motion) and frequency-domain (resolvent-operator) formulations are developed in a unified manner. In a preliminary semiclassical perturbative treatment of the electromagnetic interaction, compact Liouville-space operator expressions are derived for the linear and the general (n'th order) non-linear electromagnetic-response tensors. These expressions are valid for coherent atomic excitations and for the full tetradic-matrix form of the collision operator in the Markov approximation.   

Matter-Wave Decoherence in an Atom Interferometer: Which-Way vs. Classical Fluctuations

We present a theoretical treatment of decoherence in an atom interferometer by two seemingly disparate approaches. One arm of the interferometer contains a background gas; any collision with the background gas would constitute a ``which way'' detection and decoherence. Yet coherence can largely be retained in spite of large phase shifts acquired by atoms passing through the background gas. How can the atoms ``interact'' with the gas, producing a phase shift, without ``touching'' (i.e., giving even a gentle shove to) the atoms of the background gas? This story has a prosaic but instructive resolution in terms of the quantum cross section for scattering; however, a semi-classical treatment of the fluctuating forces experienced by the initially coherent atom traversing the background gas may give essentially the same result for both the phase shift and the decoherence. We discuss the generality and limitations of this method. Our results resolve the fundamental question of when the scattering interactions that cause the index of refraction constitute a which-way measurement for the path of atoms through the interferometer.   

Quantum Accelerator Modes at higher order resonances

Quantum Accelerator Modes (QAM) are produced by subjecting cold atoms to standing wave pulses, when these pulses are applied in the direction of gravity. A group of atoms get accelerated. Normally, QAMs are seen whenever pulse period is choosen close to an integer multiple of a time called the half Talbot time. These times are referred to as primary resonance times [1]. We, using BEC, show for the first time that QAMs can be observed at rational fractions of Talbot time, called higher order resonance times. The details and the latest experimental data will be presented. [1] G. Behin-Aein, V. Ramareddy, P. Ahmadi, G. S. Summy, Phys. Rev. Lett. (accepted for publication), arXiv physics/0609203.   

Studies of Ion Dynamics and RF Heating in Miniature Ion Traps

RF Paul ion traps have been an important tool in mass spectrometry and trapped ion quantum information processing. Arrays of ion traps have been proposed as the basis for a quantum information processor.$^{1}$ An important issue related to the use of these arrays is determining how difficult it will be to control ion heating as the traps are miniaturized and made less symmetric to accommodate the technological design needs of an actual quantum computer. Experiments performed recently have made important progress in measuring the heating rate of miniature ion traps as a function of the electrode distance from the ion position, which point to the existence of fluctuating patch potentials on the electrode surfaces as the main source of anomalous ion heating$^{2}$. We will present results from numerical and analytical studies that identify and characterize the heating processes of ions in miniature ring and end-cap traps as functions of trap size and other variations in trap design. We will also explore the suitability of these traps for use in cavity QED experiments. $^{1}$ Kim, J., et al., ``System design for large-scale ion trap quantum information processor,'' \textbf{QIC, 5 (7)}, 2005. $^{2}$ Deslauriers, L., et al., ``Scaling and suppression of anomalous quantum decoherence in ion traps,'' \textbf{\textit{Phys. Rev. Lett.}}\textbf{ 97}, 2006   

Visualization of vortex bound states in polarized Fermi gases at unitarity

We theoretically analyze a single vortex in a spin polarized 3D trapped atomic Fermi gas near a broad Feshbach resonance. Above a critical polarization the Andreev-like bound states inside the core become occupied by the majority spin component. As a result, the local density difference at the core center suddenly rises at low temperatures. This provides a way to visualize the lowest bound state using absorption imaging. As the polarization increases, the core expands gradually and the energy of the lowest bound state decreases.   

Periodic Numerical Grid Method for the Maximally Even Kronig-Penny Model

Energy band calculations for quasi-periodic crystals is an important field in condensed matter physics.~ Efficient and stable methods of calculation are necessary in the study of long quasi-periodic crystalline structures.~ The band gap structure of quasi-periodic crystals can be studied using a cell structure and applying a modified Kronig-Penney model.~ We show how a finite-element method using the discrete variable representation can be implemented with periodic boundary conditions on a Gauss-Lobatto Quadrature numerical grid.~ Because of the Gauss-Lobatto Quadrature numerical grid and the discrete variable representation, the local potential operators are represented as diagonal matrices and the kinetic energy matrix is quasi-block diagonal.~ This sparsity and block diagonal nature of the system greatly reduces the number of calculations necessary for the eigenvalue problem and increases the stability of the system as the size of the crystal chain grows.~ We implement the periodic finite-element discrete variable representation using a maximally even distribution of potentials as a modification to the usual Kronig-Penny model.   

Near-Field Acoustic Holography of Chladni Plate

Characteristic modes of a Chladni plate can be visualized by sand gathering along the nodal lines. Although one can view two-dimensional vibration patterns, the sand pattern does not show a bending of antinode. We used near-field acoustic holography to reconstruct a three-dimensional image of circular and rectangular Chladni plates. The sound pressure was measured using four scanning microphones located 3 mm above a Chladni plate. A reference microphone was placed close to a speaker which drives the plate. A total of 60x60 data were collected from 30x30 cm$^{2}$ area and processed using MATLAB in accordance with the algorithm of near-field acoustic holography. The three-dimensional graphic image of the vibrating plate reconstructed this way not only matched the sand pattern but also visualized the bending of the plate. Propagation of the sound pressure could be also visualized three-dimensionally. The plot of sound pressure against the distance showed the 1/distance$^{2}$ dependence as expected. Sound is a difficult subject in physics class because it is invisible. Our results demonstrate that near-field acoustic holography combined with computer graphic is an effective tool to visualize the generation of a sound.   

Strategies for combining physics videos and virtual laboratories in the training of physics teachers

Among the multimedia resources used in physics education, the most prominent are virtual laboratories and videos. On one hand, computer simulations and applets have very attractive graphic interfaces, showing an incredible amount of detail and movement. On the other hand, videos, offer the possibility of displaying high quality images, and are becoming more feasible with the increasing availability of digital resources. We believe it is important to discuss, throughout the teacher training program, both the functionality of information and communication technology (ICT) in physics education and, the varied applications of these resources. In our work we suggest the introduction of ICT resources in a sequence integrating these important tools in the teacher training program, as opposed to the traditional approach, in which virtual laboratories and videos are introduced separately. In this perspective, when we introduce and utilize virtual laboratory techniques we also provide for its use in videos, taking advantage of graphic interfaces. Thus the students in our program learn to use instructional software in the production of videos for classroom use.   

SPS Outreach with Amusement Park Physics

The Department of Physics and Astronomy helps host an annual outreach event at a local amusement park for students in grades 6-12. The Physics Night Program is made up of several parts primarily focused on having students apply physics principles and estimate measurements at the park. That evening, DU sponsors undergraduate student volunteers in queue lines to discuss physics concepts with students, and the Society of Physics Student (SPS) chapter stages a more elaborate collection of physics demonstrations at a central gathering place. In 2006, we expanded our outreach efforts to include interactive data collection and on-site interpretation of data. A team of graduate and undergraduate DU SPS students outfitted high school volunteers with vests and hardware designed to return three-axis acceleration information. Students wore the sensors on the rides, returned the equipment to the staging area, and received immediate feedback about the accelerations their hardware recorded. We hope to expand the range of experiential sensing to include more physiological as well as physical data collection, and to provide greater accuracy utilizing the technology we are implementing at the park.   

Space Physics: a new undergraduate program in physics

In this paper, we describe a new undergraduate physics program entitled ``Space Physics.'' Graduate programs in space physics have been around since 1959, however, this is the first undergraduate program of its kind. The B.S. in Space Physics offers the traditional core of physics courses along with four areas of concentration: Astrophysics, Particle Physics {\&} Cosmology, Exotic Propulsion, and Remote Sensing. The program has over 90 students with the first senior class graduating this Spring 2007. The students are actively engaged in undergraduate research projects that prepare them for careers in the aerospace industry as well as graduate school in physics or space physics. The positive \textit{employer feedback} from student internships already indicates that our upper-division students are prepared to move onto careers in the aerospace industry. The demographics as well as the details of undergraduate research projects will be presented in this paper.   

PR$^{2}$EPS: Preparation, Recruitment, Retention and Excellence in the Physical Sciences.

The PR$^{2}$EPS program is a multidisciplinary effort to increase the number of majors attending (and graduating) from SUNY Oneonta with degrees in chemistry, physics, biochemistry, astronomy, secondary chemistry or physics education and related areas. Components of the program include a walk-in tutoring center, a free, weeklong summer science camp, scholarship opportunities and an equipment loan program for regional secondary science teachers. 2006 was the third year of this NSF-DUE funded program. Evaluation of our progress to date shows that the program is effective at steering students (or at least reinforcing their desire) to studying the sciences in college. A summary of our goals, challenges and accomplishments, including tutoring center operation and efficacy, activities and operational details for the summer camp and other facets of the program will be presented.   

Changing Student Attitudes using Andes, An Intelligent Homework System

The size of introductory physics lectures often inhibits personal homework assistance and timely corrective feedback. Andes, an intelligent homework help system designed for two semesters of introductory physics, can fill this need by encouraging students to use sound problem solving techniques and providing immediate feedback on each step of a solution. On request, Andes provides principles-based hints based on previous student actions. A multi-year study at the U.S. Naval Academy demonstrates that students using Andes perform better than students working the same problems as graded pencil and paper homeworks. In addition, student attitude surveys show that Andes is preferred over other homework systems. These findings have implications for student attitudes toward, and mastery of, physics. See http://www.andes.pitt.edu for more information.   

Should female enrollment in science and engineering increase?

Many people hold this truth to be self-evident, that there should be more female students in science and engineering. However, the proposed arguments are generally vague and unsatisfactory, they motivate more than they really justify. `There should be more female students in science and engineering' then means that women should want to study science and engineering and that science and engineering should want to attract women. This is only a contingent win--win situation, there does not \emph{have to} be more female students in these fields for moral reasons. Moreover, this applies only to women interested in science- related fields. In particular, outreach programs ---designed to attract more females to science and engineering--- must respect the right of female students to choose freely (even if they decide to major in a non-scientific field). It would certainly be paradoxical to trample this right in order to enforce it.   

Accessing the Education Literature through the ADS

The ADS and ComPADRE in collaboration are developing a portal to the education literature in the ADS. A large part of the education and education research literature are now available through the ADS Abstract Service. The new portal makes accessing and searching of this part of the literature easy and intuitive, while also providing access to the more sophisticated search capabilities of the ADS.   

Validation Studies of a Physics Problem Solving Survey

Researchers have created several tools for evaluating conceptual understanding as well as students' attitudes and beliefs about physics; however, the field of problem solving is sorely lacking a broad use evaluation tool. This missing tool is an indication of the complexity of the field. The most obvious and largest hurdle to evaluating physics problem solving skills is the physics content knowledge necessary to solve problems. We are tackling this problem by looking for the physics problem solving skills that are useful in other disciplines as well as physics. We will report two sets of interviews that compare physics problem solving skills to the results of a problem solving survey developed at Colorado. The first set of students participated in a series of interviews on solving complex introductory mechanics problems and a second set of students were involved in a semester long series of interviews where they solved quantum mechanics problems. Students were characterized on their skills in either set of interviews and these results were then compared to independent evaluation of their skills using the problem solving survey. \begin{enumerate} \item Supported in part by funding from National Science Foundation DTS. \end{enumerate}   

Trends in Student Thinking Among Undergraduates Studying Thermodynamics and Kinetics

Student misconceptions in thermodynamics and kinetics have been explored in a number of studies. Here, we surveyed all the students in two undergraduate physical chemistry classes to determine how widespread different misconceptions are. Sixty students were given a short answer test, in which they wrote out their solutions to 34 conceptually oriented questions. The student responses were analyzed to determine how students were thinking about problems, and if some methods for solving problems were linked to getting the correct answer. This study is part of validating a conceptual survey for thermodynamics and kinetics.   

Development of a Physical Science Course for Elementary Education Majors

We have developed a physical science content course for elementary education majors that is aligned with state standards. The course incorporates several hands-on activities related to the Virginia Standards of Learning that have already been implemented as one-hour lessons in elementary classrooms. Topics include measurement, properties of matter, motion {\&} energy, electricity {\&} magnetism, sound {\&} light, chemical {\&} physical processes, weather, and the solar system. In addition to the hands-on activities, course content is discussed in a small lecture format with questions posed throughout the material. The students discuss these questions in assigned groups and then answer them using remote answer devices (see www.einstruction.com). Lastly, conceptual ideas in class are reinforced using online LON-CAPA homework questions that are individually randomized for each student and provide immediate feedback (see loncapa.org). Those questions which indicate a high degree-of-difficulty are reassigned during following weeks to provide multiple opportunities for practice. All of these active learning approaches reinforce basic concepts necessary to teach physical science at the elementary level.   

Studying the importance of students' beliefs in physics education

We have developed and used a new survey instrument -- the Colorado Learning Attitudes about Science Survey (CLASS)$^{1,2 }$-- to extensively study the importance of students' beliefs about physics and about learning physics to physics education. Since Fall 2003, we have surveyed over 10000 students in 50 physics courses ranging from courses for non-science majors to graduate courses in physics. In this poster, we characterize the range of student beliefs across the undergraduate physics curriculum from non-science majors to physics majors. In addition, we examine the relationships between students' beliefs about physics and learning physics and other important education outcomes, including their conceptual learning, their interest in physics, and pursuit of science study. Finally, we examine the relationship between students' beliefs and classroom teaching practices. 1. W.K. Adams, K.K. Perkins, N. Podolefsky, M. Dubson, N.D. Finkelstein and C.E. Wieman, ``A new instrument for measuring student beliefs about physics and learning physics: the Colorado Learning Attitudes about Science Survey'', Phys. Rev ST: Phys. Educ. Res. 2, 1, 010101 (2006). 2. See \underline {http://per.colorado.edu} for relevant papers.   

Students' Opinions of Physicists' Beliefs about Physics Versus Their Own

The Colorado Learning Attitudes about Science Survey (CLASS)$^{1,2}$ is a 42 statement questionnaire designed to elicit student beliefs about physics. Previous studies show that many students do not agree with the expert responses, raising the question of whether students know how experts would respond. In this study, students were asked to choose their opinion (from a 5 point Likert scale) and their opinion of what a physicist would believe. Students from three introductory physics classes (courses for engineers and physics majors, pre-meds, or non-science majors) were surveyed and 11 students were interviewed to provide a deeper understanding. Results from the surveys and interviews will be presented. Also considered is how students' opinions of physicists' beliefs are related to gender, current physics course, and previous physics experiences and how these opinions change throughout the semester. 1. W.K. Adams, K.K. Perkins, N. Podolefsky, M. Dubson, N.D. Finkelstein and C.E. Wieman, ``A new instrument for measuring student beliefs about physics and learning physics: the Colorado Learning Attitudes about Science Survey'', Phys. Rev ST: Phys. Educ. Res. 2, 1, 010101 (2006). 2. See \underline {http://per.colorado.edu} for relevant papers.   

Orientation of surfactant self-assembled aggregates on graphite

Micellar aggregates on surfaces can provide a self-healing corrosion protection or lubrication layer. It has been observed experimentally that on a single crystal surface this layer often consists of oriented hemi-cylindrical micelles which are aligned with the underlying crystal lattice (``orientation effect''). A key feature of this self-assembly process is the interplay between detergent--detergent and detergent--surface interactions. Since the dimensions of the detergent molecules and the unit cell of the surface are typically quite different, the origins of this orientation effect remain unclear. Here we address the question and present the results of Molecular Dynamics simulations of sodium dodecyl sulfate (SDS) self-aggregation on graphite. We employ both single-molecule and multi-molecule simulations of SDS to unravel the origins of the orientation effect. We report that the underlying graphite surface is sufficient to impose orientational bias on individual SDS molecules diffusing on the surface. This produces collective effects that give rise to the oriented hemi-micelles.   

Liquid crystalline order of carbon nanotubes

Topological defects formed during phase transitions in liquid crystals provide a direct proof of the standard Cosmological model and are direct links to the Early Universe. On the other hand in Nanotechnology, carbon nanotubes can be manipulated and oriented directly by changing the liquid crystalline state of the nanotubes, in combination with organic liquid crystals. Currently there are no nano-assemblers, which makes the liquid crystal state of the nanotubes, one of the few ways of controlling them. We show the design of a fast and efficient polarized light ellipsometric system (a new modification of previous optical systems) that can provide fast quantitative real time measurements in two dimensions of the formation of topological defects in liquid crystals during phase transitions in lab settings. Our aim is to provide fundamental information about the formation of optically anisotropic structures in liquid crystals and the orientation of carbon nanotubes in electric field.   

Nonequilibrium-molecular-dynamics measurement of the Leslie coefficients of a Gay-Berne nematic liquid crystal

We carried out nonequilibrium molecular dynamics (MD) simulations to measure the six Leslie coefficients of a nematic liquid crystal composed of molecules interacting via the Gay-Berne potential. In the presence of a simple shear flow, an external field is applied to control the molecular orientation, and a uniform director is stabilized in the central region of the channel in which the liquid crystal is confined and sheared. With the director tuned by varying the applied field, a number of orientational states are stabilized in the presence of a shear flow, and various viscous stress components are measured in these states of different directors. The six Leslie coefficients $\alpha_i$ are determined by interpreting the MD measurement data for viscous stress according to the constitutive relations in the Ericksen-Leslie-Parodi (ELP) theory. The Parodi relation $\alpha_2+\alpha_3=\alpha_6-\alpha_5$ is well satisfied. Given the values of the Leslie coefficients, liquid crystal orientations are evaluated for different field directions and shear rates. Comparison with those directly measured in MD simualtions demonstrates a quantitative agreement, showing that in the Gay-Berne nematic liquid crystal, the viscous stress and the coupling between orientation and flow are well described by the ELP theory.   

Enhancement of luminescence of nematic liquid crystals doped with silver nanoparticles

The photoluminescence of nematic liquid crystals (5CB) doped with silver nanoparticles has been investigated. The profiles of luminescence with various concentrations of silver nanoparticles remain the same and possess the same peak position around 390 nm. The peak intensity of luminescence of nematic liquid crystals increases with the increasing concentration of silver nanoparticles. The quantum yield of enhancement is 1.5 for 1{\%} of concentration of silver nanoparticles. The polarization dependence of photoluminescence was also performed by rotating the director of nematic liquid crystals relative to the polarization of pump beam. The intensities of photoluminescence with various concentrations of nanoparticles decrease and converge to the background luminescence from the director of nematic liquid crystals parallel to the polarization of pump beam to the perpendicular case. The ratio of photoluminescence intensity with director parallel to the perpendicular to the polarization of pump beam increases sharply with the concentration less than 0.1{\%} and slowly with the concentration in the range from 0.25{\%} to 1{\%}.   

Nano-Particle Doped Hybrid Organic-Inorganic Photorefractives

We present results of beam coupling from hybrid photorefractive cells comprising a liquid crystal layer doped with nano-particles of ferroelectric materials adjacent to doped inorganic photorefractive windows. The presence of the nano-particles significantly enhances the optical coupling between the signal beam and pump beam. Full Bragg matched amplification of a signal beam is demonstrated for grating spacings from 5$\mu $m to less than 800nm.   

Non-contact AFM Imaging of the Surface of Freely Suspended Liquid Crystal Films

The surfaces of freely suspended thick films of 7O.7 in modulated crystalline-B phases have been imaged using non-contact mode atomic force microscopy. Large-scale images show 3 nm steps at the edges of layers on the surface of thick films that are adjacent to large flat areas. Previous models of x-ray diffraction measurements indicate that some of the crystalline-B phases of 70.7 have static modulations with amplitudes of 0.4 nm and a period of about 10 nm[1]. No surface modulations are seen, suggesting either that the modulations are not static or that the surface structure differs from that of the bulk. [1] E. B. Sirota, P. S. Pershan, and M. Deutsch, Phys. Rev. A 36, 2902 (1987).   

Annihilation of Point Defects in Smectic-C Liquid Crystal Films

An experimental study of the pair annihilation of c-director defects with topological strength +1 and -1 in a freely suspended Sm-C film is described. Many pairs of +1 and -1 point defects are produced mechanically on the film by transient generation of compressive in-plane stress, and their subsequent behavior is studied using polarized video microscopy. The defects show local positional fluctuations and are attracted by long-range elastic forces. Immediately following their generation several hundred defects are observed, which attract and annihilate until only a few are left. The subsequent dynamics of isolated pairs of defects are then studied. It is found that r(t)\textbullet v(r(t)), the product of defect separation and the mean velocity of attraction v(r)=dr/dt, decreases as r-$>$0. The behavior of r(t)\textbullet v(r(t)) is not understood theoretically. This work is supported by a NSF and by NSF MRSEC Grant DMR0213918. \newline [1] D. Svensek and S. Zumer, Phys.Rev.Lett. \textbf{90}, 155501 (2003). \newline [2] G\'eza T\'oth, Colin Denniston, and J. M. Yeomans, Phys.Rev.Lett. \textbf{88}, 105504 (2002). \newline [3] Patrick Oswald and Jordi Ign\'es-Mullol, Phys.Rev.Lett. \textbf{95}, 027801 (2005).   

Optical bandgaps for a Cholesteric Elastomer Slab Under Stress

Using the equilibrium configuration adopted by a cholesteric elastomer elongated by the influence of a uniaxial transverse stress, we calculate the solution of the boundary--value problem for the reflection and transmission of obliquely incident plane waves due to a elastomer slab. We find a left-circularly polarized thin reflection band immersed in a wider right- circularly polarized band reflection, when the elastomer is under a stress near to the critical value. Also, a deformation of the reflection band and the reduction of its bandwidth were obtained. These bandgap features suggest to design a finely tuning polarization-universal optical filter.   

Modeling the Collective Behavior of Soft Colloidal Particles

Recent developments in the fabrication of soft colloids open up the possibility for designing new, responsive materials that are composed of these colloidal particles. Such colloidal systems can have properties tailored not only to reproduce molecular systems on the nano- and micro- scale, but also show novel collective behavior. This is due to the fact that the range of interactions between the particles can be significantly smaller than the size of particle. This is in contrast to certain molecular systems. A significant challenge here is to identify the key parameters that control the system dynamics and the underlying physical mechanisms. By numerical modeling, we study materials composed of soft particles that are closely packed into an amorphous state. We focus on the dynamical behavior and mechanical properties of such material, which behaves like a soft elastic solid at low stress but flows like a viscous liquid at stresses above the critical yielding value. Such behavior governs materials as different as pastes, foams, and biological tissues.   

Computer simulation of anisotropic diffusion of colloidal particles between flat plates

We have created a computer simulation of the anisotropic diffusion of charged colloidal particles trapped between charged or uncharged flat plates. Anisotropic diffusion is simulated using a random displacement in each time interval appropriate for diffusion constants taken from the theory of Happel and Brenner [1]. For charged flat plates, the electrostatic force on the particle was derived from measured potential energies taken from C.H. Sow, \textit{et al.,} [2]. The code and analysis techniques have been checked for the case of free diffusion in 3D and in simulating the experiments of Lin, Yu, and Rice [3]. We are now working on using the code to test new analysis techniques for the case of charged plates where the electrostatic force further suppresses diffusion in the direction perpendicular the plates. [1] J. Happel and H. Brenner, \textit{Low Reynolds Number Hydrodynamics,} (Kluwer, Dordrecht, 1991) [2] C.-H. Sow, \textit{et al.,} in preparation [3] B. Lin, J. Yu, and S. Rice, \textit{Phys. Rev. }\textbf{E62,} 3909 (2000)   

Many-body Hydrodynamic Effects in Collective Diffusion of Colloids in a Quasi-One-Dimensional Channel

We report the results of experimental, theoretical and Brownian dynamic simulation studies of particle displacements in quasi-one-dimensional colloid suspensions. We infer, from a comparison of theory and experiment, that many-body hydrodynamic interaction determines the long wavelength behavior of the collective diffusion coefficient. The consequence of the many body hydrodynamic interaction is an apparent divergence of the so called hydrodynamic factor in the limit of infinite wavelength.   

Visualizing X-ray Beam Damage of a Langmuir Monolayer via GIXD and Brewster Angle Microscopy

The extent and form of radioactive beam damage from high brilliance x-ray sources has been debated among researchers who study biological membranes. It has been shown that radiation damage increases during x-ray measurements as a function of time. However, this damage has not been optically observed on the micrometer scale for lipid membranes. Here we report the observation on the effect of radiation on a lipid monolayer of DMPS (1,2-Dimyristoyl-sn-Glycero-3-[Phospho-L-Serine]) with grazing incident x-ray diffraction in conjunction with \textit{in situ }Brewster Angle Microscopy (BAM). The measurements were done in an oxygenated atmosphere, at a surface pressure of 25 mN/m and at room temperature. Under these conditions the monolayer is fully condensed, and the GIXD measurement shows a single first order diffraction peak. When the surface pressure is held constant, the GIXD peak height decreases over time. In addition, the BAM shows patches of lowered refractive index for the monolayer, indicating that these areas no longer contain lipids in the condensed phase. When the surface area is held constant, irradiation of the monolayer leads to a dramatic change in surface morphology as part of the condensed phase of the monolayer becomes disordered.   

In-situ x-ray scattering and Brewster-angle microscopy studies of 2D streptavidin crystals bound to a lipid monolayer at the solution/vapor interface

Adsorption and two-dimensional (2D) crystallization of soluble protein streptavidin on a biotinylated lipid monolayer at an aqueous solution/vapor interface have been studied using \textit{in-situ} x-ray and optical methods. For a given subphase and lipid condition, surface-normal and in-plane structures at molecular length scales were elucidated via synchrotron x-ray reflectivity (XR) and grazing-incidence diffraction (GID) measurements, respectively, at the solution/vapor interface. For GID, CCD was used for fast data collection while a microfocusing mirror was used to enhance the lateral resolution by reducing the illuminated footprint area. The 2D crystalline structures thus revealed were correlated with the morphologies of growing 2D crystal domains observed optically by carrying out Brewster-angle microscopy (BAM) under exactly the same lipid, protein, and subphase conditions. The results show that at high salt concentration (0.5 M NaCl) and moderate biotin surface density (130 or 650 {\AA}$^{2}$/biotin), streptavidin nearly always forms 2D crystals, but both the unit cell structures and the crystal domain shapes are different at low, intermediate and high subphase pH values (3.2, 5.5, and 8.2).   

Reactive Multi-component Membranes: From Dynamic Reconstruction to Gradient Sensing

Via computer simulations, we study two- and three-component membranes in which an external stimuli initiates a chemical reaction that inter-converts two of the components, A and B. The third component, C, is assumed to be non-reactive and is incompatible with the A and B. We also assume that the A and B have specified spontaneous curvatures. The dynamics of the system is described in terms of the three order parameters, two of which represent the composition and the third one is the height of the film. The binary (AB) part of our model is based on the recent model proposed by Reigada et al. (PRE 72,(2005) 051921); we have extended the latter approach by explicitly considering the effects of the lateral interfacial tension. By performing a linear stability analysis, we predict which state is realized for the given bending elasticity and interfacial tension of the membrane; the results of our computer simulations are in an agreement with the analytical calculations. For the three-component membrane, we illustrate how the presence of the non-reactive component affects the final patterns within the membrane. We also show how such multi-component ``smart'' membranes respond to changes in external stimuli and how they can be used to perform some simplified biomimetic functions, such as a gradient sensing.   

Phase Behavioral and Structural Properties of an Efficient Solvent-Free Model of Lipid Bilayers

High-power computers facilitate the study of lipid bilayer membranes. Any computer model used to simulate such membranes must account for their spontaneous self- assembly due to hydrophobic interactions between lipid tails and water. This is usually done by simulating lipid molecules in explicit solvent. In such simulations most of the system is occupied by the solvent. It is therefore computationally desirable for equilibrium studies of lipid membranes to develop a model that leads to self-assembly of lipids without explicit solvent. We designed such a model, and show that its use leads to faster simulations than what can be achieved with current solvent-free models. This model utilizes soft interactions to account for hydrophobic effects (instead of the Lennard-Jones potential). Investigation of the lipids' diffusion coefficient, single-lipid orientational order parameter and internal energy as functions of temperature reveal a structural phase diagram in the membrane from a gel-like hexatic phase to a fluid phase. The characterization of membrane elastic properties from this model will also be presented.   

Modeling the motion of non-adhesive and adhesive capsules through channels with posts

We study numerically the motion of compliant microcapsules in microchannels. These microchannels have two adhesive posts on the walls that form an orifice with a separation distance roughly comparable in size to the diameter of the capsules. Each microcapsule consists of an elastic shell that is filled with a viscous fluid. The capsules model synthetic polymeric microcapsules or biological cells, such as leukocytes. The microcapsules are driven to move through the channels by an imposed pressure gradient. To model this multi-component system, we combine the lattice Boltzmann model for fluid dynamics and the lattice spring model for the micromechanics of elastic solids. We probe the effect of capsule stiffness and adhesion between the posts and capsule on the motion of capsules within the channel.   

Geometrical effects on helical flow in grooved microchannels

Due to the enhancement of surface effects on the microscale, patterned grooves on a microchannel floor remain a powerful method to induce helical flows within a pressure driven system. Although there have been many numerical studies on geometrical effects on the staggered herringbone mixer (SHM), all have mainly focused on the groove periodicity and depth, two factors that contribute greatly to the magnitude of helical flow. Here we present a new geometrical factor that more directly affects the generation of helical flow over patterned grooves. By varying the ratio of the length of the grooves to the neighboring ridges, helical flow can be optimized for a given groove depth and channel aspect ratio. Helical flow is characterized by the magnitude of transverse flow rate per unit axial length of channel, normalized by the bulk flow through the channel. A full numerical study details the magnitude of helical flow over uneven patterned grooves in a slanted groove micromixer (SGM), and the optimized parameters are shown to be equivalent to the SHM. These optimized systems are shown to have a 50{\%} increase in helical flow over the previous geometries.   

Direct Measurement of Ion Accumulation at Metal-Electrolyte Interface using differential interferometry

The ionic charge accumulation at the metal-electrolyte interface is directly measured by using differential interferometry as a function of magnitude and frequency (2-50 kHz) of external electric field. The technique developed probes the ion dynamics confined to the electrical double layer. The amplitude of modulation of the ions is linearly proportional to the amplitude of applied potential. The linearity is observed up to high electrode potentials and salt concentrations. The frequency response of the ion dynamics at the interface is interpreted in terms of the classical RC model. Further extension of the technique to probe Faradaic reactions at the metal-aqueous solution shows higher sensitivity (two orders of magnitude) to the electrochemical reaction compared to the conventional current measurement. Analogous to AC Cyclic Voltammetry, the change in refractive index is observed with the (applied) potential being a DC Ramp on which a small AC signal (probe) is superposed. The widely used Faradaic couple K4Fe(CN)6/K3Fe(CN)6 is used as the model system for the study.   

Influence of Particle Size Dispersity in Dense Packings on the Void Structure and the Interparticle Contact State

We explore both the effect of packing history prior to application of compressive strain, and the variation in the response with the size distribution of the component particles in a dense packing. We generate the packings by allowing the particles to settle under gravity for a fixed interval of time, or until a cut-off packing fraction is attained, followed by application of a compressive strain for a fixed interval of time. We repeat these studies using numerical experiments for samples of discrete size distributions (200 microns, 195-225 microns, 170-260 microns, 150-295 microns) and random (100-300 microns, 100-400 microns, 100-500 microns). We find the number of particles with fewer than 4 contacts to increase with size dispersity of the sample after the particles settle under gravity. In addition, the fraction of plastic contacts decreases with increasing variation in particle size during the compression (Dutt, Hancock, Bentham, Elliott, submitted). We use a combined approach of determining the porosity and the internal packing structure to obtain insight into the effect of particle size dispersity. The former is determined by the use of a dynamically tesselating algorithm to calculate the pore network in dense particulate systems (Benedict, Dutt \& Elliott, to be published Physica A).   

Experimental Verification of the Reverse Brazil Nut Effect

In the Brazil nut problem (BNP), hard spheres with larger diameters rise to the top. In a previous paper, [Phys. Rev. Lett. \bf{86} \normalfont, 3423(2001), A theory was presented for the crossover from BNP to the reverse Brazil nut problem (RBNP) based on the competition between the percolation effect and the condensation of hard spheres. We experimentally test the crossover condition as predicted by the theory. Our results show that the the RBNP does occur under certain conditions. We then verify the crossover conditions as predicted by the theory in three dimensions.   

Self-Organized Criticality in a Bead Pile

This experiment examined a conical bead pile and the distribution of avalanche sizes when using uniform 3mm zirconium spheres (``beads''). A bead pile is built by pouring beads onto a circular base where the bottom layer of beads had been glued randomly. Beads are then individually dropped from a fixed height after which the pile is massed. This process is repeated for thousands of bead drops. By measuring the number of avalanches of a given size that occurred during the experiment, the resulting distribution could be compared to a power law description as predicted by self-organized criticality. We had found in an earlier experiment that glass beads dropped from a small height were consistent with a simple power-law, but if dropped from larger heights then a power-law times an exponential was needed. The zirconium beads always had a distribution that deviated from a simple power-law with a power-law times an exponential when the beads were dropped from larger heights, but the distribution showed a distinct enhancement of the probability for large avalanches when beads were dropped from smaller heights. We compare our experimental results to a numerical simulation. We acknowledge support from NSF-REU DMR 0243811.   

Yeast Cell Encapsulation in Microfluidic Devices

Polydimethylesiloxane (PDMS) microfluidic devices were used to encapsulate single S. cerevisiae cells that contained a protein tagged with green fluorescent protein for studies on cell growth rates. It was observed that drops stored in PDMS devices shrank over time due to the permeability of water in PDMS. Drop shrinkage was characterized and alternative methods to storing drops were tested. Pre-saturating PDMS devices were also tested and found that drop volume decreases much less that unsaturated PDMS devices. Cells were found to bud in pre-saturated PDMS devices.   

A simple model system to study forces when virus interacts with cell membrane

Virus needs to attach itself to the cell membrane in order to force the cell to replicate its DNA and multiply. To better understand the fundamental adhesion mechanism between the virus and the cell membrane, we work on a much simplified system, charged hard spheres that interact with phospholipid liposomes. The interactions between the liposome and charged nanoparticles were examined using colloidal probe microscopy. The local and global mechanical properties of these ``attacked'' liposomes were measured using atomic force microscopy and a homebuilt surface forces apparatus.   

Release of nanoparticles from mobile microcapsules: designing artificial ``leukocytes'' for microfluidic devices

We present a novel algorithm to simulate the release of nanoparticles from a microcapsule as it rolls along an adhesive substrate, as well as the subsequent particle adsorption on the wall. The microcapsules consist of an elastic shell that encloses a fluid with a suspension of nanoparticles. This microcapsule is immersed in an external host fluid. A pressure gradient is applied to drive the flow. We focus on a single capsule, which interacts with the substrate through an adhesive interaction that keeps the capsule rolling along the surface. We examine how the adsorption of nanoparticles is affected by the adhesion strength between the capsule and substrate, as well as by the membrane stiffness. To regulate the movement of capsules, we exploit the fact that the adhesion strength between the capsule and the substrate could be different for an ``untreated'' surface and a surface with a coating of nanoparticles. This could be utilized to repair an area where the coating is damaged. Releasing nanoparticles from the microcapsule allows one to ``repair'' the area, before the capsule continues its motion along the surface. Our findings yield guidelines for efficient localized delivery of an active ingredient onto a substrate.   

Analytical Model of Advection and Erosion in a Rectangular Channel

We consider the Boussinesq pressure driven creeping flow in a rectangular channel. We assume a particle to be made of primary fragments bound together. Particles are advected by the flow and they erode because of the shear stresses imparted by the fluid. The time evolution of the numbers of particles of different sizes is described by the Bateman equations of nuclear radioactivity. We find, by solving these differential equations, the numbers of particles of each possible size as functions of time.   

Kinematics of a Sphere Moving through a Yield Stress Liquid

We measure the drag force exerted on spheres with different diameters (d) being pulled at constant speed through a suspension of Carbopol polymer in water in tubes of various diameters (D). We find that the drag force increases with sphere speed over the range of velocities studied. The drag force also increases with decreasing tube size for D/d $<$ 2. Thus, we can estimate the size of the envelope of sheared liquid that surrounds the moving sphere. Interestingly, we find that the envelope size does not depend on the sphere velocity.   

Study of Dense Granular Shear Flow in 3D with Discrete Element Method

The steady-state behavior of dense granular shear flow in the simple shear geometry is studied numerically with the 3D Discrete Element Method. Spheres of diameter $d$ and density \textit{$\rho $}$_{g}$ are confined in the $z$- direction between two parallel rough walls, with periodic boundary conditions in both the $x$- and $y$- directions. The bottom wall is held stationary and the top wall is moved with constant velocity in the $x$- direction, while maintaining a constant normal load per area, $p$. Beyond a thin boundary layer near the walls, the strain rate $\dot {\gamma }$ becomes independent of position and the medium achieves a uniform macrostate with no spatial gradients, allowing exploration of constitutive relations and collision kinetics with good statistics. The perturbative effect of adding interstitial fluid at low Reynolds number and low fluid density is also considered by incorporating lubrication forces into the simulation. The dimensionless shear rate, $I\equiv \dot {\gamma }{\kern 1pt}d\sqrt {\rho _g /p} $ , controls the bulk density in the flow, with and without interstitial fluid, whereas the effective traction coefficient, i.e., the ratio of shear force to normal force at the wall, increases with increasing viscosity of the fluid due to increased dissipation. The sensitivities of the results to particle stiffness, fluid density and the surface roughness of the spheres are also studied. The results are insensitive to stiffness as long as the spheres are sufficiently stiff compared to $p$.   

The Solid-Liquid Interface in a Colloidal Hard Sphere System

The solid-liquid interface between a crystal and its melt is of great interest for both statistical physics and materials science. It constitutes the transformation front in solidification and has thereby considerable influence on the microstructure of metals or alloys. For example, the weak ($\sim$1\%) anisotropy of the interfacial free energy is believed to be the origin of dendrite formation in solidifying metals.\\ We study the solid-liquid interface in a hard sphere colloidal suspension by confocal microscopy. Single crystals are grown by sedimentation of silica colloids in an index matched water-dimethylsulfoxide solution to a PMMA template prepared by lithographic techniques. The interplay between gravity and Brownian motion generates a volume fraction gradient in vertical direction, yielding a liquid layer on top of the crystalline sediment. Using confocal microscopy, this model interface can be studied at convenient time and length scales. In the same way as in computer simulations we try to obtain the interfacial stiffness from the Fourier spectrum of the capillary fluctuation waves. This type of simulation complements existing numerical work and offers the advantage of a large system size.   

Jamming of Confined Colloids in Aqueous and Non-polar Media

We contrast the jamming behavior of confined colloidal particles suspended in either aqueous or non-polar media. The systems we explore are: 1) colloidal poly(methyl methacrylate) (PMMA) suspended in non-polar media and 2) Synthetic poly($N$-isopropylpolyacrylamide) (PNIPAM) microgel in aqueous media. We examine the effects of volume fraction and film thickness on the structure and rheological properties of both systems. By using a home-built microrheometer integrated with confocal microscopy, we visualize the packing configuration and mobility of confined colloids simultaneously during shear force measurements. For the PNIPAM system, our results show that confinement induces the formation of three-dimensional tenuous PNIPAM aggregates. By applying large shear amplitude, the reorganization of colloidal gel structure is also observed. For the hard-sphere PMMA system, we observe glass-like behavior as gap spacing approaches 10-15 particle layers. Upon applying large shear amplitude, the `melting' of glassy PMMA thin films is observed.   

Non-affine deformations and floppy modes in random fibrous networks

We study the class of heterogeneous elastic networks composed of crosslinked fibers. These systems have recently been suggested as model systems for studying the mechanical properties of paper sheets or biological networks of semiflexible polymers. While these networks are known to have a rigidity percolation transition at low densities, we show here that even networks in the high-density regime in many ways resemble the behaviour of fragile matter, despite the fact that they are far away from the percolation threshold. In these networks highly non-affine deformations have been found, however, few is known about the actual nature of this non-affinity. This work tries to fill this gap by characterizing in detail the non-affine deformation field present in fibrous networks. By relating non-affinity to the low-energy excitations (``floppy modes'') we can, starting from a microscopic picture, calculate the macroscopic elastic moduli both in a scaling theory and a self-consistent effective medium theory.   

Nonlinear energy transport in one-dimensional lattices

We present a simple lattice model consisting of a one-dimensional chain, where the masses are interconnected by linear springs and allowed to move in the horizontal direction only, as in a monorail. In the transverse direction each mass is also attached to two other springs, one on each side of the mass. The ends of these springs are kept at fixed positions. The nonlinearity in the model arises from the geometric constraints imposed on the motion of the masses, as well as from the configuration of the springs. In the transverse directions the springs are either in the extended or compressed state depending on the position of the mass. Under these conditions we show that solitary waves are present in the system. In the long wavelength limit an analytical solution for these nonlinear waves is found. Numeric integrations of the equations of motion in the full system are also performed to analyze the conditions for the existence and stability of the nonlinear waves. Nonlinear supratransmission is examined and shown to exist in the model and an explanation of its mechanism is presented.   

What is going on in liquid metals?

We present neutron scattering data on a range of liquid metals that demonstrate that there is a contribution to the total scattering of a neutron by a metal that is in excess of the coherent and incoherent cross-sections. We show by means of polarized neutron scattering data on liquid Gallium that this additional contribution is not magnetic in origin, ruling out a possible explanation that appeared to fit all the facts [Patty et al., Phys. Rev. E 73, 021202 (2006)].   

Liquid metal flow in a spherical shell: recent results.

Motivated originally to study dynamo action in an Earth-like geometry, our group has performed a series of experiments using liquid sodium. An externally applied magnetic field probes the underlying flow field in a spherical Couette configuration revealing a series of inertial wave modes. The frequencies and wave numbers of these modes closely match those predicted for a full rotating sphere and numerical calculations of the magnetic field based on those modes also agree very well with the observed induced field. The modes seem to be excited by virtue of the over-reflection instability. Implications for the Earth's core are briefly discussed.   

Interfacial dynamics of a liposome deforming in an axisymmetric extensional flow

Liposomes are self-enclosed structures composed of curved lipid bilayer membranes which entrap part of the solvent in which they freely float. They are predominantly made from amphiphilic molecules, a special class of surface-active molecules. Liposomes have various applications in science and technology including drug delivery systems, medical diagnostics and they can also be used as simple cellular models for basic research. We simulated the deformation of a liposome in an axisymmetric extensional flow using the boundary integral method. The liposome deforms due to hydrodynamic loading on the interface. The dynamics of the system are characterized by the competition between the hydrodynamic and interfacial forces. The lipid bilayer membrane can be modeled as a hyperelastic continuous material or a liquid-liquid interface with a highly packed surfactant layer. We compare the deformation behavior of liposomes with both types of interfaces and identify similarities and differences between the two models.   

Deposition Pattern of a Pinned Circular Evaporating Coffee Drop

When a coffee drop dries on a solid surface, a ring-like, finite-width stain remains. We establish a simple mathematical model to study the density distribution of the stain across its width. The asymptotic form of the distribution does not depend on the specific evaporation profile, i.e., diffusion-controlled evaporation vs. uniform evaporation. We also compare our findings with the simulation results based on truncated one-dimensional transport process.   

Two-dimensional turbulence experiments in sheared flow using circular Couette cell: initial results.

An experiment to study turbulence in quasi-two-dimensional flows with a controlled mean flow shear has been built. Experiments are underway to investigate the suppression of turbulent transport by sheared flow as seen in geostrophic flows and laboratory fusion plasmas. The apparatus, a circular Couette cell, uses a liquid film of dilute soap solution suspended freely in an annular channel with a rotating outer boundary. The channel is 7 cm wide with an average radius of 46.5 cm, and can be rotated at angular speeds exceeding 10 rad/s. Turbulence is driven independently via electromagnetic forcing. The rate of turbulence injection can be varied continuously, and its spatial scale corresponds to the spatial frequency of an array of NdFeB magnets. Diagnostics include particle imaging velocimetry, two-point laser Doppler velocimetry, and thickness measurements via reflection interferometry. Initial analysis will be presented which indicates the existence of both turbulent suppression and expansion in high and low frequency regimes respectively. Plans for further analysis will also be presented.   

On the quantum master equation for Bardeen-Cooper-Schrieffer pairing models

A master equation symmetric with respect to particles and holes has been introduced for systems composed of non-interacting identical fermions. [C. F. Huang and K. -N. Huang Chinese J. Phys. 42, 221 (2004); R. Gebauer R and R. Car R Phys. Rev. B 70, 125324 (2004).] Extensions to such an equation, in fact, can be obtained by incorporating two anti-hermitian terms for the lifetimes of particles and holes to construct the quantum relaxation term. In this poster, we focus on the extended equation for the interacting Fermi systems modeled by Bardeen- Cooper-Schrieffer (BCS) pairing theory. A constraint on the relaxation term is taken into account to preserve the pairing relation. Such a constraint, in fact, is also important when the coupling between quasiparticles and quasiholes is introduced to unify the BCS and antiferromagnetic/ferromagnetic models.   

Fluctuation-Dissipation Theorem and the Dynamic Response of a Fractional Relaxor-Oscillator

The so called fractional relaxor-oscillator system exhibits a rich variety of relaxation and damping characteristics. While the `free' relaxation/oscillations of the system are characterized by the so called `intrinsic' relaxation/ damping parameter, when driven by a sinusoidal driving force, the system exhibits frequency dependent relaxation/oscillation characteristics. This dynamic response of the fractional relaxor/oscillator is examined in the light of the fluctuation-dissipation theorem.   

Derivatives of Mittag-Leffler Functions with Respect to their Parameters

The Mittag-Leffler functions are natural extensions of the exponential function and appear often as solutions of differential equations of non-integer order in much the same way as exponential functions appear as solutions of differential equations of integer order. This ubiquitous nature of Mittag-Leffler functions underscores the importance of understanding the properties of these functions. In this regard, the derivatives of the Mittag-Leffler function E$_{\alpha ,\beta }$(-x) with respect to its parameters $\alpha $ and $\beta $ have been investigated. Particularly interesting are the derivatives of t$^{\alpha -1}$E$_{\alpha ,\alpha }$(-t$^{\alpha })$, which occurs as the fundamental Green's function solution to certain dynamic problems.   

Properties of the Mittag-Leffler Function

The Mittag-Leffler function E$_{\alpha ,\beta }$(z), which is a generalization of the exponential function, arises frequently in the solutions of differential and integral equations of fractional order. In order to better understand the physical systems described by these equations it is important to understand the basic properties of the Mittag-Leffler function. This paper focuses on the Mittag-Leffler function E$_{\alpha ,\alpha }$(z), the location and distribution of its zeros, and its inverse denoted by Ln$_{\alpha ,\alpha }$(z).   

Variational and Coupled Cluster Method for Many-Particle Systems

The use of canonical transformations in both quantum chemistry and physics in the construction of effective Hamiltonians has long been a useful tool in the hands of theoreticians. Here we wish to revisit the basic tenets of the Coupled Cluster Method, wherein the exponentials appearing in the transformed Hamiltonian $e^{-S}He^{S}$ are expanded out. These terms are then recombined to form the basis states of a recently developed variational scheme. Here the operator $S=\sum_{n}\lambda_{n}s_{n}$ represents the excitations of the system. We then apply this new method to a number of Hamiltonian systems.   

Analytic Properties of Moments Matrices

Associated with each matrix element of the recently developed Generalized Moments Expansion, \textrm{GMX}$(n,m)$ there is a unique expansion for the ground state energy in terms of the \textquotedblleft connected moments\textquotedblright\ $I_{k}$ of the Hamiltonian (Phys. Lett.~\textbf{A}% 349, 320 [2006]). That is, for any set $\{n,m\}$ a polynomial in the $I_{k}$'s may be generated to any desired order $L$, which is dependent upon the highest moment calculated. Here we wish to study the eigenvectors and eigenvalues of the \textrm{GMX} matrix itself. Furthermore we investigate the interplay between the set $\{n,m\}$ and the order $L$ of the matrix in determining which combination $\{n,m,L\}$ yields the \textquotedblleft best\textquotedblright% \ (i.e.~most convergent) result for the ground state energy.   

Excited States of Generalized Moments Expansion

A newly developed generalized moments expansion, \textrm{GMX}$(m,n)$, based on the \textquotedblleft\emph{t}-expansion\textquotedblright\ of Horn and Weinstein has proved useful in the calculation of the ground--state energy of a number of Hamiltonian systems in both quantum chemistry and physics. As has been shown elsewhere (Phys.~Lett.~\textbf{A}349, 320 [2006]) the well-known Connected Moments Expansion (\textrm{CMX}) of Cioslowski (PRL 58, 83 [1987]) is just a special case of $\mathrm{GMX}=\mathrm{GMX}(1,1)$. A number of years ago Marko\v{s} and Olejn\'{\i}k were able to generalize the \textrm{CMX} to include excited states (PRD 42, 2943 [1990]). Here we have extended this work to the \textrm{GMX} with applications to a number of systems.   

Unique properties of parallel-coupled double quantum dots in an Aharanov-Bohm ring

Stimulated by recent intriguing experiments with a parallel-coupled double quantum dot (QD) [1] and coupled QD's embedded in an Aharonov-Bohm (AB) ring [2], we investigate the electron tunneling conductance and resonance structure in the presence of inter-dot tunnel coupling of double QD's. First, when direct interaction (either Coulomb repulsion or tunnel coupling) between the dots is neglected, lattice and square-like transmission probability features are demonstrated as a function of the size of the two QD's. A modulation of magnetic flux in this system at fixed electron energy, which is equivalent to tuning the inter-dot coupling in experiments, results in the distortion of the lattice-like transmission probability. Second, we show the effect of direct coupling between the QD's, using an exactly-solvable tight-binding model. Contour plots of the transmission properties of the coherently coupled states of the QD's will be presented as a function of coupling strengths and magnetic flux variation through the AB-ring. [1] J. C. Chen \textit{et al}, PRL \textbf{92}, 176801 (2004). [2] A.W. Holleitner \textit{et al}, PRL \textbf{87}, 256802 (2001). * One of the authors (E. R. H.) is partially supported by a grant from the Center for Energy Research, Education, and Service (CERES) at Ball State University.   

Numerical ansatz for solving integro-differential equations with increasingly smooth memory kernels

We present an efficient and stable numerical ansatz for solving a class of integro-differential equations. We define the class as integro-differential equations with increasingly smooth memory kernels. The resulting algorithm reduces the computational cost from the usual T$^2$ to T*C(T), where T is the total simulation time and C(T) is some function. For instance, C(T) is equal to ln T for polynomially decaying memory kernels. Due to the common occurrence of increasingly smooth memory kernels in physical, chemical, and biological systems, the algorithm can be applied to quite a wide variety of situations. We demonstrate the performance of the algorithm by examining two cases. First, we compare the algorithm to a typical numerical procedure for a simple integro-differential equation. Second, we solve the NIBA equations for the spin-boson model in real time. Work supported in part by NSF and Sigma Xi. See also, cond-mat/0611412   

Absorbing Boundary Conditions for the Finite-Difference Time Evolution of the Wigner Function using the Vlasov Equation

Absorbing boundary conditions for the finite-difference time evolution of the Wigner function using a constant flux condition through the 2D region of calculation in phase space are presented. Numerical results on the time evolution of a Gaussian wave packet in phase space will be presented and discussed.   

Absence of certain exchange driven instabilities of an electron gas at high densities

In the high density limit the exchange energy easily overcomes the correlation energy in both two and three dimensional electron liquids. It is therefore reasonable to inquire if the class of exchange driven instabilities first discussed by Overhauser within the Hartree-Fock theory, could be of relevance in this limit. Our analysis shows that this is not the case.   

Landau Diamagnetism and Meissner effect in the BCS Theory

When the energy gap becomes zero, the BCS wavefunction leads to the free electron gas wavefunction. The free electron gas shows the Landau diamagnetism, whereas the superconducting electrons show the perfect diamagnetism, i.e., Meissner effect. However, the magnetic response of the BCS theory does not lead to the Landau diamagnetism for zero gap. It was argued that the small Landau diamagnetism would appear only in a higher order, although the Landau diamagnetism of free electrons is obtained in a linear order in the magnetic field. Here we show that a gauge-invariant derivation of the Meissner effect from the BCS wavefunction leads to the Landau diamagnetism for the zero gap case. We discuss the implications of this study on the electrodynamics of BCS superconductors and the Anderson-Higgs mechanism.   

Electronic excitations in Vanadium Dioxide (VO$_2$)

Vanadium dioxide has a metal-insulator transition at T$_C$ = 340 K. The role of correlation in this first-order transition has been debated for a long time: is VO$_2$ a Peierls or a Mott-Hubbard insulator? Despite a good agreement with experimental lattice parameters, DFT-LDA yields a negative gap in the insulating phase and hence is not able to give a good answer to this question. This seeming failure of LDA is discussed and different possible approaches to solve this problem are reviewed: from standard perturbative G$_0$W$_0$ to self-consistent GW. Also results concerning other kinds of electronic excitations (in particular: electron energy loss spectra) are presented.   

Reheating the Universe in brane world cosmological models

The transition of the Universe from the inflationary era to the standard $\Lambda $CDM phase is studied in the framework of the brane world cosmological models. In this scenario the presence of the large extra-dimensions determine the cosmological dynamics. The brane Universe is initially in an inflationary phase driven by a scalar field. The decay of the scalar field heats up the Universe. The evolution on the brane is generally modified due to the new terms in the energy momentum tensor. We investigate the reheating process, by assuming that the quadratic corrections, a consequence of the non-compactified geometry, give the dominant contribution to the energy-momentum tensor. The reheating process is studied by using both analytical and numerical methods. We generalize the standard reheating model by allowing the exchange of mass-energy between the bulk and the brane. In particular, we carefully consider the effect of the presence of a scalar field in the bulk on the reheating process. Constraints from various cosmological observations on the flow of energy/matter from the bulk into the brane are also obtained.   

Special Relativistic Clock Comparisons

Time mappings of a stationary clock's time points onto a moving clock's time line heuristically resolve certain temporal asymmetries in time dilation. Time mapping postulates are identified and transforms are derived. `Clock Re-phasing' vs. `Time Leap' is discussed.   

Monte Carlo simulation of degenerate semiconductors

A modified algorithm is proposed to include Pauli exclusion principle in Monte-Carlo simulations. This algorithm has significant advantages to implement in terms of simplicity, speed and memory storage. We show that even in moderately high applied fields, one can estimate electronic distribution with a shifted Fermi sphere without introducing significant errors. At high fields, the quasi Fermi level is valley dependent and the free-flights must be coupled to state availability constraints. With this algorithm we are now able to simulate inhomogenous devices accurately, which cannot simulate using traditional methods.   

A rigorous approach to nonequilibrium Monte Carlo simulations

Methods exist for mapping Monte Carlo time to real time in kinetic simulations, but these are not rigorously self-consistent because they are based on Boltzmann transition probabilities, which do not apply out of equilibrium. I present a rigorous method in which both transition probabilities and the time scale are self-consistently specified for any energy-based kinetic Monte Carlo simulation. I propose a method in which transition probabilities are still derived from an energy function, but not based on Boltzmann statistics. Although appropriate for nonequilibrium systems, this method recaptures detailed balance and Boltzmann statistics in the equilibrium limit.   

Instability studies of a spherical electrostatic confinement

The spherical inertial electrostatic confinement concept offers an alternative fusion plasma confinement scheme, where charged particles are accelerated and confined electrostatically with a series of biased spherical concentric electrodes. It is very attractive for a power plant due to its mechanical simplicity and high power-to-mass ratio. However, its beam-plasma interactions are not clearly understood. In order to evaluate the concept, a perturbative ($\delta f$) particle simulation technique\footnote{S. E. Parker and W. W. Lee, {\em Phys. Fluids B} {\bf 5}, 77 (1993).} for a kinetic analysis is applied to simulate completely the dynamic evolution of perturbed Vlasov-Poisson equations. This model is used to study the behavior of two-stream-like instabilities related to the trapped spherically converging ions. Results show that steady-state solutions of the self-consistent Vlasov-Poisson equation in which angular momentum of positively charged particle becomes lower correspond to the formation of a deep potential well. Also, it is shown that the growth rates are a decreasing function of angular momentum spread and an increasing function of longitudinal velocity spread.   

Modeling of a Seeded Table-Top Soft X-Ray Laser Amplifier

Key to the development of compact soft x-ray lasers for applications is the understanding of the plasma physics and amplification behavior. For this purpose we have developed a two temperature 1.5D hydrodynamic code with complete atomic model and radiation transport. The propagation of the amplified light is simulated with a 3D ray tracing post processor code. The code was used to model the amplification of high harmonic seed pulses in a dense transient collisional soft x-ray laser plasma amplifier created by heating a solid titanium target. The results of seeded amplification in the 32.6 nm line of Ne-like Ti are compared to experimental results which demonstrated the generation of a very high brightness soft x-ray laser beam with nearly full spatial coherence. Work supported by the NSF ERC for Extreme Ultraviolet Science and Technology under NSF Award EEC-0310717.   

Development and testing of cut-cell boundaries for electromagnetic particle-in-cell codes.

The finite difference time domain (FDTD) approach for electromagnetic particle-in-cell (EM-PIC) is a proven method for many problems involving interactions of charged particles with electromagnetic fields. However accurately modeling fields and particle process at complex boundaries with such methods is still an active research topic. A variety of methods have been developed for this purpose but the testing and application of these methods to real world problems in fairly limited. We have recently implemented the Dey-Mittra boundary algorithm into our EM-PIC code VORPAL. Convergence tests comparing how the frequency of cavity oscillations converge to the physical values for simulations run with stair-step and Dey-Mittra algorithms will be presented. These tests demonstrate how the Dey-Mittra algorithm provides considerable improvements over stair step boundaries. A method to correct for the image charge accumulation from removing particles at complex surfaces will also be presented. Applications to superconducting RF cavities and high-powered microwave devices will be presented.