Session Q1: Poster Session III

Effect of Hydrogen Bonding End Groups on the Bulk Diffusion of Polymers

Multiple hydrogen bonding (MHB) groups are being incorporated into an increasingly wide variety of polymer architectures, from short difunctional oligomers to high molecular weight multifunctional random copolymers. The addition of even weakly binding groups can have a drastic effect on rheological properties; generally this is due to dimerization of complementary groups, but these can also interact with the surrounding polymer matrix. To better understand this process we have synthesized linear polymers with a single hydrogen bonding end group and compared their bulk diffusion behavior to non-functional linear polymers. Deuterated poly(benzyl methacrylate) was synthesized via RAFT polymerization; the dithioester chain end was displaced by heating in the presence of an excess of radicals derived from AIBN or a functional azo compound. Thin films ($\sim $100nm) of deuterated polymer were cast onto thick films ($>$500nm) of protonated polymer and these bilayers annealed at various temperatures. The depth profiles of the bilayers were measured by dSIMS and fit to the solution of the diffusion equation to determine diffusion coefficients and the effect of a single hydrogen bonding end group on motion through the polymer matrix.   

Thermal Conductivity of Carbon Nanotube/Liquid Nanofluid

Nanofluids have been shown to be a potential heat-transferring medium due to nanofillers’ superior thermal conductivity and larger surface areas. Multi-wall carbon nanotubes (MWNT) have quickly attracted much attention in this nanofluid field because of their unique thermal conductivity. Our studies on MWNT nanofluid show that there is a modest improvement in thermal conductivity at a high nanotube loading, $\sim$35\% increase for a 1wt\% MWNT nanofluid. We attribute this increase to the formation of a nanotube network with a higher thermal conductivity. However, at low nanotube loadings, $<$ 0.03wt\%, we observe a decrease in thermal conductivity with the increase of the loading, $\sim$30\% decrease for a 0.02wt\% nanofluid. A thermal void mechanism is proposed to explain this unexpected decrease. Molecular size and polarity of the matrix liquids have been studied to gain more insights on the heat transportation inside nanofluids.   

Experimental and Theoretical Study of Raman Spectra of Polyisobutylene

The effects of strain induced partial crystallization, partial deuteration, molecular weight and temperature on the Raman spectra of polyisobutylene (PIB) are determined. The Raman spectra of stretched PIB are correlated with the development of crystallinity as found by X-ray diffraction. One of the spectral regions altered by the stretching is in the vicinity of 200 cm$^{-1}$ which is often attributed to disordered longitudinal modes. The results are compared with spectra computed from 1$^{st}$ principles electronic structure calculations using Density Functional Theory and Hartree-Fock approaches on model PIB systems.   

Examining the origin of spatial shear-rate variation in Couette flow of entangled polymer solutions

The objective of this work is to determine the time-dependent velocity profile in Couette flow of entangled polymer solutions. In comparison to other flow apparatuses including cone-plate and sliding plate assemblies, the Couette flow can potentially avoid the complications arising from the boundary conditions. Both rheological and particle imaging velocimetric measurements have been carried out on model solutions to compare with results obtained for a cone-plate cell [1] and to shed light on the nature of non-homogeneous shear flow in the stress plateau region. \newline \newline [1] Tapadia, P.; Wang, S. Q. \textit{Phys. Rev. Lett. }, in press (2005).   

Opening the black-box of entangled polymers in flow: A first time-resolved velocity profile determination upon step shear

We have recently revealed that entangled polymer solutions undergo a discontinuous bulk flow transition under controlled shear stress due to a chain entanglement-disentanglement transition (EDT) [1]. The implication of such results is far reaching, the least of which is that the assumed \textit{homogeneous} shear would not occur in the stress plateau region, invalidating the previous efforts to explore the constitutive behavior of entangled polymers. The present work [2] applied a particle-tracking velocimetric method developed in our lab and confirmed the inevitable consequence of the EDT: presence of a spatial variation of the shear rate across the sample thickness in a cone-plate shear cell. The explicit velocity profile evolution sheds light onto such common features as shear overshoot during startup shear. [1] Tapadia, P.; Wang, S. Q. \textit{Phys Rev. Lett}, \textbf{91}, 198301 (2003); Tapadia, P.; Wang, S. Q. \textit{Macromolecules }\textbf{37}, 9083 (2004). [2] Tapadia, P.; Wang, S. Q. \textit{Phys. Rev. Lett. }, in press (2005).   

Influence of Neutralization Methods on the Self-Assembly of Nanoscale Ionic Aggregates in Ionomers.

Recent results show that scanning transmission electron microscopy (STEM) and small angle x-ray scattering (SAXS) have been used to obtain complementary morphological information for solvent cast poly(styrene-\textit{ran}-methacrylic acid) (SMAA) ionomers neutralized with copper. In order to systematically explore the affect of neutralization methods on the nanoscale morphology, materials were isolated by three different methods: solvent casting, solution annealing, and precipitation. Each of these methods allows for a different rate of polymer structure formation. While solvent cast and solution annealed Cu-SMAA films exhibit isotropic ionomer scattering peaks and similar morphologies in STEM, the precipitated powder shows no scattered intensity in the region of the ionomer peak.. These data suggest that isolating the polymer by precipitation induces a metastable morphology in which the ionic aggregates are only partially formed or are not uniformly distributed throughout the material. Upon thermal treatment, the scattering pattern of the precipitated powder begins to resemble that of the solvent cast films. SAXS and STEM experiments are underway to further investigate the nanoscale self-assembly mechanism.   

Cylindrical Polyelectrolyte Brushes

Polyelectrolytes tethered to a cylindrical surface are investigated. The surface can either be formed by e.g. another polymer, a carbon nanotube, or protein chain. The cylindrical polyelectrolyte brush is of interest to us because it forms a model system for the description of the properties of aggrecan, which is one of the main components of cartilage. We used a molecular theory to theoretically investigate the properties of aggrecan. We computed the potential of mean force between two aggrecan molecules. The influence of the cylindrical geometry, the salt concentration, and the pH of the solution upon the conformation of the aggrecans, was investigated. The findings for cylindrical electrolyte brushes were contrasted with the behavior of planar electrolyte brushes, which revealed a large influence of the geometry upon the properties of the brush layer.   

Poly(Ethylene-Methacrylic Acid) Ionomers Neutralized by Solution and Melt Methods

Poly(ethylene-methacrylic acid) has been traditionally neutralized in the melt although other methods, including solution, are also possible. Different neutralization methods may introduce morphological differences in the polyethylene crystallites and ionic aggregates. While the crystallization of the polyethylene chains can be controlled through thermal history, the ionic aggregates form during neutralization and there is no evidence to date of morphological changes after neutralization. In this study, we examine the effect of neutralizing 50{\%} of the acid groups in P(E-\textit{ran}-3.87{\%}-MAA) with a weak zinc base in solution as compared with in the melt. We have found that solution neutralization with slow precipitation as compared with melt neutralization does not affect the size and distribution of the ionic aggregates. STEM indicates spherical aggregates in a liquid-like disorder with diameter of 2.84 nm $\pm $0.42 in the solution neutralized polymer as compared with 2.83 nm $\pm $0.56 in the melt neutralized polymer and X-ray scattering interpreted with the Yarusso-Cooper model has parameters of 0.88 nm in diameter versus 0.89 nm.   

Structure calculations for hydrated ionomer membranes

The structure of the perfluorinated ionomer Nafion is determined by a process of partial phase separation in which the ionic components form clusters within a matrix of hydrophobic material. The dipoles associated with the ionic groups tend to order in such a way as to form head-to-tail sequences. In this study we are investigating the effects of hydration on the structure and formation of the ionic clusters and their associated internal electric fields by a combination of atomistic molecular-dynamics simulations and analytical theory. We find that a useful concept is the vorticity of the electric dipole moments and its dependence on hydration. Various procedures have been used in order to develop realistic pictures of the nature of the ionic clusters and their interconnectedness, as the presence of continuous pathways through a Nafion membrane is a determining factor in the transport of protons in polymer-electrolyte membrane fuel cells.   

Direct observation of enhanced mobility near the surface of polymer nanocomposite thin films

The x-ray photon correlation spectroscopy (XPCS) technique with grazing-incident geometry is currently being used to probe surface dynamics of a planer film on microscopic length scales. Sinha et al.$^{1}$ have shown that the viscosity of a polymer thin film could be derived from the relaxation rate of thermally induced surface roughness. As a further application of XPCS, we studied surface dynamics of polymer nanocomposite thin films. A combination of thiol-functionalized gold nanoparticles (4 nm in diameter, 4wt{\%}) and polystyrene was chosen as a model system. The high x-ray contrast of Au nanoparticles enabled us to monitor their Brownian motion associated with polymer chain dynamics. In addition, making use of the advantage that the x-ray penetration depth can be tuned by varying the incidence angle of the incoming beam, we could measured the Brownian motion at the topmost 10 nm of the film and in the bulk separately. As a result, it was found that the diffusion coefficient for the Brownian motion was 50{\%} greater at the surface than in the bulk. That is, the viscosity of the polymer is much lower at the surface. [1] H. Kim, et al. \textit{Phys. Rev. Lett}., 90, 683020 (2003).   

Viscosity Measurements of Very Thin Polymer Films

We reported that the viscosity of ultra thin polymer films can be measured by three independent techniques: Bilayer dewetting measurements where the viscosity is derived from dewetting velocity; X-ray Photon Correlation Spectroscopy (XPCS) where the viscosity is determined from the relaxation rate of thermally induced surface roughness and Dynamic Secondary Ion Mass Spectroscopy (DSIMS) where the viscosity is derived from the measurements of the tracer diffusion coefficient. The results from three different techniques are in good agreement with each other. The scaling relationship, $\eta \sim $Mw PS, yielded from dewetting and XPCS measurement was in excellent agreement with the bulk scaling of 3.4 and the prediction from reptation theory.   

Tethered Ligand-Receptor Binding in Confined Environments

The binding of small proteins with ligands that are attached to polymers tethered to a surface is studied using a molecular theory. The effects of changing the intrinsic binding equilibrium constant, the polymer surface coverage, the polymer molecular weight, and protein size are studied. The results are compared with the case where the ligands are directly attached to the surface without a polymer acting as a spacer. Within biological range of binding constants the protein adsorption is enhanced by the presence of the polymer spacers. There is always an optimal surface coverage for which ligand-receptor binding is a maximum. This maximum increases as the binding energy or the polymer molecular weight increases. The presence of the maximum is due to the ability of the polymer bound proteins to form a thick layer by dispersing the ligand in space to optimize binding and minimize lateral repulsions. The binding of proteins is reduced as the size of the protein increases. The orientation of bound proteins can be manipulated by proper choice of the grafted layer conditions. A surface modified with a mixture of long ligand-conjugated polymers in low surface density and short polymers in high surface coverage will present optimal binding properties at the same time as nonspecific adsorption of proteins onto the surface is suppressed. The results of the theoretical calculations can be used in the design of experimental systems, providing the kind of polymer and the surface coverage required to optimize ligand-receptor binding.   

Growth of poly-DL-lysine hydrobromide single crystal on mica(001) surfaces.

We present a method for controlling the initiation and kinetics of polymer crystal growth using atomic force microscope tip coated with poly-DL-lysine hydrobromide (PLH). PLH form triangular prisms on freshly cleaved mica(001) surfaces. These prisms grow in a parallel or anti parallel fashion, which indicates epitaxial growth of the crystal with respect to underling mica lattice. Back reflection Laue on mica (001) surface along with optical microscopy measurements guided us to propose a model which showed that two preferred prism orientations exist along [100] direction of the mica lattice. Grazing incidence oscillation x-ray diffraction measurements revealed that the prisms grown on mica are single crystal with a lattice that has an in-plane orientational epitaxy with the underlying mica lattice. We have observed two growth exponents for the growth of the edge length of the PLH prisms. This reveals that in the early stage of the growth, the prisms grow faster and then the growth rate decreases.   

Electrostatic conformation and hydrodynamic properties of a polyelectrolyte studied using static light scattering and viscosity

Using a combination of light scattering and viscometry we are able to observe the changes in the second and third virial coefficients as well as hydrodynamic properties for a continuous concentration gradient. The concentrations of various salts are observed in order to better describe how the molecule behaves when in the presence of different ionic species ranging from simple salts such as NaI to complex salts such as tetraethylammonium chloride. The ionic strength is varied up to 1M which is near physiological conditions. The goal of this work is to obtain a detailed explanation of how these molecules behave so that we will be able to use these models to describe the functions of these molecules within biological systems such as the kidney.   

Transport along freely suspended actin cortex models in a controlled microfluidic environment

Arrays of microfabricated pillars are constructed to serve as a template for mimicking the actin cortex of cells. The three-dimensional template surface prevents interaction of the actin filaments hanging between pillars. A special flow-cell design enables applying flow around a network of actin freely suspended between polydimethylsiloxane pillars. This opens new possibilities to study the mechanics of two-dimensional actin networks as a function of actin-crosslinkers, to observe the active diffusion of molecular motors operating on pending networks and to investigate the alternations in the transport of microscopic particles, coated by different proteins and molecular motors, along these actin cortex models under the drag of flow. The stiffness of the F-actin can be tuned by bundling through various cross-linkers. Additionally, actin filaments act as tracks for guiding passive and active transport of cargo such as organelles or microspheres by molecular motors like myosin-V. These transport problems are biomimetic studies of tracks and external driving force on a statistical process of two-dimensional networks isolated from the complicated and undetermined cellular environment.   

Time-resolved electric force microscopy of charge traps in polycrystalline pentacene films

The microscopic mechanisms by which charges trap in organic electronic materials are poorly understood. Muller and Marohn recently showed that electric force microscopy (EFM) can be used to image trapped charge in working pentacene thin-film transistors [E. M. Muller \emph{et al}, \emph{Adv. Mater.} {\bf 17} 1410 (2005)]. We have extended their work by imaging trapped charge in pentacene films with much larger grains. In contrast to the previous study in which charge was found to trap inhomogeneously throughout the transistor gap, we find microscopic evidence for a new trapping mechanism in which charges trap predominantly at the pentacene/metal interface in large-grained devices. We have also made localized measurements of the trap growth over time by performing pulsed-gate EFM experiments. Integrated-rate kinetics data supports a charge trap mechanism which is second order in holes, e.g., holes trap in pairs, although the charge-trapping rate appears to depend on gate voltage.   

Shear flow induced unfolding of collapsed polymers.

In the process of clotting in small vessels, platelets form a plug in an injured zone only in the presence of a protein known as the von Willebrand Factor (vWF). The absence or malfunction of the vWF leads to a bleeding disorder, the so-called von Willebrand disease. It is believed that the protein is collapsed (or globular) when released into the blood flow, and that it undergoes a transition at high shear rates that allows it to bind platelets. Using hydrodynamic simulations of a simple model of the vWF in shear flow, we show that a globular polymer undergoes a globule-stretch transition at a critical shear rate. Below this threshold shear rate the polymer remains collapsed and slightly deformed, while above it the chain displays strong elongations in the direction of the flow. Finally, we discuss the relevance of our results in the case of blood flow, and compare them to the physiological values present in the body.   

Dynamics of polar guest molecules contained in cryptophane molecular crystals

Cryptophanes are a class of molecules with an interior cavity which can be occupied by smaller guest molecules non-covalently. In cases where the guest molecules are free to tumble within the cryptophane, and where the cryptophanes can be crystalized, it is possible to create ordered arrays of reorienting guest molecules. By using dipolar guests such as the methyl halides, the guest motions can be observed by dielectric spectroscopy and guests may order via strain or dipole-dipole interactions. We will present the results of dielectric spectroscopy experiments on single crystals of iodomethane in cryptophane-A. The observed rotational barriers are compared to computational molecular mechanics calculations based on the X-ray structure of the complex.   

Tethered Polymer Interactions with Attractive Surface Potentials

Due to their surface modification capabilities, tethered polymer chains have been a research focus for several years. Experimental and theoretical work has been done to understand the conformations and interactions of these systems at varying tethering densities. A new technique developed in our group makes it possible to use single crystals of crystalline-amorphous diblock copolymers to study tethered polymer chains. These systems have controlled density through defined fold numbers and controlled MW through living polymerization. Zheng and coworkers found that the reduced tethering density is 3.7 for the onset of interchain interaction and 14.3 for the highly-stretched brush. This work provides, for the first time, the values of these two onsets with high certainty; however, the systems studied do not provide general results. Both systems had repulsive interactions between the surface and the tethered chains. In order to broaden the scope of the physics behind tethered polymer chains, the study should include systems with attractive potentials. Preliminary studies of one such system, PMMA-b-PLLA, will be presented.   

Single crystal engineering of block copolymers

In the past two decades, tethered polymer systems have attracted attention due to, not only their theoretical interests, but also their applicable potentials. Usually, physical adsorption, ``grafting to'' and ``grafting from'' methods have been used to fabricate polymer brushes on solid substrates. Recently, we have proposed a novel method which can achieve narrow molecular weight distribution, and precisely control the tethering density by using amorphous-crystalline block copolymers. With a constant molecular weight of the crystalline block and crystallization temperature, the thickness of single crystals and thus, the number of folds are fixed. This leads to a constant tethering density. We can thus adjust the tethering density by controlling the thickness of the single crystals by changing undercooling and molecular weights of the crystalline blocks. Systems of triblock copolymers with one crystalline block at one end or in the middle will generate diblock copolymer brushes or mixed and/or unmixed polymer brushes. The morphologies of polymer brushes can be controlled by the crystallization condition of single crystal. Mutually, the tethered polymer brushes could also affect crystallization of the crystalline block.   

Overall Crystallization Kinetics of Polymorphic Poly (propylenes)

The isothermal crystallization of propylene ethylene random copolymers evolves with a simultaneous formation of two polymorphic forms, monoclinic crystals (alpha form) and orthorhombic crystals (gamma form). The relative content of each modification changes during crystallization. The content of gamma crystals developed at high levels of transformation increases with the concentration of ethylene and with the crystallization temperature. The overall crystallization kinetics of copolymers with an ethylene content ranging from 0.8 to 7.5 mol {\%} were followed by DSC and analyzed according to classical Avrami kinetics. For most copolymers, fits with single stage nucleation and growth models were poor. Following structural models for lamellar growth that account for epitaxial gamma branching from alpha surfaces, the experimental data were modeled with parallel two-stage crystallization kinetics with excellent fits up to 80 {\%} transformations. The Avrami exponents obtained from the fits are consistent with a 3-D spherulitic growth with instantaneous nucleation for alpha and homogeneous nucleation (linear with time) for the gamma phase.   

Melting in Copolymer Blends

It is observed that the melting temperature T$_{m}$ of polyethylene is insensitive to blending with a melt-miscible random ethylene copolymer. On the other hand, the T$_{m}$ of an ethylene-rich copolymer is depressed strongly when blended with a less ethylene-rich copolymer. If A is a homopolymer that does not cocrystallize with B, the standard thermodynamic prediction for T$_{m}$ employs the small entropy of mixing of entire chains. The situation is quite different if A and B are random copolymers with different amounts of non-crystallizable comonomers. Copolymer melting is based on the mixing of crystallizable and non-crystallizable monomers during fusion, calculated for sequences, not for entire chains. In essence, T$_{m}$ for the copolymer blend depends on the total concentration of crystallizable monomers from both blend components, regardless of the chain to which a particular sequence belongs. This concept adequately accounts for the observations above.   

Ultra-Small Angle Neutron Scattering Study of Polyethylene Crystallization from Solution

Crystallization of polyethylene in deuterated toluene solution has been investigated using time-resolved ultra small angle neutron scattering for the first time. Both slit-smeared and de-smeared scattering spectra were analyzed using Guinier’s law to obtain time-dependent structural information. The average lateral dimension of crystallites grows linearly with time, while the degree of crystallinity increases as square of time. The results suggest that the number density of nuclei remains constant during the crystal growth.   

Holographic Patterning and Crystallization of a Semicrystalline polymer

Manufacturing dynamically controlled multifunctional photonic structures for application in optical elements and waveguides is crucial to control the flow of light. Holographic Patterning (HP) is a simple, fast and attractive means to fabricate complex photonic structures. During the HP process, a photopolymerizable syrup is exposed to two or more coherent laser beams. The resulting anisotropic photopolymerization leads to the spatial distribution of a crosslinked network polymer and homopolymer poly ethylene glycol (PEG). The HP of low molecular weight PEG and thiol-ene reactive monomers produced a long range uniform layered structure. The difference in refractive indexes of PEG and crosslinked polymer network results in unique transmission spectra and diffraction efficiencies. Furthermore, upon heating the holographically patterned PEG produces a red shift in the reflected wavelength of the material which reverses upon cooling. Closer analysis through X-ray scattering has found that PEG crystals contained within the confined layers preferentially crystallize.   

Comparative Determinations of Orientation in Injection-Molded Thermotropic Liquid Crystalline Copolyester (TLCP) Plaques

Two-dimensional wide-angle X-ray scattering (2D-WAXS) in transmission and C K edge near edge X-ray adsorption fine structure (NEXAFS) spectroscopy were used to characterize anisotropy and catalog orientation states in injection-molded plaques fabricated under varying conditions using two commercial TLCP copolyesters. The observed 2D-WAXS patterns are usually bimodal in character due to contributions from the “core” and “skin” regions that result from extensional and shear flow, respectively, in the mold. Deconvolution of the 2D- WAXS patterns permits one to gain a measure of the Herman’s orientation parameter of the core and skin for finite thicknesses of the plaques. The NEXAFS in partial electron yield mode is sensitive to the orientation of the molecular pi orbital of backbone phenyl groups of the top 3 nm of a surface. Updated analyses of the NEXAFS results indicate much greater localized molecular orientation at the very surface of the samples than for the more general orientations for the core and skin derived from the WAXS. The NEXAFS results are generally consistent with those obtained by using surface specific attenuated total reflectance Fourier transform infra-red spectroscopy.   

Coupling of Lithium-Polymer Complexes and Electric Field: Routes to Enhance the Alignment of Block Copolymer Thin Films

Recently, the unexpected experimental results indicated that lithium ionic impurities in block copolymer thin films might assist applied electric filed to overcome the interfacial interactions so that the alignment of microdomains was enhanced.1 But some questions are still open: whether lithium ions aid in overcoming interfacial interactions, what is the nature of the interactions of the lithium ions with the polymer chains, and what is the driving forces in this process? We designed a method to introduce lithium chloride into polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) copolymer thin films. The IR results show that lithium-PMMA complexes in copolymers were formed. These lithium-PMMA complexes markedly enhanced the dielectric constant of PMMA block. Thus the critical electric field strength for aligning the microdomains is significantly decreased so that the external electric field can overcome the interfacial interactions and increase the alignment of microdomains. The addition of ionic salts into one block of a diblock copolymer opens up a potential route to fabricate the long-range ordered nanostructures of block copolymer thin films. [1] Xu T.; Goldbach J. T.; Leiston-Belanger J.; Russell T. P. Colloid Polym. Sci., 2004, 282, 927.   

Structure of Rod-Coil Block Copolymer Thin Films

The self-assembly of rod-coil block copolymers confined to thin films demonstrates new and unusual surface effects. Since the rod block does not experience chain stretching and instead undergoes liquid crystalline interactions, the surface induced order of the block copolymer system is drastically modified. Thin films of a model rod-coil block copolymer, poly(alkoxyphenylene vinylene-b-isoprene), form islands or holes with lamellae oriented primarily parallel to the substrate. These parallel lamellae form grains bounded by defect regions consisting of lamellae oriented out of the plane of the film. These defects appear to have long-range interactions with the surrounding grains resulting in regular, angular grain boundaries. As film thickness is increased toward 10 lamellar spacings, the surface induced order dies off and the lamellae at the vacuum interface are entirely oriented perpendicular to the substrate. We will discuss the kinetics of grain growth and the penetration depth of surface induced ordering as a function of thermal history.   

Renormalized One-Loop Theory of Fluctuations in Homopolymer Blends and Diblock Copolymer Melts

We construct a perturbation theory for the effects of composition fluctuations in polymer mixtures, which we apply to both binary homopolymer blends and diblock copolymer melts. The inverse structure function is divided into an inverse intramolecular correlation function and a direct correlation function, which are calculated separately to first order in a loop expansion. We show that corrections to mean-field theory that are sensitive to local fluid structure, as reflected by a dependence on the value of a microscopic cutoff length, can be absorbed into $q$-independent changes in the value of the direct correlation function (which is closely related to the Flory-Huggins $\chi$ parameter) and changes in statistical segment lengths. Predictions are presented for the changes in radii of gyration of chains in a binary solution near the critical point (which are extremely small) and for the behavior of the stucture factor in a block copolymer melt near the order-disorder transition.   

Investigation of the Phase Behavior of Amphiphilic Triblock Copolymers (PAA-\textit{b}-PMA-\textit{b}-PS in Mixed Solvents

Unique morphologies have been prepared from amphiphilic triblock copolymers of poly(acrylic acid)-b-poly(methyl acrylate)-b-polystyrene in water/tetrahydrofuran (THF) solvent mixtures. The length of the acrylic acid block and the methyl acrylate block were held constant for each copolymer while the polystyrene block length was varied. For self- assembly, the block copolymers were dissolved in THF in the presence of a divalent, organic counterion and water was added slowly. This resulted in a variety of unique structures including polymer nanoparticles with internal block phase separation, bulk phase separation, spherical micelles, cylindrical micelles, disks, as well as toroidal (ring-like) assemblies. The specific structure formed was dependent on the architecture of the triblock copolymer, the amount of counterion present, and the water to THF volume ratio. The focus of this work is the basic understanding and characterization of the phase separated structures present in low water content solutions. The understanding of this bulk phase behavior and its effects on the assemblies formed at higher water contents will be presented. Transmission electron microscopy (TEM), cryo-TEM and neutron scattering were used to examine the copolymer solutions.   

Overall static conformation of chain molecules in nanoscopic cylinders

We explore the overall static conformation of chain molecules in nanopores. We used anodized aluminum oxide membrane with hexagonally packed, regular-sized nanopores with the pore diameter of 15 nm. Via strong capillary action, we have successfully filled different size polystyrene (molecular weight range 20,000 to 3,000,000) in these membranes. Polymers, whose size in the bulk is much larger than the diameter of the nanopores, should deform due to the confinement and the spatial configuration of the chain must deviate from that seen in the bulk. By examining the single chain scattering of the polymer confined within the nanopores via small angle neutron scattering, we examine whether the chain retains its random conformation or elongate along the nanopore.   

Effects of shear flow on reactive coupling of polymer chains at melt interfaces

The coupling reaction of functional polymers at static polymer-polymer interfaces is typically much slower than that at interfaces formed during mixing (see, for example, Reference 1). We have demonstrated that the imposed simple shear can accelerate coupling reactions at flat interfaces. For amine-terminal polystyrene (PS-NH$_{2})$/anhydride terminal poly(methyl methacrylate) (PMMA-anh), the PMMA-anh conversion under dynamic oscillation even at strain amplitude as small as 1{\%} is seven times that under static conditions. Reaction time and temperature and the total interfacial area were maintained the same. Similar behavior was found for both bilayer and multilayer samples. Under steady simple shear, the reaction conversion and the rate of interfacial area generation are comparable to that of batch mixing. \begin{enumerate} \item Macosko, C. W.; Jeon. H. K.; Hoye, T. R. \textit{Prog. Polym. Sci.} \textbf{2005}, 30, 939. \end{enumerate}   

Cation Effects on Electroactive Responses of Conjugated Polymers

The ions present in the electrolyte in which a conjugated polymer actuator is cycled are known to affect performance such as response time and volume changes. Understanding how speed, force, strain, etc. are affected by ion size and other ion characteristics is critical to applications, but is not yet well understood. In this paper, we first present effects of monovalent and divalents cation mass on transport velocity and volume change in polypyrrole doped with dodecylbenzenesulfonate, PPy(DBS), which is a cation-transporting material. Volume change, measured by mechanical profilometry, was greatest for Li$^{+}$ and decreased in order of atomic mass: Na$^{+}$, K$^{+}$, Rb, and Cs$^{+}$. This was expected because prior studies had shown that the ions are hydrated when they enter the PPy, and Li$^{+}$ has the largest hydration shell. Ion transport, measured by phase front propagation experiments, was also fastest for Li$^{+}$, however, contradicting the expectation that larger species would move more slowly. Possible explanations based on hydration shapes and ion-chain interactions will be discussed. In this paper, we also present effects of ion mixtures: Li$^{+}$/K$^{+}$ and Li$^{+}$/Mg$^{2+}$. The results show that small amount of slow moving ions increases the response time drastically. The experiments also show that the Mg$^{2+}$ ions are trapped in the polymer films and influence electroactivity of conjugated polymers.   

Modeling self-oscillation and waves in a reactive polymer gel

We model the self-oscillation and wave propagation phenomena in a swollen polymer gel that participates in the Belousov-Zhabotinsky (BZ) reaction. The BZ reaction causes variations in the gel volume through reduction-oxidation changes of a metal catalyst, which is covalently bonded to the polymer chains. We employ the Oregonator model to describe the BZ reaction kinetics. The Flory and Flory-Huggins models are used to describe the gel elasticity, and the polymer-solvent interactions, respectively. We identify the model parameters that yield the oscillatory behavior, and demonstrate that the kinetics of the BZ reaction can be significantly affected by coupling the reaction to the polymer gel dynamics. To simulate the traveling waves of swelling-deswelling, we utilize the gel lattice spring model, which we have developed recently, and is equivalent to the two-fluid model. We demonstrate and discuss the effects of mechanical constraints on the generation and propagation of 1D and 2D swelling-deswelling waves in the reactive gel.   

Measuring the Local Modulus of Soft Polymer Networks

Biological tissues often rely upon local ``heterogeneities'' to define their structure --property relationship. An example is the integrated layered structure of the mitral valve. For most native tissues, these ``heterogeneities'' are attributed to the local arrangement and structure of the collagen fibril network. To guide the development of tissue scaffolds, we characterize and understand these structure-property relationships on local length scales. In our research we have developed a method to determine the local modulus at specific points within a material. The method involves inducing cavitation and monitoring the pressure of the cavity instability. This pressure is directly related to the local modulus of the material. Initial results focus on the network development of poly vinyl alcohol hydrogels. We monitor the process of gelation and the mechanical response of these hydrogels on length scales similar to their pore structure.   

Monte Carlo Simulation of Reversibly Associating Networks

We applied Monte Carlo simulations to study the reversible network formation through oligomers end-functionalized by ligands capable of complexation in 3:1 ratio with metal ions acting as crosslinkers. By varying the oligomer concentration and metal-to-oligomer ratio, we studied the fractions of different associating species and the molecular weight distribution. The conditions for network formation (over the percolation threshold) were determined using three different criteria, which lead to similar results. The molecular weights of species in the corresponding sol and gel phases were obtained along with the fractions of small rings, dangling ends, and network mesh sizes. An analytical model based on the equilibrium among different associating species and classical gelation theory modified to account for unequal reactivity/cooperativity was developed. The predictions of the analytical model are in good agreement with the simulation results, which compare favorably with experimental observations.   

Formation of nanoparticles during melt mixing a thermotropic liquid crystalline polyester and sulfonated polystyrene ionomers

The formation of nanoparticles and the mechanism of their formation in a blend of a thermotropic liquid crystalline polyester (LCP) and the zinc salt of a lightly sulfonated polystyrene ionomer (Zn-SPS) were investigated using Fourier transform infrared, thermogravimetric analysis, and gas chromatograph-mass spectroscopy. Transmission electron microscopy and wide-angle X-ray scattering were used to study the morphology of the blends and structure of nanoparticles. The origin of nanoparticle formation appeared to be related to the development of phenyl acetate chain ends on the LCP that arose due to a chemical reaction between the LCP and residual catalytic amounts of zinc-acetate and/or acetic acid that were present from the neutralization step in the preparation of the ionomer. The origin of formation and kinetics of the nano-particle formation and the mechanical and rheological properties of these nanocomposites are briefly discussed.   

Characterization of Release Mechanism in Polymeric Drug Delivery Systems

Our polymeric drug delivery system is based on our understanding of phase behavior of polymers [e.g poly(lactic acid)], low molecular drugs and various solvents used in processing. Clearly the different morphologies achieved, based on different phase separation kinetics, can affect drug release rates. Release of drugs, in most cases, involves the exchange between the extraction media and drug. We have characterized the transport behavior using a number of unique techniques. Reflectance infrared spectroscopy has given us a detailed description of the release rate of drugs into the extraction media. Surface plasmon resonance has shown the overall mass loss. UV-visible spectroscopy has yielded the concentration of drug in the solution. These measurements are compared to the release mechanism based on Fickian diffusion. The two step release rates observed can only be explained by taking into account differences in the morphological features of the phase separated films.   

Transport behavior of small molecules out of various poly (vinylidene fluoride) copolymer morphologies

Due to its biocompatibility, durability and existence of various crystalline states, poly (vinylidene fluoride) (PVdF) have been used in medical applications. In this study, the hydrophobic poly (VdF-hexafluoropropylene) polymeric matrix was used to develop a durable matrix system and to control the transport of small molecules by diffusion. Films of this family of copolymers have been prepared from various solutions and with different thermal history. Various morphological features in terms of degree of crystallinity, crystalline phase and segmental orientation can be obtained. These morphological features at different length scales have been characterized by AFM, X-ray and confocal Raman microscopy. The changes in morphology and microstructures are especially interesting when the films of this family of copolymers are prepared using the electrospray technique. The dispersion of small molecular weight component in the various films has been evaluated. Their transport behavior has also been characterized.   

An Analysis of the Solidification Process in Immiscible and Crystallizable Polymer Blends.

Despite the tremendous number of studies in this area, the solidification process of binary and ternary blends is not clearly understood. Here we report the solidification process of immiscible and crystallizable blends, composed of crystallizable polyester and noncrystallizable polyether, in terms of crystallization kinetics, morphological features, and interaction between domains and matrix. All blends exhibited phase-separated morphology with polyester-rich dispersed domains and polyester-poor continuous matrix. The local composition, morphological features, crystallization and solidification process were characterized using $^{1}$H-NMR, optical microscopy, DSC, and small-amplitude oscillatory viscometry, respectively. The local composition and crystallinity in polyester-rich and --poor phases were varied, depending on blend composition and/or crystallization temperature. The solidification process of blends with low polyester content of 10-30 wt{\%} was dominated by crystallization of the polyester-poor phase. The ultimate viscosity of the overall blend was also lower than that of the polyester-poor phase. The high ultimate viscosity in the polyester-poor phase was associated with the percolated morphology of `curved leaf'-shape crystallites.   

The Role of Percolation in the Rheological Behavior of Binary and Ternary Blends.

The rheological behavior of binary and ternary blends involving polyether [e.g. poly(propylene glycol) (PPG)], crystallizable aliphatic polyester [e.g. poly(hexamethylene adipate) (PHMA)] and acrylic copolymers has been studied. The phase behavior of these polymer mixtures at different temperatures is extremely fascinating depending on the nature of the polymers used. The morphological features formed during cooling from elevated temperatures involve phase separation and polyester crystallization. The existence of a percolating solid was established based on evidence from rheological (oscillatory shear) and morphological studies. Typically the blends form a phase-separated droplet-matrix morphology. The transition from the liquid to the solid state has been explicitly correlated to the crystallization behavior of the polyester poor phase. A step increase in the elastic (storage) modulus is observed at a precisely determined PHMA content. The high elasticity can be explicitly explained in terms of the percolation model. The contributions of the domains, interfacial layer and the matrix to the percolation ability of the blend are also discussed.   

Production of Exfoliated Polyethylene-Organoclay Nanocomposites and Its Effect on Mechanical Properties and Thermal Stability.

Low density polyethylene (LDPE)-organoclay nanocomposites as well as linear low density polyethylene (LLDPE)-organoclay nanocomposites are prepared using solid-state shear pulverization (SSSP). The organoclay content in each nanocomposite is 5 wt{\%}. X-ray diffraction indicates that the nanocomposites are exfoliated, because the basal beak at 3.6 degrees for organoclay is nearly eliminated in the nanocomposites made by SSSP. Transmission electron microscopy (TEM) provides further evidence that the nanocomposites are significantly exfoliated. Upon 2 hour annealing at temperatures 25 K above the melt transition of the polymer, x-ray diffraction patterns remain unchanged, indicating that the exfoliated structure of the nanocomposites is kinetically stable in the melt state of the polymer. Due to the stiffness of the clay sheets, the Young's modulus is increase $\sim $ 20{\%} by nanocomposite formation. Thermogravimetric analysis reveals that the onset temperature of degradation is increased by $\sim $ 5 K in the nanocomposites and that the initial degradation of the nanocomposites is retarded relative to neat polymer up to 5{\%} loss of sample weight.   

Oxygen Permeation as a Quantitative Means of Ranking Exfoliation in Polymer-Clay Nanocomposites.

A series of polymer-clay nanocomposites have been made by solid-state shear pulverization (SSSP), which yields good exfoliation of clay based on x-ray diffraction and transmission electron microscopy results, and by melt mixing, which yields relatively little exfoliation of clay based on similar characterization. Here we demonstrate the utility of oxygen permeation as a quantitative tool for ranking exfoliation levels in polymer-clay nanocomposites; the basis for this approach is related to the fact that greater exfoliation of clay will lead to a more tortuous diffusion path for oxygen in the polymer matrix and thereby a reduction in permeability coefficient. A greater than factor of 3 reduction in permeability coefficient has been achieved in polymer clay nanocomposites processed by SSSP. A comparison of permeation results will be made to other characterization and properties that are known to reflect exfoliation.   

Origins of Linear Viscoelastic Behavior of Polymer-Nanoparticle Composites

We use computer simulations to study the mechanisms governing the linear viscoelasticity behavior of composites of spherical nanofillers dispersed in polymer melt matrices. Our results suggest that particles can influence the viscoelastic properties of the system by a variety of different mechanisms. On the one hand, the particle-induced effects on the dynamics of polymer segments modify the relaxation spectrum of the polymers. Secondly, particle jamming effects lead to slow relaxations and substantial enhancements in elasticity. Finally, our results suggest that the strain field distortion caused by the presence of rigid inclusions also affects the overall modulus of the composite. For our model system, we delineate the regimes and frequencies at which the different effects manifest and also suggest how the picture can be generalized for parametric conditions different from our simulations.   

Effect of Molecular Weight on Load Transfer in Nanotube / Polymer Composites

The tensile moduli of nanocomposite fibers are being investigated with attention to the molecular weight of the polymer. Nanocomposites composed of single wall carbon nanotube (SWNT) and poly(methyl methacrylate) (PMMA) were prepared by our coagulation method and processed into composite fibers using melt fiber spinning. SWNT in the fibers are aligned and the nanotube - nanotube interactions are diminished, so that, the mechanical load on SWNT is mainly from polymer - SWNT interactions. The tensile moduli along the direction parallel to the SWNT were characterized at 1.0 mm / sec with the fiber length of 25.4 mm. At a weight-average molecular weight (Mw) 25 kDa, the tensile moduli of PMMA are the same with the composites. However, when the Mw is increased to 100kDa, the tensile moduli are improved greatly by adding SWNT. This indicates that the load in the composites is transferred to the SWNT more efficiently at 100 kDa molecular weight. A micromechanics model was used to relate the elastic shear stress on the polymer - SWNT interface to the polymer chain length. It showed that with increasing polymer chain length, the interfacial shear stress was enhanced. This study demonstrates the importance of the molecular weight of the polymer matrix to the load transfer in nanocomposites.   

Templated Nanocarbon Black Nanocomposite Electrodes for Rechargeable Lithium Batteries

In this work, the fabrication of high energy density electrode materials for solid-state rechargeable batteries via block copolymer templating schemes was investigated. Atom transfer radical polymerization was used to synthesize the copolymer template poly((oligooxyethylene) methacrylate)-\textit{block}-poly(butyl methacrylate), POEM-b-PBMA. Continuous, nanoscale phases of vanadium oxide were subsequently grown within the POEM domains of the microphase-separating block copolymer using sol-gel synthesis from a vanadium alkyoxide precursor. The in situ growth of cathodic components in ion-conducting POEM domains allows for control of morphology and increases the interface-to-volume ratio, thereby escalating the specific electrode area over which faradaic reactions can occur and decreasing ion diffusion distances within the electrode. Films incorporating up to 34 wt{\%} V$_{2}$O$_{5}$ were flexible and semi-transparent. To achieve necessary electronic conductivity, the incorporation of nanocarbon black was investigated. Hydrophilic surface modification of carbon black nanoparticles provided a mechanism for their selective incorporation into POEM domains. Transmission electron microscopy (TEM) and small angle x-ray scattering (SAXS) were performed to probe the morphology of the nanocomposite electrodes.   

Creep and Recovery Behaviors of a Polythiophene-based Electrorheological Fluid

We investigate the creep response of poly(3-thiopheneacetic acid) (PTAA) particles doped with perchloric acid. With increase of applied stress, these suspensions exhibit an evolution from a linear viscoelastic response, with three components of instantaneous elastic strain, retarded elastic strain and viscous strain, to a nonlinear viscoelastic response, where the retarded elastic and viscous strains monotonically decrease and a plastic contribution to the instantaneous strain grows, followed by a viscoplastic solid behavior, with fully plastic instantaneous strain, and finally a transition from plastic solid to a plastic liquid at the yield stress. With increase of electric field strength at fixed particle concentration and applied stress, the viscoplastic response diminishes, and more elastic behavior ensues. For highly-doped samples, at high electric field strengths, a fully elastic solid response is observed in the linear viscoelastic regime. The equilibrium compliance, J$_{C}$ and steady state recoverable compliance J$_{R}$, were investigated as a function of electric field strength, particle concentration and particle conductivity. The results are interpreted in terms of the field-induced formation of thick fibrillar aggregates spanning the gap between the electrodes, each consisting of bundles of particle strings.   

Electric Field Generated Stress Moduli in Polythiophene/Polyisoprene Elastomer Blends

The effects of crosslinking ratio and electric field strength on the rheological properties of polyisoprene and polythiophene/polyisoprene (Pth/PI) blendss were investigated as potential electroactive actuator. Electrorheological properties of polyisoprene and blends were measured under the oscillatory shear mode with the applied electric filed strength varying from 0 to 2 kV/mm. The dynamic moduli, G$'$ and G$''$, of the pure polyisoprene depend on the crosslinking ratio and the electric filed strength; the storage modulus (G$'$) increases but the loss modulus (G$''$) decreases with increasing crosslinking ratio. The storage modulus (G$'$) and the loss modulus (G$''$) of the pure polyisoprene fluid exhibit no change with increasing electric field strength. For PI with the crosslinking ratios of 2, 3, 5 and 7 (PI{\_}02, 03, 05 and 07), the storage modulus sensitivity, $\Delta $G'/G'$_{o}$, increases with electric field strength and attains maximum values of 10{\%}, 60{\%}, 25{\%}, and 30{\%} at the electric field strength of 2 kV/mm, respectively. The loss modulus (G$''$) of the PI with the crosslinking ratios of 2 and 3 increases with the electric field, but for the blends of the crosslinking ratios of 5 and 7, it decreases. For the blends of polythiophene with PI at concentrations of 5{\%}, 10{\%} and 20{\%} by vol, G$'$ and G$''$ are generally higher than those of pure polyisoprene.   

Electromechanical response of a silicone elastomer containing PPV

Electrorheological properties of PDMS gel and PPV/PDMS blend were investigated experimentally under an oscillatory shear mode at the temperature of 27$^{\circ}$C to determine the effects of crosslink ratio, electric field strength and doping level. For the pure PDMS gels, the storage modulus, G$'$, increases with increasing crosslinking ratio and electric field at all frequencies between 0.1-100 rad/s. When an electric field is applied, the polymer molecules become polarized resulting in the interaction through the electrostatic force between the polarized PDMS molecules. The PDMS gel system with the crosslinking ratio of 0.01 possesses the highest G sensitivity to electric field. For the PPV/PDMS blends (PPV/PDMS{\_}10), the dynamic moduli, G$'$ and G$''$, are higher than those of pure PDMS in the absence of electric field because PPV particles act as a filler in PDMS matrix. The G$'$ sensitivity of PDMS increases up to 35{\%} at the electric filed strength of 2 kV/mm. Moreover, the doped PPV/PDMS blend (doped PPV (1:10)/PDMS{\_}10) shows the highest G$'$ sensitivity (170{\%}) due to interacting electrostatic forces between electric field induced dipole moments of the conductive molecules.   

Creep and Recovery of Electroactive Polyaniline Suspension

Creep and recovery behaviors of the PANI/silicone oil suspensions were investigated under applied electric field to explore the effects of field strength and particle concentration. The data show that, at any applied shear stress, the creep curves of this ER fluid showed a large instantaneous elastic response, whereas the retarded elastic and the viscous responses were very small and they disappeared as applied stress was increased. After the removal of applied stress, the strain decreased but did not completely relax to its original value indicating that the fluid exhibited a partially elastic recovery. However, it was noted that the recovery after stress removals disappeared when the strain was higher than the critical values $\sim $ 0.4-0.5, independent of particle concentration and field strength. It was also found that creep resistance of EB/Silicone oil suspension were clearly dependent on both field strength and particle concentration.   

Anisotropy of Electroactive Strain in Textured Polypyrrole Actuators

Polypyrrole has been extensively studied as an electroactive material, but these studies have provided little insight to the influence that morphology has on actuation at the nanoscale. By discovering and exploiting the connection between nanoscale transport events and macroscale active strain, we can learn how to process polypyrrole and other conducting polymers for improved electroactive device performance. We show that by controlling polymer chain configuration and packing, a conducting polymer actuator can be engineered that shows a significantly larger macroscopic electroactive response for a given set of driving conditions. We utilize different modes of deformation to impart orientation textures that can be observed via synchrotron x-ray diffraction and electronic and ionic resistance measurements. Certain textures enhance pathways for ion transport between polymer chains, resulting in an anisotropic electroactive strain response that can be harnessed when making polypyrrole-driven devices. This response provides valuable insight to the mechanism of polypyrrole actuation on the nanoscale, supporting a mechanism where counterions migrate to locations between the oriented polymer chains.   

Detecting composition and monomer sequence distribution in random copolymers with interaction chromatography

We demonstrate that interaction chromatography (IC) is capable of discriminating among both the chemical composition and monomer sequence distribution in random copolymers. By fine-tuning the separation conditions in the IC (solvent type and stationary phase type), we were able to delineate the effect of both their chemical composition and the monomer sequence distribution of partially brominated polystyrenes on chromatographic retention. The degree of bromination and the 4-BrS sequencing was controlled by varying the bromine concentration in the reaction vessel, bromination reaction time, and solvent temperature. Our experiments suggest that 1) the blockiness of 4-BrS adsorption segments can further enhance the surface affinity of the copolymer chains at a fixed copolymer chemical composition, and 2) the adsorption-based molecular recognition of copolymer chains occurs by cooperative and synergistic adsorption of segments on surfaces along with neighboring adsorptive segments.   

Interplay between polymer and nanopore sizes for polymer adsorption in nanopores

We investigated the adsorption and desorption of monodisperse polystyrene (Mw: 4K $\sim $ 3M) in cyclohexane into nanoporous silica (Rp, radius of pore: 4, 7, 14, and 24nm; particle diameter: 7 micron) to understand the interplay between polymer coil size and nanopore size for the polymer adsorption in nanopores. Regardless of the pore sizes, two regimes of polymer nanopore adsorption have been identified to universally describe the surface access (adsorbed mass per unit area) in terms of the relative size interplay, Rg/Rp. When polystyrene is much smaller than pore, the surface access increases with the radius gyration, Rg, essentially following the adsorption behavior on the flat surface. When polystyrene is similar or larger than pore, the surface access in nanopore surface is smaller than that on the flat surface. From SEM, we found morphological evidence to support that the steric crowding of polystyrene chains occurs at the nanopore entrance to offer the limited accessibility of polymers into the pore surfaces.   

Recent developments in multivariant surface-tethered polymer assemblies

Surface-tethered polymer brushes prepared via surface-initiated controlled/``living'' radical polymerization represent an effective tool for tuning the physico-chemical properties of surfaces. We have recently developed methods leading to assemblies of surface-tethered polymers with continuously varying grafting density, molecular weight, and composition. In this presentation, we will demonstrate how these assembling concepts can be utilized to: 1) study the kinetics of surface-initiated controlled/``living'' radical polymerization, 2) systematically study the phase behavior of diblock and triblock copolymers upon exposure to selective solvents, 3) study the behavior of block copolymer polyampholyte brushes, and 4) explore the wettability transitions (superhydrophobic/superhydrophilic) using thermoresponsive polymer brushes on topologically corrugated substrates.   

Evaporation and interdiffusion of solvent in polymer films

Solvent evaporation from homopolymer and heteropolymer films along with the interdiffusion of solvent into these films is studied using large scale molecular dynamics and grand canonical Monte Carlo simulations. As the solvent evaporates, a sharp increase in polymer density at the film/vapor interface is observed. The rate of solvent evaporation, for both homopolymer and heteropolymer films is found to decrease exponentially with time. For multiblock films the resulting domain structure is found to be strongly affected by the relative stiffness of the two blocks. In the interdiffusion study, the shape of the solvent concentration profile and the weight gain by the film can be related to the diffusivity. For homopolymer films, although the diffusivity is found to be strongly concentration dependent especially as one approaches the glass transition temperature of the polymer, the weight gain scales as $t^{1/2}$ for all cases studied, which is expected for Fickian diffusion. For a multiblock copolymer film in which the stiffer block is below the glass transition temperature is also studied, even though the solvent swells only the softer block of the copolymer, the weight gain by the film remains Fickian.   

Adsorption of Biodegradable Diblock Copolymers PEO-PLA and PEO-PCL to Colloidal Polystyrene

A series of amphiphilic block copolymers composed of poly (ethylene oxide) and either poly(lactide) or poly($\epsilon$- caprolactone) (PEO-PLA, PEO-PCL respectively) were synthesized and their solution properties studied using dynamic light scattering. The synthesis was performed using the same PEO macroinitiator while varying the fraction of the hydrophobic block. These materials were found to self-assemble in aqueous media with the hydrodynamic radius increasing with increasing hydrophobic fraction. In order to ascertain the potential for use of these materials as degradable coatings in delivery applications, block copolymers were adsorbed to colloidal polystyrene and the thickness of the adsorbed layer determined from changes in the hydrodynamic size. The adsorbed thickness ranged from 4 - 10 nm with varying block ratio, polymer concentration and colloid concentration. In general, the adsorption of PEO-PCL to colloidal polystyrene resulted in larger adsorbed thickness compared with PEO-PLA.   

Optical Properties of Molecules and Molecular Aggregates Adsorbed on Solid Surfaces. First Principle Study.

Equilibrium atomic geometries of Ferrocene and Rhodamine 6G (R6G) dye molecules adsorbed on Si(111) and Ag(111) surfaces respectively are studied using density functional theory. Equilibrium atomic geometries are obtained through total energy minimization method. Electron energy structure and optical properties are calculated using generalized gradient approximation method with \textit{ab initio} pseudopotentials. Modifications of electronic surface structures of Ag(111) and Si(111) caused by the adsorption of molecules as well as charge transfer between molecules and solids are studied. Red or blue shifts of optical absorption spectra of R6G dye molecules after aggregation in $J$- or $H$-dimmers respectively are predicted. Calculated optical absorption spectra are interpreted in terms of inter-molecular and molecular-solid interactions. Results of the calculations are discussed in comparison with experimental data.   

Wetting Morphologies in Triangular Grooves

We studied the wetting behavior of liquids in triangular grooves with chemically homogeneous walls. Droplets form elongated morphologies with negative mean curvature for contact angles, $\theta$, smaller than 90$^{\circ}$ minus half the opening angle of the groove. For larger $\theta$, the liquid either forms elongated filaments of finite length and positive mean curvature or drop-like morphologies. For in situ manipulation of small amounts of liquid on this substrate topography, we used electrowetting which allows varying $\theta$ as a function of the applied Voltage. The filling and drainage behavior of these grooves were studied as a function of time and $\theta$. In contrast to grooves with rectangular cross section, the liquid filaments in triangular grooves undergo a dynamic instability when being quenched from a filling to a non-filling situation. The liquid filament breaks up into isolated droplets with a preferred distance which compares favorably with a straightforward theoretical model.   

Freezing of Polymer Thin Films and Surfaces: The Small Molecular Weight Puzzle

Experimental observations (ellipsometry, scanning force microscopy (AFM), and nuclear magnetic resonance (NMR)) of the freezing behavior of thin supported films as well as the free surface of atactic polystyrene are reported, taken at a particularly small molecular weight of 2 kg/mol. Remarkably, we find the same effect of reduction of the glass transition temperature, Tg, as observed earlier with much longer molecules. Furthermore, surface melting is observed by NMR, with the molten layer thickness similar to what has been observed with larger molecular weight. We conclude that molecular geometry effects cannot account for these observations, and that a consistent explanation must be presentable in a continuum picture. Based on the capillary wave spectrum of the free surface and of the supported films, we present such a model, and find that it accounts very consistently with all observations made so far with polystyrene.   

Film Formation with Reactive Hydrophobic and Polar Groups in Aqueous Solution: A Bond-Fluctuating Computer Simulation Model

We study film formation with reactive hydrophobic ($H$) and polar ($P$) components in evaporating aqueous ($A$) solution by Monte Carlo simulation to model the polyurethane film growth. Each component is represented by mobile particles with appropriate molecular weight, interaction, and reaction functionality on a simple three-dimensional lattice $L_x \times L_y \times L_z$ with an adsorbing substrate. $H$ and $P$ react by forming fluctuating covalent bonds proceeding from the substrate with probability $P_B$. Bonds may also be formed between $H$ and $A$ when $A$ is considered reactive. Growth of the film thickness ($h$) and surface roughness ($W$) are studied at a range of temperature ($T$). With non-reactive $A$, the saturated film thickness ($h_s$) and roughness ($W_s$) decay first before increasing linearly on raising the temperature. With reactive $A$, a fast increase of $h_s$ at low temperature is followed by a slow increase at high temperature. $W_s$ also shows non-monotonic dependence on temperature.   

Enhancement of X-ray Reflectometry for the Depth-Profiling of Polymer Films using Polymeric Substrates

The investigation of the internal structure of thin polymer films on conventional substrates by X-Ray reflectometry is restrained due to the high electron densities of these substrates compared to those of most polymers. A methodology is proposed and applied to circumvent this problem by utilizing polymeric substrates of appropriate thickness and roughness, which can be reproducibly deposited on the standard substrates in order to render them “invisible” to the X-rays. The resolution of the X-ray reflectometry is significantly enhanced as illustrated utilizing thin films of PS-b-PMMA diblock copolymers of various thicknesses. The proposed methodology will allow the study of internal polymer interfaces using in- house diffractometers/reflectometers.   

Switchable Adhesion Properties From Low and High T$_{g}$ Polymeric Brushes

We investigated the adhesive properties of mono-component and bi-component polymeric brushes chemically grafted on silicon wafers from end-functionalized hydrophilic and/or hydrophobic chains by using the technically simple `grafting to' method. The charge state and the molecular organization of high T$_{g}$ brushes could be varied reversibly by exposure to appropriate solvents or by varying the grafting density. Adhesive properties were tested against soft pressure-sensitive-adhesives (hydrophobic {\&} hydrophilic) with a probe method. In the case of low T$_{g}$ brushes, an increase in contact time could selectively improve the adhesion with the soft hydrophobic or hydrophilic adhesive layer. This is due to reorganization of the interface at the molecular scale leading to the formation of entanglements or favorable interactions between the polymer brush and the polymers in the soft adhesive.   

Principles of resonant soft x-ray reflectivity -- a novel tool in polymer research

Resonant soft x-ray reflectivity (RXR) has been reported to be a potentially excellent tool for the study of polymer thin films [1]. To further delineate the applicability of this method, detailed principles about RXR will be discussed. Near the carbon 1-s absorption edge, the complex index of refraction n = 1-$\delta $-i$\beta $ is changing rapidly as a function of photon energy in a manner that strongly depends on the chemical moieties of the polymer. This leads to enhanced contrast between two polymers at specific energies when compared to hard x-rays. This contrast enhancement mimics the contrast enhancement achieved through deuteration used in neutron reflectivity, yet does not require special chemical procedures. PS/PMMA, PBrS/PMMA, PS/SAN, PS/P2VP and PCHMA/PMMA bilayers were characterized by RXR at beamline 6.3.2 at the Advanced Light Source. For a subset of these samples, RXR results were directly compared to hard x-ray reflectivity characterization that uses a fourier analysis method [2], and the results showed good agreement. [1]C. Wang, T. Araki, and H. Ade, Appl. Phys. Lett. 87, 214109 (2005). [2]O. H. Seeck et al., Appl. Phys. Lett. 76, 2713 (2000).   

Calorimetric Tg and Heat Capacity of Polystyrene Thin Films

The glass transition temperature and the absolute heat capacity of polystyrene thin films were measured using the step-scan method of differential scanning calorimetry. The glass transition temperature is found to be depressed 8 K for a sample of stacked 17 nm thick films and Tg is depressed 3 K for a sample of stacked 61 nm thick films. The results are consistent with data in the literature for the Tg depression in supported polystyrene films although our films are expected to be ``freely standing'' for the initial DSC scan. In addition, the absolute heat capacity in both the liquid and glassy states decreases with decreasing film thickness, the step change in heat capacity at the glass transition temperature decreases with decreasing film thickness, and the breadth of the transition region increases with decreasing film thickness. The effect of heating the thin film samples to 135 \r{ }C, approximately 40 \r{ }C above their Tgs, is a slight increase in the absolute heat capacity and a slight increase in Tg. No significant changes occur on subsequent scans. The ``thin film'' morphology is maintained in spite of scanning to above Tg where the stacked film sample is expected to lose its free surface.   

Dissipative particle dynamics simulations of dilute polymer solutions confined in a slit with interactive surfaces

Dynamics of polymer chains in confined geometries are significantly different from that in a bulk solution. Understanding the confinement effect on the chain dynamics is of great value to applications of microfluidic devices. We applied dissipative particle dynamics (DPD) to study polymer chain dynamics confined in slits with interactive surfaces. We first examined the dynamics and the static properties of polymers in dilute bulk solutions. After correcting for the effect of finite box size, our results unambiguously confirmed that the DPD has correctly accounted for hydrodynamic interaction within a polymer chain. We then extended the simulation to polymer solutions confined in a slit with interactive surfaces. For purely repulsive surfaces, the dynamics and the static properties of polymers show a broad crossover from a free solution to a confined solution. The attractive interactions of polymers with the walls were found to retard the dynamics of the chains significantly. The influence of surface interactions on polymer dynamics will be presented.   

Dissipative particle dynamic simulations of solvent flow through nanometer sized smart channel coated with stimuli responsive polymer brush

Various efforts have been focused on the development of smart materials which can respond to environmental stimuli such as pH or temperature. Nanometer sized channels coated with such stimuli responsive polymers can serve as smart gating or smart valve that are of interest for applications such as controlled drug release and tunable permeation and separations of toxic solutes. These systems involve the interplay of solvent flow with the structure of polymer brush in response to the external stimuli. We applied dissipative particle dynamic (DPD) method to investigate solvent flow through such smart channels. The external stimuli are modeled by the change in solvent quality that causes polymers undergoing a coil-to-globular transition. We investigate the ability of such smart channel to control the solvent flow under different stimuli. The interplay of the solvent flow and the properties of polymer brush are studied at the molecular level. The effects of the grafted density and the layer thickness of polymer brush on its controlling ability have also been investigated.   

Exfoliation of stacked sheets: effects of temperature and platelet size by a Monte Carlo simulation

Can a layer of stacked sheets (coarse grained description of clay platelets) exfoliate in a solvent? Computer simulations are performed to address this question with a stacked layer of four sheets. A sheet is modeled by nodes tethered together by fluctuating bonds on a cubic lattice; sheets of sizes $12^2 - 64^2$ are considered. The initial distance between sheets is small and kept constant for different sets of stacked sheets. There is an attractive interaction between sheets, i.e., their nodes and the empty lattice sites, the effective solvent medium. Nodes execute stochastic movement with the Metropolis algorithm subject to bond fluctuation and excluded volume constraints. Simulations are performed at different temperatures. Visual analysis shows that the exfoliation depends on the size of the sheet and temperature. We find that it is easier to exfoliate smaller sheets and that the exfoliation is enhanced by raising the temperature. The exfoliation becomes very slow for large sheets at low temperatures due to longer relaxation times.   

Correlations of System Mobility with Various Scalar Metrics

Molecular dynamic simulations of chain systems were performed in order to investigate the relationships between the system mobility and thermostatic quantities. Systems consisted of pearl-necklace chains along with single site penetrants. Both attractive and repulsive systems (based on the cut-off of the Lennard-Jones potential) were simulated. The diffusion coefficients, D, for the chains and penetrants were then found for a variety of temperatures (T) and density combinations. D/T was found to be a single-valued function of a thermostatic quantity that we denoted as a ``scalar metric.'' Four scalar metrics were found. Since, through the master curve, the mobilities for all temperature-density points can be extrapolated to a single zero, a unique ideal glass transition can be proposed to exist. Consequently, a scalar metric can be used as a ``distance'' measure to this ideal glass transition.   

A Monte Carlo Study of Frozen Lattices on Curved Surfaces

We compare the defect structure and density of particles confined on 2d surfaces with different curvatures as well as different surface morphologies. All simulations are performed using a smart Monte Carlo algorithm while the particle system is cooled from a melted state to zero degree temperature. The surfaces show similar defect patterns. At low maximum Gaussian curvature a hexagonal lattice wraps defect-free onto the curved surface. We then observe a critical maximum curvature at which the creation of defects lowers the potential energy of the lattice with respect to the defect-free lattice. At this critical value we observe free dislocations located at the inflection points of the surface. Further increase in curvature results in the creation of more free dislocations which organize in grain boundaries aligned along the shortest connection between the positive and negative Gaussian curvature areas. A stronger curvature leads to the creation of free disclinations. The $-60^{o}$ disclinations are pinned to the area of positive curvature while the $+60^o$ disclinations occupy the negative areas. The transition from the defect free lattice to a defected lattice as the lowest energy state is compared to an analytic expression derived from continuum mechanics.   

A Molecular Dynamics Study of the Odd-Even Effect in Self-Assembled Monolayers

Classical Molecular Dynamics has been used to examine the friction in model systems consisting of an amorphous hydrocarbon tip sliding across self-assembled monolayers consisting of well-ordered densely-packed pure linear hydrocarbon chains. An odd monolayer composed of C13 chains is compared against an even monolayer composed of C14 chains at a number of loads. Periodic Boundary Conditions imposed in the horizontal plane are used to model infinite monolayers. The friction of the odd monolayer is seen to be roughly twice as large as that of the even monolayer at all loads. The difference is attributable to the differing orientation of terminal chain groups in odd versus even monolayers. Analysis of net forces exerted by the entire set of tip atoms on individual monolayer atoms lends insight into specific properties associated with the observed frictional difference.   

Fabrication of Biopolymer Nanofibers of Hyaluronic Acid via Electrospinning

Electrospinning is a novel technology that uses an electric field to form fibrous materials from a polymer solution. Unlike traditional spinning techniques, electrospinning can produce fibers on the order of 100 nm that can be utilized in applications where nanoscale fibers are necessary for successful implementation, including tissue engineering. Hyaluronic acid (HA) is a widely used biopolymer found in the extracellular matrix and currently marketed in medical applications for joint lubrications and tissue engineering. The high viscosity and surface tension of HA make it an unlikely candidate for electrospinning processes as viscosity is an important parameter in successful electrospinning. To promote HA fiber formation by electrospinning, the effects of salt (NaCl), which is used to reduce the viscosity of aqueous HA solutions; molecular weight of the HA; and an additional biocompatible polymer (e.g., PEO) are under investigation.   

Influence of D-Penicillamine on the Viscosity of Hyaluronic Acid Solutions

Polyelectrolyte hyaluronic acid (HA, hyaluronan) is an important component in synovial fluid. Its presence results in highly viscoelastic solutions with excellent lubricating and shock-absorbing properties. In comparison to healthy synovial fluid, diseased fluid has a reduced viscosity. In osteoarthritis this reduction in viscosity results from a decline in both the molecular weight and concentration of hyaluronic acid HA. Initial results indicate that D-penicillamine affects the rheology of bovine synovial fluid, a model synovial fluid solution, and its components, including HA. In order to understand how D-penicillamine modifies the viscosity of these solutions, the rheological properties of sodium hyaluronate (NaHA) in phosphate-buffered saline (PBS) with D-penicillamine were studied as function of time, D-penicillamine concentration (0 -- 0.01 M), and storage conditions. Penicillamine has a complex, time dependent effect on the viscosity of NaHA solutions---reducing the zero shear rate viscosity of a 3 mg/mL NaHA in PBS by \textit{ca}. 40{\%} after 44 days.   

Electrospinning of Biocompatible Nanofibers

Artificial scaffolds for growing cells can have a wide range of applications including wound coverings, supports in tissue cultures, drug delivery, and organ and tissue transplantation. Tissue engineering is a promising field which may resolve current problems with transplantation, such as rejection by the immune system and scarcity of donors. One approach to tissue engineering utilizes a biodegradable scaffold onto which cells are seeded and cultured, and ideally develop into functional tissue. The scaffold acts as an artificial extracellular matrix (ECM). Because a typical ECM contains collagen fibers with diameters of 50-500 nm, electrostatic spinning (electrospinning) was used to mimic the size and structure of these fibers. Electrospinning is a novel way of spinning a nonwoven web of fibers on the order of 100 nm, much like the web of collagen in an ECM. We are investigating the ability of several biocompatible polymers ($e.g$., chitosan and polyvinyl alcohol) to form defect-free nanofiber webs and are studying the influence of the zero shear rate viscosity, molecular weight, entanglement concentration, relaxation time, and solvent on the resulting fiber size and morphology.   

Structure and interactions of human respiratory mucin

Human respiratory mucin plays a crucial role in the pathology of Cystic Fibrosis lung infections. Mucin is a flexible, linear polyelectrolyte, characterized by its many charged oligo-carbohydrate side chains that give it its bottle-brush structure. The macroscopic properties of a mucin suspension are known to change drastically with changes in ion concentration and solution pH, but little is known about the effect of these variables on individual mucin structure. We present preliminary results on the structural response of individual human respiratory mucin molecules to variations in concentration of ions of different valences via small angle x-ray diffraction.   

Theoretical simulation of compression of single bovine carbonic anhydrase II molecule by AFM tip

Based on all-atom force field model called CHARMM, we calculated the force distance curves of the single bovine carbonic anhydrase II molecule adsorbed on the graphite surface in the UHV condition. The AFM tip is modeled by the graphite sheet of monolayer thickness. The force distance curves show the nonlinear feature which can be classified into three regions, i.e., the attractive, weak repulsive, and strong repulsive force regions. We found that in the second region the surrounding alpha helices are compressed while in the third region the core beta sheets are also compressed. This finding indicates that the heterogeniousity in a single protein molecule can be measured by the AFM. We will also discuss the effect of the temperature on the force distance curves.   

Self Assembly of $\beta $-Hairpin Peptides into Hydrogel Networks: Tuning Supramolecular Properties Through Molecular Design

Monomeric peptides were designed to undergo reversible, intramolecular folding with external stimuli (e.g. pH, temperature, salt) to form $\beta $-hairpins that consequently self assemble into a hydrogel network rich in $\beta $-sheet. The design was composed of a turn sequence (V$^{D}$PPT) flanked by extended strands containing alternating lysine and valine residues. The hydrophobicity of the peptides was altered through replacing valine residues in the arms with residues such as, norvaline, norleucine and isolecine. Circular dichroism spectroscopy illustrated that random-coil to $\beta $-sheet transition could be tuned from 35$^{o}$C to below 5$^{o}$C at pH 9, while the transition pH at T$_{room}$ could be shifted from pH 9 down to pH 7. TEM illustrated that all peptides self-assembled into fibrilar networks. Single fibril dimensions were 3 nm as measured using TEM and small-angle neutron scattering, consistent with the proposed self-assembly mechanism$^{ }$of fibrils with a molecular bilayer cross-section. There was a direct correlation between fibril morphology and consequent changes in the nature of junction points and gel rigidity as observed by TEM, and oscillatory rheology, respectively.   

Flory $\chi $ of the homologous series of deuterated polystyrene-b-poly(n-alkyl methacrylates): small angle neutron scattering and theoretical studies

We have performed small angle neutron scattering and theoretical studies of the homologous series of deuterated polystyrene-b-poly(n-alkyl methacrylates) from methyl to n-hexyl groups that cover from UODT (upper order-disorder transition) to LODT (lower DOT), and barotropic to baroplastic behavior.~ It was shown that the effective Flory $\chi $ from the measurements reveal a monotonic dependence on 1/T for UODT (methyl, n-hexyl) systems and a complicated dependence for LDOT (ethyl to n-pentyl) systems. The recently developed compressible random-phase approximation (RPA) analysis has been applied to the copolymers to interpret in a unified way such diversified temperature and pressure dependence of $\chi $ and the transitions. Specific interactions (SI) and compressibility were incorporated through the RPA interaction fields in this theoretical approach. It was argued that the SI gives a major contribution to the LDOT behavior and the complicated dependence of $\chi $ on 1/T. The compelling changes in $\chi $ and the pressure responses of transitions according to the pendant group size were also interpreted successfully with the theory.   

A new field-theoretic simulation method for compressible systems containing block copolymers

A new field-theoretic simulation method based on a compressible random-phase approximation (RPA) theory has been suggested to understand the self-assembly behavior and its pressure responses of compressible systems containing block copolymers. Finite compressibility is incorporated in the free energy functional for the dissipative dynamics through effective RPA interactions that account for the excluded volume and the attractive nonbonded interactions. It was shown for A-b-B copolymer melts in unconfined or confined geometry that basic equation-of-state parameters completely characterizing given block components readily yield stable and metastable morphologies without any presumed symmetry over a wide range of temperature-pressure-composition space. It was demonstrated that the simulation tool is capable of predicting in a unified way the self-assembly behavior not only exhibiting nanoscale ordering either upon cooling or reversely upon heating, but also revealing barotropicity and baroplasticity. The simulation of the self-assembly behavior of multiblock copolymers with diverse molecular architectures and also of block copolymers in a solvent covering from complex patterns to micellar structures was also discussed.   

Stability of the orthorhombic $Fddd$ phase in diblocks using Landau theory of weak crystallization

Recent numerical SCFT caculations by Tyler and Morse [{\em Phys. Rev. Lett.}, {\bf 94}, 208302, 2005] predict a stable orthorhombic network phase with space group $Fddd$ in weakly segregatd diblocks. In this work, we examine the stability of the $Fddd$ phase using Landau theory. Our analysis and results suggest that $Fddd$ structure with a special unit cell is expected to be a stable phase not only in weakly segregated diblocks but in any other weakly ordered material.   

Depletion interaction effects on diblock copolymer micelles in selective solvents

Block copolymers form micelles when dissolved in a selective solvent for one block. At higher concentrations the micelles pack into ordered microstructures. When non-adsorbing homopolymer is added to the solutions, we observe that the ordered morphologies can be disordered. Due to the addition of homopolymer the repulsive inter-micellar interactions are screened, which leads to the melting of the ordered microstructures. This phenomenon is analogous to depletion interactions in colloid/polymer mixtures. We have chosen the polystyrene-b-polyisoprene dissolved in dialkyl phthalates, as our model system. To these solutions polystyrene homopolymer is added. The molecular weight of the homopolymer has a significant effect on the phase behavior of the system. Our goal is to study the changes in phase behavior induced by addition of homopolymers and explore the underlying parameters which control the phase behavior.   

Order-Disorder Transition and Critical Micelle Temperature in Concentrated Block Copolymer Solutions

The phase behavior of symmetric styrene-isoprene diblock copolymers in selective solvents in the vicinity of the order- disorder transition (ODT) was investigated by SANS and rheology. To understand morphological changes in more detail, one of the blocks is deuterated and the scattering length density of the solvents used were identical to the corona chains. We examined two different ODTs: body-centered cubic/disorder and hexagonal cylinder/disorder, with similar SANS results in both cases. With increasing temperature, the micellar aggregation number decreases, and the core radius is roughly maintained by the increased solvent swelling of the core. The critical micelle temperature is experimentally described by an abrupt decrease in the size, aggregation number, and volume fraction of micelles, which is approximately 20-30 °C higher than the TODT. Rheological measurements, however, revealed a distinct plateau in elastic modulus just above the temperature of cylinder/disorder. The structure is attributed to the micellar congestions due to the slow break-up of cylindrical domains into an equilibrium distribution of micelles. An isothermal frequency sweep in this regime indicated a very long relaxation time and the transient response significantly depends on solvent selectivity. Possible origins of the remarkable solvent selectivity dependence are also discussed.   

Crystallization studies of polyethylene -poly(ethylene glycol) graft copolymers

Structure and crystallization behavior of three copolymers obtained by grafting poly (ethylene glycol) (PEG) chains to polyethylene (PE) main chain was investigated by variable temperature x-ray diffraction and thermal analysis. The results show that PEG side chains and PE main chains crystallize into separate domains. This is especially true when grafted chains are long (50 and 100 repeat units), in which the PEG domains are same as in PEG homopolymer both in structure and in melting behavior. In the copolymer with shorter chains (25 repeat units), the PEG crystals are not distinct and melting is broad. The PEG domains can be dissolved in water or ethanol without altering the mechanical integrity of the film. PE crystallites in both samples are similar to that in PE homopolymer. For instance, the thermal expansion of the basal cell plane (a- and b-axes) of the PE domains agrees well with that of PE homopolymer over the entire temperature range from ambient to melt. However, the chain-axis dimension PE-lattice in the copolymer is shorter by $\sim $ 0.05 {\AA} and the basal dimensions are larger by $\sim $ 0.05 {\AA}. The changes in these dimensions due to the changes in the length of the grafted PEG chains were investigated.   

Electroluminescence Emission of Fully Conjugated Heterocyclic Aromatic Rigid-rod Polymer Doped in \textit{Multi}-wall Carbon Nanotube

Carbon nanotube and poly-$p$-phenylenebenzobisoxazole (PBO) contain fully conjugated rodlike backbone entailing excellent optoelectric properties and solvent resistance. Rigid-rod polymer PBO is only soluble in methanesulfonic acid or Lewis acid. \textit{Multi}-wall carbon nanotube ($M$WNT) was dissolved in a Lewis acid solution of PBO for dispersion of nanotube, and then spun for thin film. $M$WNT concentration in the films was from zero up to 5 wt. {\%}. Compared to that of pure PBO film, composite films of $M$WNT doped PBO showed same UV-Vis absorption peaks which were enhanced with $M$WNT concentration but without overlapping electron orbitals to effect their energy gaps. All films were excited by He-Cd laser with excite wavelength of 325 nm for photoluminescence (PL) response. All PL spectra had maximum wavelength peak at 540 nm indicative of yellow-green light emission. For light emitting diodes, $M$WNT doped PBO would decrease threshold voltage for about 2 V. At 0.1 wt. {\%} of $M$WNT, the composite device emission current was increased up to two orders of magnitude than the PBO diodes without $M$WNT. Further increase of $M$WNT caused a successive decrease in electroluminescence emission intensity attributed to a quench effect form aggregation of $M$WNTs.   

Layer Effects of Photovoltaic Heterojunction of Fully Conjugated Heterocyclic Aromatic Rigid-rod Polymer Poly-$p$-phenylenebenzobisoxazole

Poly-$p$-phenylenebenzobisoxazole (PBO) contains fully conjugated rod like backbone entailing excellent optoelectronic properties$^{ }$and superior stabilities. Polystyrenesulfonate:poly (2,3-\textit{di}hydrothieno-1,4-\textit{di}oxin) (PEDOT:PSS) is a hole transferring medium which could be spun into a thin-film between indium-tin-oxide (ITO) and PBO to exhibit highly sensitive photovoltaic (PV) effect. PEDOT:PSS and PBO formed a donor-acceptor interlayer and made photoinduced charge transfer. Optimal absorption PV cell thickness for PBO was about 71 nm. By using a layer of lithium fluoride (LiF) as an electron transferring layer, the most open circuit voltage (V$_{oc})_{ }$and short circuit current (I$_{sc})$ were achieved at a LiF thickness of 1 nm. The V$_{oc}$ increased from 0.7 V to 0.9 V and the I$_{sc}$ increased one and half times.   

Induced Interaction between Polypyrrole and SO$_{2}$ via Molecular Sieve 13X

Electrical conductivity sensitivity and interaction mechanisms between polypyrrole(Ppy)/molecular sieve 13X composites and CO$_{2}$, CO, and SO$_{2}$ were investigated. Polypyrrole was synthesized and composites were fabricated from dry mixing and dispersing zeolite particles into the Ppy matrix particles, and followed by compressing into a pellet form. Effects of zeolite concentration, cation type, and cation concentration were investigated. The electrical conductivity in air of Ppy doped with naphthalene-2-sulfonic acid($\beta )$ sodium salt increased monotonically with the doping level. There were negligible negative electrical conductivity responses of Ppy and its composites when exposed to CO$_{2}$, and CO in contrast to definite positive responses towards SO$_{2}$ in which the interaction was irreversible. Undoped Ppy and doped Ppy composites at 10{\%} v/v of zeolite13X content possessed the highest sensitivity to SO$_{2}$; beyond this volume fraction the sensitivity was reduced with increasing molecular sieve 13X content. The composites of unmodified 13X, with Na$^{+ }$ fully present within its cavity, gave the greatest electrical conductivity sensitivity towards SO$_{2}$. The sensitivity of Ppy/13X composite to SO$_{2}$ diminished when the cation Na$^{+}$ was exchanged to other alkali cations in this decreasing order: Cs$^{+}$, K$^{+}$, and Li$^{+}$.   

Influence of Green Fluorescent Protein (GFP) Nanoparticles on the Optical and Mechanical Properties of Silk in Bio-Nanocomposites with Photonic Properties

We are examining the influence of Green Fluorescent Protein (GFP) nanoparticles on optical properties and mechanical properties of silk in bio-nanocomposites membranes. Liquid solutions of GFP have previously been shown to exhibit significant intrinsic two-photon absorption, coupled with fluorescent emission in the visible. The nonlinear absorption and emission of GFP have been shown to saturate at high pump intensities, and upconverted lasing of a GFP-doped polymer film has previously been demonstrated. The observed saturation intensity increases cubically with increasing GFP concentration, suggesting that better access to the intrinsic nonlinearity might be possible at higher concentrations than are possible in liquid solution. Silk and GFP solutions were mixed and cast at different concentration. The nonlinear optical properties of these membranes were examined by two-photon absorption measurements using near-infrared femtosecond pulses. Transmission and upconverted fluorescence of focused near-infrared pulses (780nm, 160fs pulsewidth) is being investigated.   

Fluctuations and Phase Transition-Like Phenomena of a Filled Elastomer Under Deformation

Fluctuations, critical phenomena and phase transitions have drawn much attention for many years. But, no report anticipates that an elastomer containing fillers under gentle deformations will show similar effects. In this presentation, we show that a filled rubber system under about 2{\%} strain may display feature fluctuations that could be associated with a transition from an elastic solid state to a dispersed fluid state and that is reminiscent of critical phenomena.   

Controlling the interaction between two rolling microcapsules on elastic substrates

We study the interaction between two rolling capsules, which model polymeric microcapsules or blood cells. Each capsule consists of an elastic shell that is filled with a viscous fluid. The capsules are driven by an imposed flow to roll along a compliant substrate. 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. This technique allows for a dynamic interaction between moving, elastic walls and the surrounding fluid. We determine how the mechanical properties of the substrate and the capsules, and the characteristics of the imposed flow, affect the dynamic behavior of two closely placed capsules. We find that the stiffness of the capsules and the substrate affects the relative motion of the capsules on the surface, i.e., by tuning the mechanical properties, the separation between the capsules can be increased or decreased as they move along the surface. The results provide guidelines for designing micro-reactors that utilize elastic capsules to transport reagents and carry out reactions.   

Tunable Nanoparticle Arrays by Immobilizing Nanoparticles on Polymer Single Crystal Surface

2-Dimensional nanoparticle (NP) array has been extensively investigated and a number of techniques are available for fabricating this unique structure. One remaining technical challenge is controlling the inter-particle spacing, which could directly leads to numerous applications. We herein report a novel means to achieve tunable NP arrays by immobilizing AuNPs on polymer single crystal surface. The single crystals of thiol-terminated polyethylene oxide (PEO) were incubated in a monodisperse gold sol. Strong Au-S chemical bonds were formed between the AuNPs and the PEO single crystal surfaces. The inter-particle spacing was controlled by the crystallization temperatures, thus the thickness of the PEO single crystals, the incubation time, and the annealing temperatures after incubation. This research might lead to a novel method to fabricate NP arrays with controlled inter-particle distance from a few nanometers to $\sim$100 nm distances.   

Assessment of structure, dynamics, and stability of HPC microgel nanoparticles in good and poor solvents

Microgel nanoparticles formed in aqueous solutions of neutral polymer hydroxypropylcellulose (HPC) is a promising drug carrier system due to their ability to solubilize hydrophobic drugs and to serve as vesicles for controlled drug delivery and release. Microgel was synthesized through self-association of amphiphilic HPC molecules at room temperature by lowering polymer phase-transition with salt addition and consequent cross-linking and dialysis. Dynamic Light Scattering and Optical Probe Diffusion techniques were used to study structure and dynamics of microgel of different polymer composition in good and poor solvents. We found the size distribution of microgel to be broad and multimodal in good solvents and narrow and unimodal in poor solvents, indicating shrinking of the nanoparticles. We present comparative analysis of microgel and HPC chain dynamics in good solvents and near good-poor solvent transition. The comparison with polymer transport properties gives important insights into the structure, dynamics, and, potentially, drug delivery capabilities of HPC microgel.   

Optical Effects on Laser Ablated Polymer Surfaces

Laser ablation of poly (ethylene terephthalate) and polyimide films were investigated using Excimer-UV laser. SEM analyses indicate the presence of rings for a wide range of ablation parameters (fluence, frequency and number of pulses). It is proposed that the particles present in the plasma plume could cause the incident laser light to diffract, similar to the optical effects observed in the femtosecond laser ablation of solids. The polymer surface provides a perfect medium to register the optical signatures as seen in the SEM images. The fringe-spacings observed in the images are compared with the theoretical diffraction patterns and the height of the plasma particles above the surface is estimated using an optimization scheme. The results of the analysis are consistent with experimentally observed dynamics of the plasma plume. It is proposed that such optical effects could be a routine feature in the laser ablation of polymers. The significance of such artifacts for lithography is discussed.   

Lessons from Biology: How Patterns Control Adhesion

Polymer adhesion impacts applications from biomedical devices to semiconductor processing, yet our fundamental control of this property remains a challenge. Nature has shown us through numerous examples (e.g. gecko to lotus leaf) that a powerful strategy for tuning adhesion lies in the development of hierarchal structures at an interface. Although nature has demonstrated this solution, our understanding on using this approach effectively is relatively unexplored. In this work, we seek to understand the inter-relationship between material structure, patterns, and performance (i.e. adhesion) for soft, polymer materials. We present an overview of the importance of length scales in adhesion, our overall strategy, and results on the adhesion of soft elastomers to inorganic surfaces. Through these results, we explain critical scaling relationships that demonstrate the potential for using patterns to tune adhesion, while also illustrating the wide range of mechanisms through which patterns can interact with a polymer interface.   

Elastomer Friction: Understanding Schallamach Waves.

From the dynamics of biomaterial interfaces to the interpretation of nanoscale characterization of polymer interfaces, the friction of soft polymer layers is critical to a wide range of advanced materials. In 1971, Schallamach discovered that friction of soft, elastomeric interfaces is often dominated by the onset and propagation of elastic instabilities in the form of surface waves$^{1}$. Although significant contributions have been made, the fundamental relationship between the polymer structure and Schallamach waves has not been established. We present our results on the development and propagation of Schallamach waves at model, soft interfaces using a custom-built instrument. Our goal is to decouple the interfacial and bulk contributions to the onset and propagation of these friction-dominating waves. Using interfaces of polydimethylsiloxane and fused silica, we observe critical transitions in the behavior of Schallamach waves and develop qualitative models to link these transitions to interfacial properties. (1)Schallamach, A.;Wear 1971,17, 301-312.   

On Fundamentals of Filler Reinforcement: an Investigation Using Polymeric Nanoparticles

We investigate the filler reinforcing mechanisms and the interactions using polymer-grafted nanoparticles [1,2] of various brush lengths and densities in polymer matrices of various molecular weights. We find that the modulus enhancement and its nonlinearity vary and depend on the distance of the system approaching the critical state, and that simple laws of corresponding state could describe the phenomena. [1]. Chem. Phys, 2004, 16, 121.; ACS Proceedings: PMSE, March, 2006. [1]. USP6437050; 6689469; 6872785; 6875818; 6956084.   

AFM-TEM observations of effect of ``melt'' time on polytetrafluoroethylene morphology.

TEM observations of PTFE dispersion particles dispersed on glass and held at 350 \r{ }C or above for various times indicates that individual, $>$ 0.1 mm long molecules wander individually on the substrate and can, with time in the ``melt,'' aggregate and form either flat-on or on-edge, folded chain single crystals. If ``trapped'' by cooling before aggregation, on-edge, single molecule, single crystals can form. All on-edge crystals, both individually and as the shish of shish-kebabs, have a ``double-striation'' appearance, suggested to arise from nucleation of the Pt/C shadowing material, used for the TEM image, on the folds at the top edge of the crystals.\footnote{P. H. Geil, \textit{et al}., Adv. Polym. Sci., 180, 89 (2005).} AFM observations have confirmed these suggestions and, furthermore, indicate the nascent, rod-like dispersion particles of a ``nano-emulsion,'' with a volume corresponding to a single molecule, have faceted ends. Combined with the TEM and ED observations that the molecular axis is parallel with the rod axis, not only must chain-folding occur during polymerization but the chain folds must be staggered on the end surfaces.   

Depth Profiling of $^{13}$C Labeled Polymers using Secondary Ion Mass Spectrometry

Deuterium labeling is known to change the thermodynamic properties of polymers and polymer blends (e.g. surface tension and bulk phase behavior). Anomalous segregation of dPS to an hPS:dPS/hPMMA interface has been recently observed, where the hPS:dPS blend is well within the single phase region of the phase diagram. Therefore, to probe various physical phenomena at polymer interfaces, such as chain mobility or reactive chain coupling, alternative labels must be established in order to provide a true tracer. It has been found that $^{13}$C labeling provides a true tracer for depth profiling of $^{13}$C-PS in $^{13}$C-PS:hPS and $^{13}$C-PS:hPS/hPMMA blends, with no observable segregation of $^{13}$C-PS to the $^{13}$C-PS:hPS surface or $^{13}$C-PS:hPS/hPMMA heterogeneous interface (Harton et al. Patent Pending). $^{13}$C-PS was synthesized with $\alpha $,$\beta -^{13}$C substituted styrene (33{\%} v/v with 77{\%} unlabeled styrene) using atom transfer radical polymerization with a low polydispersity (M$_{w}$/M$_{n}$ = 1.2). A magnetic sector instrument was used for this analysis to separate $^{13}$C from $^{12}$C$^{1}$H. This requires a mass resolution (m/$\Delta $m) of $\sim $ 3000, which cannot be achieved with a quadrupole (typical m/$\Delta $m $\sim $ 300) thereby eliminating its use for depth profiling of $^{13}$C labeled polymers. Mass spectra of labeled and unlabeled PS were also analyzed using time-of-flight (ToF) SIMS to demonstrate the possibility for detection of high molecular weight fragments as a function of depth.   

Solvent mediated assembly of Nanoparticles confined in Mesoporous Alumina

In-situ small angle x-ray scattering measurements of the solvent mediated assembly of 2~nm diameter Au-core colloidal nanoparticles inside mesoporous alumina are presented. The evolution of the self-assembly process was controlled reversibly via solvent condensed from vapor. Measurements of the absorption \& desorption of solvent showed strong hysteresis upon thermal cycling. In addition, the capillary transition for the solvent in the nanoparticle-doped pores was shifted to greater under-saturation by a factor of four relative to the expected value for the same system sans nanoparticles. Analysis indicated that a cylindrical shell super-structure of the nanoparticles is maintained throughout the addition and removal of liquid solvent. Nanoparticle nearest-neighbor separation increased and the in-shell order decreased with the addition of solvent. The process was reversible with the removal of liquid. Isotropic clusters of nanoparticles were also observed to form temporarily during desorption of the liquid solvent and disappear upon complete removal of liquid.   

Beam propagation through a thermo-optic waveguide switch

We present simulations and experiments of a thermo-optic waveguide switch using a novel 3-d wide angle beam propagation algorithm. The switch is based on a Y-branch directional coupler, with two microheaters embedded beneath both sides of the Y-branch. These two microheaters act to generate an inhomogeneous temperature profile, by which most of the energy of the light signal propagating in the waveguide will be shifted to one of the two branches. The oblique sections of the waveguides require that the second order terms in the beam propagation method be included in the simulation. Both the thermo-optic effects and beam propagation will be investigated to optimize the parameters of the device.   

Semiconductor lasers in optical communication band with very broadband tunability

Because the optical fiber has the low-loss window covering from below 1300 nm to above 1600 nm, devices used for optical communication are required to have the broadband characteristics. Several types of non-identical multiple quantum wells had been experimented. One has five 6 nm In$_{0.67}$Ga$_{0.33}$As$_{0.72}$P$_{0.28}$ quantum wells and two 15 nm In$_{0.53}$Ga$_{0.47}$As quantum wells, bounded by 15 nm In$_{0.86}$Ga$_{0.14}$As$_{0.3}$P$_{0.7}$ quantum barrier. One type of the non-identical multiple quantum wells has been used to fabricate the semiconductor optical amplifiers for semiconductor lasers with broadband tunability. Inserting the semiconductor optical amplifier in an external-cavity configuration with a grating as the feedback, the external-cavity semiconductor laser is tunable from 1295 nm to 1570 nm, which covers almost the entire low-loss window of the optical fiber. Also, using reflected-type grating telescope configuration, we are able to simultaneously generate two wavelengths. The two lasing wavelengths are tunable from a few nm separation to 170 nm separation. Simultaneous generation of two wavelengths at 1344 nm and 1514 nm in optical communication band is demonstrated.   

Near Field Measurement of Broad Area Laser Diodes by Utilizing Near-Field Scanning Microscope

It is well known that the filamentation in the broad area laser diodes (BALD) is closely related to the linewidth enhancement factor ($\alpha $-value). While normal $\alpha $ values in the multiple-quantum well (MQW) structure are 2$\sim $4, it is expected and calculated that $\alpha $ values in quantum dot (QD) are under 1. This means that the filamentation in the BALD with QD structure would be delayed until higher current level. In this study, we report the difference of near field between QD and QW BALDs by utilizing near-field scanning optical microscope (NSOM). $\alpha $ value in the QD BALD was measured to be around 0.6, while in the QW around 2. It was observed that the BALD with QD kept single mode beam under higher current level than the BALD with QW. This result shows another evidence that QD structure might be more adequate for high power laser diodes with single mode beam characteristics.   

Niobium Hot Electron Bolometer Development for Terahertz Detection

We have developed both diffusion and phonon cooled Hot Electron Bolometers (HEBs) for detecting weak signals in the submillimeter or terahertz range. The device consists of a superconducting microbridge between two non-superconducting reservoirs. We use niobium as our superconductor and aluminum as our normal metal. The goal of the research is to produce very fast (100 MHz) direct detectors for spectroscopic use, and high bandwidth mixers. We will report the performance as both heterodyne and direct detectors. The microscopic properties of the devices, including the effects of the superconducting proximity effect have also been studied.   

A Magneto-Electric Microwave Filter

A new class of electric field-tunable ferrite-ferroelectric microwave band-pass filter is discussed. The electric field tunability is possible through magnetoelectric (ME) interactions. The mechanical deformation due to piezoelectric effect in an electric field manifests as a magnetic field shift in the ferromagnetic resonance (FMR) for the ferrite. The filter was fabricated with an ME resonator consisting of bilayers of 30-110 micron thick (111) yttrium iron garnet (YIG) films and lead zirconate titanate (PZT). The bilayer was positioned between input and output antenna in a microstripline structure. The device insertion loss was measured as a function of frequency f, bias magnetic field $H$ (applied parallel to bilayer plane) and the electrical field $E$ applied across PZT. The minimum insertion loss was 4-5 dB at 5-10 GHz. The off-resonance losses were about 20 dB. The E-field tunability was 120 MHz for E = 3 kV/cm for bilayers with 110 micron thick YIG. The ME microwave filters discussed here are miniature in size, would facilitate high-speed operation, and are compatibility with integrated circuit technology. -- The work was supported by grants from ARO, ONR and NSF.   

A method for anhysteretic magnetization and magnetostriction measurement of a thin ferromagnetic films as a function of an applied isotropic stresses

A new method of the stress dependant anhysteretic magnetization and magnetostriction measurements in a thin-film ferromagnetic materials is described. This method is based on conventional vibrating sample magnetometer system combined with the specially designed loading fixture providing sizable uniaxial stresses on thin film and wires. For determining uniaxial stresses in thin-film and wires anisotropic ferromagnetic samples contactless method was developed. Stresses are deduced from characteristic resonant frequency of the sample vibrating in the special fixture and verified using pulse propagation velocity monitoring. Vibration of the sample is measured remotely using a laser Doppler vibrometer. This contactless method has been demonstrated for membrane thickness down to 50 $\mu $m and stresses up to 1GPa. Estimated accuracy of this method is better than 0.2{\%}. This method was applied to measure anhysteretic permeability and magnetization curve of Ni-Fe as a function of stress and temperature. This technique was also shown to be able also used to measure magnetostriction coefficient as a function of external magnetic field.   

Characterization of Binding Strengths of DNA on Various Indium Tin Oxide Conducting Substrates Using Surface Electrophoresis for Genomic Studies/Applications

We have analyzed DNA-surface interactions on solid surfaces using surface electrophoresis with $\lambda $ Hind-III DNA. Laser scanning fluorescence microscopy on YOYO-labeled DNA was used to image molecules exiting deposited drops for different surfaces, buffer concentrations, DNA concentrations, and electric field strengths. PET Indium Tin Oxide (ITO) and ITO coated glass were studied by varying TBE buffer concentrations from 10$^{-2}$X to 3X, DNA concentrations from 5$\mu $g/ml to 50$\mu $g/ml and electric fields from 3 to 7 V/cm. We have obtained defined circular DNA droplets on these surfaces and observed DNA strands being pulled from the outer ring by the electric field. Various adsorption mechanisms and models of mobility will be discussed. Support from the NSF MRSEC program is gratefully acknowledged.   

Shift of Optical Absorbance with Pressure in Star Polymers

The optical absorbance dependence on pressure is studied for star polymers dispersed (less than 1{\%} wt./wt.) in either polymethylmethacrylate or polycarbonate. The star polymer is composed of a hyperbranched core and $\pi $-conjugated polymer arms. The optical absorbance red shifts with increasing pressure and is expected to be due to increase in the conjugation length. The sensitivity on pressure is greater with the star polymers than for a pure linear polymer of the conjugated arm. The proposed enhancement mechanism is a non-uniform distribution of strain in analogy with the impact toughening of a matrix material by multidimensional additives.   

Novel biocompatible and biodegradable ultrathin films of poly (L-Lactic acid) by plasma polymerization

Ultra-thin films ($\le $ 50 nm) of biodegradable poly (L-lactic acid) were prepared through efficient RF plasma synthesis. The surface morphology of deposited films was amorphous and molecularly uniform (Ra = 0.7 nm). The chemical compositions as determined from FTIR and NMR demonstrated extraordinarily high retention of ester groups with a small fraction of chain cross-linking that could be controlled by process parameters. The chemical routes of the polymerization were described and discussed. This versatile thin film coating technique is very useful for surface engineering of general biomedical devices and implants for improved biocompatibility. In addition, PLLA polymerized in the liquid phase by plasma was also explored and will be presented. This work is supported by National Science Council of Taiwan.   

Terawatt picosecond CO$_{2}$ laser

We present the design of a powerful CO$_{2}$ laser that generates short 3 ps pulses with energy exceeding 10 J/pulse. The output of a conventional 1 atm TEA oscillator undergoes pulse shortening via a semiconductor switching method in combination with nonlinear Kerr cell. Pressure broadening of CO$_{2 }$rotational line in the 10 atm regenerative amplifier and the 9 atm final amplifier allows amplification of the short pulse up to several terawatts in peak power without pulse distortion. Present applications of the terawatt CO$_{2 }$laser include various methods of laser acceleration of relativistic electron beams, generation of high-intensity X-rays via Thomson scattering, ion acceleration, etc.   

Synthesis and thermoelectric properties of Na$_{x}$CoO$_{2}$ single crystal

Na$_{x}$CoO$_{2}$ has been known to be a potential thermoelectric material because of its large thermoelectric power and low resistivity. Recently, Na$_{x}$CoO$_{2}\cdot $yH$_{2}$O was discovered as a superconductor with a transition temperature T$_{c}$ of about 5K. Further understanding of the electron structure and properties of this material need larger samples of Na$_{x}$CoO$_{2 }$single crystal. In our work, Na$_{x}$CoO$_{2}$ single crystals were grown by flux method. It was found that choosing an appropriate slow cooling temperature region can increase the size of the single crystals grown by this method. The size of the largest single crystal is about 5$\times $4$\times $0.02mm$^{3}$. The structure and thermoelectric properties of these crystals will be discussed.   

Excitation of surface dipole and solenoidal modes on toroidal structures

The time-dependent Schrodinger equation inclusive of curvature effects is developed for a spinless electron constrained to motion on a toroidal surface and subjected to circularly and linearly polarized waves in the microwave regime. A seven-state basis set is employed with the goal of determining the character of the surface currents as the system is driven at a resonance frequency that selects for a solenoidal mode. Trajectory methods are used as a means of visualizing the character of the induced surface currents. Optical transitions into solenoidal modes of excitation can be observed.   

Magnetically Modulated Optical Nanoprobes: Live Cells and Bioassay Diagnostics

Fluorescent techniques are frequently used for chemical analysis in biological samples because fluorescent dyes produce strong signals that can be easily read with conventional photodetectors, spectrometers, and cameras. Magnetic techniques are used for chemical analysis and measurements of physical viscosity/elasticity because strong magnetic forces and torques are easily applied to magnetic particles without shielding or interference from biological samples. Combining fluorescence detection with magnetic manipulation of metal hemisphere coated ``Magnetically Modulated Optical Nanoprobes'' (MagMOONs) provides the best features of both techniques. We describe fabrication of uniform magnetic half-coated particles and MagMOON applications for immunoassays, intracellular chemical sensing, and passive or active viscosity measurements on a single particle and ensemble level that is based, in part, on a newly discovered effect: critical slipping of rotationally driven nanoparticles.   

Phonon engineering of electronic transport in hybrid nanotubes

Recently, a number of biological nanoscale objects, including tobacco mosaic viruses (TMV), have been employed as templates for assembly of inorganic nanostructures. This approach can potentially lead to a new method of fabrication of nanoelectronic circuits beyond conventional CMOS. Here we theoretically demonstrate that in addition to their role as nano-templates [1], the elastically soft TMVs can improve electron transport in the nanotubes grown on them [2]. In the simulated hybrid nanostructures, which consist of silicon or silica nanotubes on TMVs, the confined acoustic phonons are found to be redistributed between the nanotube shell and the acoustically soft virus enclosure. As a result, the low-temperature electron mobility in the hybrid TMV-silicon nanotube can increase up to a factor of four compared to that of an empty silicon nanotube [2]. Our estimates also indicate an enhancement of the low-temperature thermal conductivity in the TMV-silicon nanotube, which can lead to improvements in heat removal from the hybrid nanostructure-based circuits. The authors acknowledge the support of MARCO and its Functional Engineered Nano Architectonics (FENA) Focus Center. [1] W.L. Liu, K. Alim, A.A. Balandin et al., Appl. Phys. Lett. 86, 253108 (2005); [2] V.A. Fonoberov and A.A. Balandin, Nano Lett. 5, 1920 (2005).   

Nanoscale Silicon Particles in Sandwich Structures Fabricated on Silica Wafers: HRTEM and EDX Techniques as Imaging Tools

The use of silicon in optical applications is limited by its small and indirect band gap. However, recent observations of photoluminescence in porous Si and in Si ultra fine particles suggest that Si nanoclusters may become a promising material for optical applications. The nanometric Si clusters have a band gap enlarged into the visible range and quantum confinement effect is believed to be the mechanism for light emission. The ability to control the size and structure of nanoparticles would allow the fabrication of structures with desired electrical and optoelectronic properties for device applications. Nanosized Si particles with controlled diameter were fabricated on silica substrate wafers using an specially designed ultra-high vacuum nanocluster deposition apparatus. We present the structural characterization and composition studies of the fabricated nanostructures using high resolution transmission electron microcopy (HRTEM), x-ray diffraction (XRD) and energy dispersive x-ray (EDX).   

Hydrogen uptake in single-walled carbon nanotubes synthesized by the hydrogen arc plasma jet method.

Carbon nanotubes were synthesized by a modified electric arc discharge method under hydrogen atmosphere using a catalytic mixture of powders with composition C/Ni/Co/Fe/FeS. The samples were characterized with transmission and scanning electron microscopy, Raman spectroscopy and thermo-gravimetric analysis. Unpurified samples contain 20 wt{\%} of carbon nanotubes, 2 wt{\%} of other forms of carbon and 78 wt{\%} of catalytic metals. Adsorption/desorption of hydrogen in unpurified samples at atmospheric pressure measured with gravimetric methods was of the order of 2 wt{\%}. Samples for these measurements were prepared with a three-step treatment: (i) oxidation in air for 30 min at 500 $^\circ$C, (ii) ball- milling for 1 hr, and (iii) second oxidation in air for 30 min at 500 $^\circ$C. This treatment was applied to eliminate other forms of carbon as well as to shorten the tubes and open their caps. Hydrogen desorption was also measured in degassed samples (400 $^\circ$C for 20 min in vacuum) which adsorbed H$_ {2} $ while cooling to room temperature. Desorption in degassed/hydrogenated samples is a two-step process which we attribute to distinct desorption characteristics of nanotubes and of residual oxidized metal catalysts.   

Theoretical investigation of catalysed MgH2

MgH2 has attracted much attention for being a good hydrogen storage material due to its light weight, low manufacturing cost and high storage capacity (7.6 wt{\%}). But its slow absorption/desorption kinetics and high dissociation temperature (nearly 300\r{ }C) limit its practical applications for hydrogen storage. To overcome this, much effort has been paid mainly by making nanocrystalline Mg and/or by adding alloying elements. In this work, we provide a theoretical investigation of the electronic and structural properties of pure and M-doped MgH2 (with M=Sc, Ti, V, Fe, Ni, Al). We have made calculations for both the crystalline state and 1.0 nm particles. The self-consistent total energy calculations are performed within density functional theory using the VASP package for crystals and TURBOMOLE package for clusters. One aim of this study is to see if the alloying elements can weaken the Mg-H bonds, resulting in improved thermodynamics and faster kinetics. Another one is to understand the differences in the thermodynamics of clusters and crystals.   

Fabrication and Structural Studies of Nanostructured Alumina Templates Using X-Ray Diffraction and EDX Techniques

We have previously developed a thin film template based nanostructure fabrication technique that is applicable for the development of high performance photonic devices. The template is created by the anodization of thin film aluminum deposited on an arbitrary substrate and contains nanoscale pores inside which a nanoscale material could be synthesized. An alumina template is characterized as a nonstoichiometric, amorphous film that is optically transparent over a wide spectral range and is also electrically insulating, which makes it a perfect embedding material for nanoscale devices. We investigated the formation of thin film alumina templates on various substrates, under different fabrication conditions. The final template structure was found to depend on the structure of the starting aluminum film (free standing or deposited on another substrate). We show that pre-anodization annealing of the substrate can dramatically alter the stoichiometry as well as the microstructure of the final template, changing it from amorphous to nanocrystalline.   

The adsorption and photo-degradation of oxalic acid at the TiO$_{2}$ surface.

Oxalic acid is the simplest model compound to study the heterogeneous photocatalytic oxidation of pollutants on TiO$_{2}$ containing more than one carboxylate group. We have carried out a study of a system of an oxalic acid solution in contact with a thin film of TiO$_{2}$ particles employing ATR - FTIR in combination with quantum chemical calculations. Thus, possible adsorption structures have been identified and molecular dynamic simulations have been used to compare their predictions with the experimental data. It was found that the adsorption of oxalic acid on TiO$_{2}$ in the dark can be explained in terms of two surface complexation modes for the anatase phase and only one surface complexation mode for the rutile phase. We have found that under illumination one of the complexes on the anatase phase preferably undergoes photo-degradation. At the same time water molecules are desorbed from the TiO$_{2}$ surface by a thermal mechanism induced by the absorption of photons. Both processes favor the adsorption of more molecules of oxalic acid at the TiO$_{2}$ surface which is thus enriched in the second complexation mode. A similar mechanism was found to occur on the rutile phase. The only complexation mode appears not to be photo-sensitive but the TiO$_{2}$ surface is enriched in oxalic acid under illumination due to the replacement of photo-desorbed water molecules.   

Infrared photodetector based on modulation-doped quantum-dot structures

We investigate a device model for the quantum-dot photodetector based on modulation-doped structures (for example, the modulation-doped AlGaAs/GaAS structure with InAs quantum dots). At room temperatures, the electron momentum relaxation is determined by electron- phonon scattering. The electron mean free path is small and the photoelectron capture is conditioned by electron diffusion in the potential relief created by modulation doping. Modeling with diffusion-limited capture is used to evaluate the photodetector performance. The results show that the modulation-doped structures provide longer lifetimes of photoelectrons, which in turn improves the photoconductive gain and sensitivity.   

Nanoscale Metal-molecule contacts

Studies of the influence of the metallic contacts to the electronic transport through single or a few molecules are reported. In order to separate these influences from intrinsic molecular properties we use a number of different, versatile contacting methods. The tested molecules vary from nominally insulating molecules to molecules with conjugated charge systems. It is important that the coupling of the molecules to the electrodes is mechanically and electronically stable throughout the experiment. As a first step two different structures allowing for the change of the mechanical coupling will be analysed to find an optimal configuration. On the one hand a mechanically controllable break-junction (MCB) technique will be studied, on the other a shadow evaporation technique based on a silicon structure will be tested. Transport through the molecules can be investigated at different temperatures. First results indicate that the interplay between the metallic electrodes and the molecules depends strongly on the coupling of the electrodes to the molecules and on the packing of the molecules on the metallic substrates.   

All Electronic Charge Carrier Transit Time Mobility Measurements and Data Analysis in Pentacene Organic Field Effect Transistors

In organic field-effect transistors (OFETs), the most common method of estimating mobility is to extract it from the transfer characteristics ($e.g.,$ I$_{d}$-V$_{ds}$ and I$_{d}$-V$_{dg}$ curves). Time-of-flight measurements via optical excitation of charge carriers in the organic material are also used, but this technique measures the mobility of carriers perpendicular to their motion in the OFET geometry. Additionally, in some polycrystalline organic semiconductors, the optically excited time-of-flight measurements are rendered difficult due to charge trapping. We present an all-electronic time of flight method for measuring the in-plane mobility of charge carriers using an applied voltage pulse to the source of the OFET with a rise time of $<$ 60 ns. The gate of the OFET was grounded and the drain was connected to ground across a small load resistor. The transit time of the charge carriers from the source to the drain was measured by recording the voltage drop across the load resistor using a high-speed oscilloscope. The fabrication of the OFET, extraction of the charge carrier transit time from recorded data and effect of the displacement current over varying temperature and voltage step magnitudes will be discussed.   

Temperature dependent spectroscopy of poly[bis-(2-ethyl) hexylfluorene]/ (9,9-di- $n$ -octylfluorene) copolymers

A series of random polyfluorene (PF8) copolymers comprised of linear di-$n$-octyl (F8) and branched bis(2-ethylhexyl) (F2/6) units has been synthesized and characterized by temperature dependent steady-state absorption and emission spectroscopy. The F2/6 polymer is already well known for forming conformationally disordered five fold helices while the F8 polyfluorene adopts a number of distinct near-planar type conformational isomers. One of these conformational sequences is an unusual low energy absorption and emission band known as the $\beta$ phase. In these copolymers the PF chains must temporize between differing interchain packing motifs, pentagonal and planar type structures, and different main chain morphologies. Increasing the content of F2/6 monomers strongly affects the formation of the $\beta$ phase conformer, enhances the extent of conformational disorder (and the effective electron-phonon coupling strengths) and also alters the bulk structural phase behavior. There are only weak correlations between the overall phase behavior and the observed spectroscopy at temperatures below 100~$^\circ$C.   

Vapor deposited organic thin films with large third order nonlinearities for integrated nonlinear optics

We used vapor deposition in high vacuum to fabricate homogenous organic thin films for third-order nonlinear optics. We used small molecules that have a high third order polarizability, are robust, and can be sublimated without decomposition. The most important example is 1,1,2-tricyano-2-[(4-dimethylaminophenyl)ethynyl]ethene (TDMEE), which has a \textit{specific} third order polarizability one order of magnitude larger than other highly nonlinear molecules. It is a member of a family of donor-substituted cyanoethynylethene molecules that has recently been found to have polarizabilities that approach Kuzyk's fundamental limit.\footnote{J. C. May et al, Opt. Lett. 30, 3057 (2005)} The organic thin films have a high, flat transmission above 1000 nm, a high two-photon absorption near 1200 nm, and an essentially real third order polarizability at 1.5 $\mu$m that is three orders of magnitude larger than fused silica.   

The Relativity of the Photon Mass

In the standard model of the electromagnetic radiation, a photon is mass-less. In [1] it was showed that the puzzling question -- whether a photon does or does not have a zero mass -- can be however answered. According to the new viewpoint, photons can exist in two states: 1) photons have energy and collide neither with a medium nor with surfaces; 2) photons collide with a medium or a surface. In the first case (1), photons possess latent or potential energy and are invisible. In the second case (2), they are visible Wirkungsquantums according to the Planck's quantum theory. The energy of a photon could in case (1) be determined by Planck's equation $E_P^1 =hf$ and in case (2) by Einstein's formula $E_P^2 =mc^2$. By a collision (Wirkung) between a photon and a medium or surface, an energy transformation takes place: the mass-less and invisible light particle (phantom) with latent energy, is converted into a visible photon, which now possesses an impulse mass and energy. For the case that the photons ``bomb'' a surface, which moves with a velocity $v$ into the same direction as the photons, a new equation for the photon mass has been derived: $m_P =2.2102186\cdot 10^{-42}\cdot \lambda ^{-1}\cdot \left[ {{\left( {1-v \mathord{\left/ {\vphantom {v c}} \right. \kern-\nulldelimiterspace} c} \right)} \mathord{\left/ {\vphantom {{\left( {1-v \mathord {\left/ {\vphantom {v c}} \right. \kern-\nulldelimiterspace} c} \right)} {\left( {1+v \mathord{\left/ {\vphantom {v c}} \right. \kern-\nulldelimiterspace} c} \right)}}} \right. \kern-\nulldelimiterspace} {\left( {1+v \mathord{\left/ {\vphantom {v c}} \right. \kern-\nulldelimiterspace} c} \right)}} \right]^{0.5}$(kg) 1. \"{U}ber die Relativit\"{a}t der Masse und Energie des Lichtquanten, S. Rei{\ss}ig, 2005, http://www.efbr.org in /Publikationen.   

Correlation Energy of A Model Problem

The search for new analytic methods of calculating details of the energy spectrum of strongly interacting systems has long been the vocation of both theoretical chemists and physicists. In particular, the accurate calculation of both the ground-state and correlation energies are important in settling issues relating to the exact nature of the ground-state and low-lying excited states. Furthermore there exist a number of physically relevant systems that cannot be treated by perturbation theory or in which other approximation schemes yield completely erroneous results. Exact diagonalization studies are well known to suffer from size effects, while the neglection of correlations in fluctuations in mean-field theories, although calculationally tractable, leave much to be desired. Here we wish to apply a recently developed Generalized Moments Expansion (GMX) [1] to the problem of $N$ coupled one dimensional harmonic oscillators given by the Hamiltonian: $ H=\frac{1}{2}\sum_{j=1}^{N}\left( -\frac{d^{2}}{dx_{j}^{2}}+\omega^{2}% x_{j}^{2}\right) +g^{2}\sum_{ij}^{N}x_{ij}. $ Comparisons are then made with other methods such as a Lanczos tridiagonalization scheme as well as a Canonical Sequence Method approach. \\ \\ \noindent\lbrack1] V.~Fessatidis, J.D. Mancini, R.~Murawski and S.P.~Bowen, Phys.~Lett.~A.   

Zero-Point Energy of the Pullen-Edmonds Hamiltonian

Here we wish to apply the newly developed Generalized Moments Expansion (GMX) to the well-known potential \[ U=\frac{1}{2}\left( x^{2}+y^{2}\right) +\alpha x^{2}y^{2}, \] which is used to model such molecular systems as formamide and \textrm{C}% $_{\mathrm{2}}\mathrm{O}_{\mathrm{3}}$. Our motivation is to investigate the numerical accuracy as well as the viability of the GMX for evaluating ground-state energies of quantum Hamiltonian systems. The zero-point energy of this potential is calculated and results are compared to an analogous Lanczos (tridiagonal) matrix truncation as well as to a Canonical Sequence Method approach.   

Ground-State Energy of A Two-Level System with Phonon Coupling

The coupling of a two-level system to a quantized boson mode has been the focus of many researchers for a number of years. Applications to exciton motion, molecular polaron formation, chaos in quantum systems as well as a number of other effects in condensed matter physics have also been studied. Here we investigate the interaction of a single bosonic mode with a two-level fermionic system given by the Hamiltonian% \[ H=-\delta_{0}\sigma_{x}+\sum_{k}\hbar\omega_{k}a_{k}^{\dag}+\sum_{k}% g_{k}\left( a_{k}^{\dag}+a_{k}\right) \sigma_{z}. \] This quantum system is used as a testing ground for a newly developed Generalized Moments Expansion, GMX$\left( m,n\right) $, of which the well-known Connected Moments Expansion (CMX) and Alternate Moments Expansion (AMX) are special cases: \textrm{CMX}$=$\textrm{GMX}$\left( 1,1\right) \mathrm{,}$ \textrm{AMX=GMX}$\left( 1,2\right) $. The convergence and viability of this scheme is discussed and comparisons are made with other methods.   

Locating Regions of Complex Zeros of the Mittag-Leffler Function E$_{\alpha ,\beta }$(z) for 2 $< \quad \alpha \quad \le $ 3

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. Moreover, the zeros of E$_{\alpha ,\beta }$(z) for some values of $\alpha $ and $\beta $ are the eigenvalues of fractional differential operators. Consequently, knowledge of the zeros and their distribution is of fundamental importance. This work focuses on the distribution of zeros in the region where the Mittag-Leffler function possesses an infinite number of real zeros and a finite number of complex zeros and is restricted to the range 2 $< \quad \alpha \quad \le $ 3 and $\beta \quad \le $ 14.   

Dynamics of a Pair of Coupled Fractional Oscillators

The integral equations of motion of a pair of coupled fractional oscillators are obtained by generalizing the equations of motion of a pair of coupled harmonic oscillators to integrals of fractional order. The Greens function dynamic response to sinusoidal forcing is given in terms of Mittag-Leffler functions. A direct and transparent derivation of the results of Ryabov and Puzenko obtained by a tedious perturbation method is presented.   

A Time Dependent Approach for Computing N-N Phase Shifts and Cross Sections

Scattering matrix elements for n-n, n-p, and p-p collisions are computed using a new time dependent wave packet technique. Using this approach, reactant and product channel packets are prepared in the asymptotic limit on the N-N potential energy surfaces. For these calculations we are using the Argonne v18 potential package (1). The channel packets are propagated in time using the split operator method together with a unitary transformation that diagonalizes the tensor potential. Scattering matrix elements are computed from the Fourier transform of the correlation function between the evolving channel packets. Phase shifts and cross sections are computed from the scattering matrix elements . (1) R.B.Wiringa, V.G.J. Stoks, and R.Schiavilla, Phys Rev C \textbf{51} (1995) 38   

Tunneling Density of States for 2D Fermi Liquid

We calculate the Green's function for an interacting two-dimensional electron liquid whose strength of interaction is characterized by the electron density parameter $r_s$. The screened electron-electron interaction is expressed in terms of a frequency and wave vector-dependent dielectric function $\varepsilon(q,\omega)$. If this screening is neglected, the tunneling density of states (DOS) is strongly modified due to electron-electron interaction. In particular, in this case, the DOS has a dip near the Fermi energy whose width increases with $r_s$. By treating $\varepsilon(q,\omega)$ in the hydrodynamical approximation, we investigate the way in which the tunneling DOS is modified at various electron densities, due to screened interaction. Preliminary results show considerable modification of the tunneling DOS at large $r_s$ for both a noninteracting and unscreened electron system. When there is screening, the dip becomes less pronounced but survives.   

On the quantum master equation with nonhermitian operators

A quantum master equation is derived for fermions by considering a relaxation term in addition to the mean-field Hamiltonian. [C. F. Huang and K.-N. Huang, Chinese J. Phys. \textbf{42}, 221 (2004); Ralph Gebauer and Roberto Car, Phys. Rev. B \textbf{70}, 125324 (2004).] The relaxation term is symmetric with respect to particles and holes, and its loss and gain factors can be obtained by incorporating nonhermitian parts to the Hamiltonian for the decays of particles and holes, respectively. Such an equation reduces to the Markoff master equation of the Lindblad form when the electrons or holes are of the low-density distribution. On the other hand, the derived equation reduces to the quasiclassical master equation in an incoherent limit. Both the loss and gain factors induce the decoherence. From Cauchy's inequality, we can see that the decoherence is important to keep both the density matrices for particles and holes as positive operators. The positivity of the density matrix for holes, in fact, indicates the validity of Pauli exclusion principle under the derived master equation. To further investigate the decoherence, we compare the master equation for fermion to that for bosons. The gain factor of the latter equation does not induce the decoherence, which is reasonable because bosons do not need to follow the Pauli exclusion principle.   

The Induced Coherent Interaction of Localized Spins via Thermal Bosonic Environment.

We obtain the indirect coherent interaction between two spins induced by the bath of bosonic modes, and demonstrate that this interaction can create entanglement. We utilize a perturbative approach to obtain a quantum evolution equation for the two-spin dynamics. The induced interaction is calculated exactly. The exact solution is then used to identify the time scales for which the spins remain entangled.   

Pair State Analysis of the Hubbard Hamiltonian in One-Dimension

Using two-electron states as the basis, we have analyzed the one-dimensional Hubbard Hamiltonian (HH) with periodic boundary conditions for many-electron systems. The N-electron energy eigenvalues are simply the sum of the pair energies (eigenvalues of the two-particle reduced HH) weighted by two-particle density matrix elements. We are investigating the possibility that this approach will lead to a useful approximation scheme. For many weakly correlated systems, the pair-energy sum can be truncated and still the ground state energy can be obtained with reasonable accuracy. For example, in the case of six sites at half-filling (with U/t = 1) we need only include 12 of the 45 triplet pair states and 6 of the 21 singlet pair states, and still the ground state energy can be found with only 6\% error. A comparison between the exact and approximate results for this system and several others are presented.   

P3/2 state in New Dirac Equation

In this talk I explain the very interesting properties of the \textit{most probable} excited state of a \textit{new} type of Dirac equation and singularity when applied to a \textit{single} type of charge 'e'. Notably this first excited state psi*psi P3/2 (state lobes) shape is a trifolium, \textit{three} lobed. Thus for each lobe there is a (1/3)e average charge given the charge 'e' spends on average 1/3 of the time in each lobe. Thus combinations of lobes have average charge, \textbf{e/3}, 2e/3 (\textit{FRACTIONAL CHARGE}) or e. The lobes \textit{can't leave (ASSYMPTOTIC FREEDOM)} or move so are \textit{NONRELATIVISTIC}. The singularity of this Dirac equation results in containment at about .6 Fermi, the TOTAL CHARGE is still `\textbf{e}'(explaining away the need for \textit{COLOR}), etc. Most importantly the Frobenius series solution to this new Dirac equation also gives accurate hadron eigenvalues. You quickly see that you are getting here all the properties of quarks (identified here as individual lobes) obtained in a very simple way from the movement of a single unit charge `e' using this new Dirac equation. There appears no simpler way to explain all these many quark properties.   

Some Interesting Symmetries of the Gravitational Stress Energy Tensor.

Some thoughts are presented on a few symmetries found in the Gravitational Stress Energy Tensor, with implications for black holes, fermion masses, and unification theories.   

``Hidden Kinetic Energy'' and ``Mass-Electric'' Principle

A hypothesis is proposed which suggests another fundamental energy equation of Special Relativity. Another relativistic variance is described which gives self-consistent definition of Lagrange Function for both classical and relativistic dynamics. This hypothesis leads to a new ``Hidden Kinetic Energy'' concept as well as a new ``Mass-Electric'' Principle.   

Acceleration of Markov Chain Monte Carlo Simulations through Sequential Updating

Strict detailed balance is not necessary for Markov chain Monte Carlo simulations to converge to the correct equilibrium distribution. In this work, we propose a new algorithm that only satisfies the weaker balance condition. The new algorithm is based on sequential updating moves with a small fraction of randomness to eliminate possible oscillatory effects. We prove analytically that the new algorithm identifies the correct equilibrium distribution. In addition, based on the properties of the diagonal elements of the underlying transition matrices, we demonstrate that the new algorithm converges faster than the Metropolis algorithm with strict detailed balance. We illustrate the efficiency of the new algorithm on the two- dimensional Ising model. The sequential update algorithm also compares well with multispin-based Monte Carlo techniques. The new method, however, is very general and can be readily extended to off-lattice continuum systems in various ensembles (canonical and grand canonical) with minor modifications. Simulation results on hard-spheres, square-well, and Lennard- Jones fluids indicate that the new method is more efficient in reducing autocorrelation times than conventional Metropolis- type of algorithms in canonical and grand canonical ensembles. Regarding off-lattice continuum fluids, autocorrelation time reduction is more substantial at higher densities due to the sequential nature of the Monte Carlo moves.   

Competing instabilities in correlated electronic systems in the FLEX framework

We consider the Hubbard model in the regime of strong correlations on bipartite and frustrated lattices. While it is well established that on a bipartite lattice at half filling the leading instability is in the SDW channel, upon varying the lattice symmetry and the electron concentration, competing instability channels emerge. We study possible channels such as pair, magnetic and charge correlations that can be either commensurate or incommensurate. We use the fluctuation-exchange approximation to investigate the development of, and competition between, different kinds of incipient instabilities in these particle-particle and particle-hole channels.   

Geometries of small cadmium selenide clusters

The sizes, shapes, relaxed atomic positions, eigenvalues, and total energies are calculated for selected ultrasmall CdSe clusters using SIESTA, a software package for electronic structure calculations and molecular dynamics simulations of molecules and solids. The properties of these bare clusters with small numbers of constituent atoms are studied using density functional theory (DFT) for energy calculations and the conjugate gradient approximation method in relaxing the structure to find the lowest energy configurations. The \textit{ab-initio }norm-conserving pseudopotentials, the exchange-correlation approximation, and parameters used in the computations are discussed. We describe ancillary software for calculating initial atomic coordinates, testing pseudopotential transferability, and conveniently generating input data files.   

The wavefunction topography in the regime of Fano interference

In a point contact the wavefunctions belong to the continuum and the imaging of quantum states is based on measurements of the local density of states or the conductance. An attractive quantum dot in the 2D-waveguide can produce a Fano resonance in the conductance. We present here analytical calculations of conductance variation with the short range attractive potential for modeling the probe experiments in the regime of the Fano interference. It was shown that the wavefunction variation in the waveguide may be expressed through the Green's function of an unperturbed system if the characteristic size of the probing potential is less than the Fermi wavelength. We have found that in general the variation of the conductance depends on the phase interference of different waveguide modes. At the same time in the Fano regime in the conductance variation, there are dominant terms proportional to a resonant wavefunction. It was shown that this property suggests the possibility of extracting the wavefunction related to the Fano resonance.   

A Numerical Investigation of the Non-Linear Interaction Forces in Tapping Mode Atomic Force Microscopy

There is an important need to measure the topography of soft materials including many of biological importance. Such soft and fragile materials can easily be damaged by the use of conventional nanoscale materials characterization instrumentation. Tapping mode atomic force microscopy routinely measures the topography of materials with atomic resolution. For soft materials this is limited by the nonlinear contact forces imposed upon the sample by the oscillating cantilever tip. A fundamental understanding of these forces could lead to techniques capable of minimizing the possibly destructive tip-sample interactions. A physical model of the contact interactions is constructed and numerical simulations of the cantilever dynamics are presented to explore these possibilities.   

Analytical solutions of the Poisson-Boltzmann equation: biological applications

Electrostatic interactions are a key factor for determining many properties of bio-molecules. The ability to compute the electrostatic potential generated by a molecule is often essential in understanding the mechanism behind its biological function such as catalytic activity, ligand binding, and macromolecular association. We propose an approximate analytical solution to the (linearized) Poisson-Boltzmann (PB) equation that is suitable for computing electrostatic potential around realistic biomolecules. The approximation is tested against the numerical solutions of the PB equation on a test set of 600 representative structures including proteins, DNA, and macromolecular complexes. The approach allows one to generate, with the power of a desktop PC, electrostatic potential maps of virtually any molecule of interest, from single proteins to large protein complexes such as viral capsids. The new approach is orders of magnitude less computationally intense than its numerical counterpart, yet is almost equal in accuracy. When studying very large molecular systems, our method is a practical and inexpensive way of computing bio- molecular potential at atomic resolution. We demonstrate the usefullnes of the new approach by exploring the details of electrostatic potentials generated by two of such systems: the nucleosome core particle (25,000 atoms) and tobacco ring spot virus (500,000 atoms). Biologically relevant insights are generated.   

Semi-Meissner state and neither type-I nor type-II superconductivity in multicomponent systems

Traditionally, superconductors are categorized as type-I or type-II. Type-I superconductors support only Meissner and normal states, while type-II superconductors form magnetic vortices in sufficiently strong applied magnetic fields. Recently there has been much interest in superconducting systems with several species of condensates, in fields ranging from Condensed Matter to High Energy Physics. Here we show that the type-I/type-II classification is insufficient for such multicomponent superconductors. We obtain solutions representing thermodynamically stable vortices with properties falling outside the usual type-I/type-II dichotomy, in that they have the following features: (i) Pippard electrodynamics, (ii) interaction potential with long-range attractive and short-range repulsive parts, (iii) for an n-quantum vortex, a non-monotonic ratio E(n)/n where E(n) is the energy per unit length, (iv) energetic preference for non-axisymmetric vortex states, ``vortex molecules''. Consequently, these superconductors exhibit an emerging first order transition into a ``semi-Meissner'' state, an inhomogeneous state comprising a mixture of domains of two-component Meissner state and vortex clusters. We also discuss a counterpart of this state which may occur in type-I/type-II superconducting bilayers.   

The St.- Petersburg State University Experiment that discovered the Photon Acceleration Effect

Using the principles of the Vortex Theory, it was theorized that when a photon encounters an electromagnetic field, both the velocity and the frequency of the photon will change. To prove this revolutionary idea an experiment was devised using a laser interferometer and two electromagnets. The electromagnets were arranged so that when the beam splitter divided the initial beam of laser light into two secondary beams; one of the two secondary beams passed back and forth between the two magnets. With the DC current to the electromagnets turned off, the two beams formed an interference pattern on the target screen. When the current to the electromagnets was suddenly turned on, the pattern fluctuated wildly until the two beams again reached a quiescent state creating a stable pattern on the screen; when the current to the electromagnets was suddenly turned off, again the pattern fluctuated wildly until it reached a quiescent state forming the initial stable pattern on the screen. It was determined that this new effect was a phenomenon created by the changing frequency of the laser light whose velocity is increasing as it passes between the expanding electromagnetic field of the magnets. Because it is a new phenomenon in science revealing that the speed of light is not a constant but indeed can be varied, it possesses great historical significance. It is called the Photon Acceleration Effect.   

The Neutral Pentaquark

Using the principles of the Vortex Theory, it was discovered that when the gamma ray strikes a nucleon, the positively charged pentaquark [and the K$^{-}$ meson] had to be created by the collision with neutron. This discovery further reveals that if the gamma ray strikes a proton it can create a Neutral Pentaquark [and a D$^{+}$ meson]. The neutral pentaquark will consist of an up, up, down, down, and an anti-charm quark, while the D$^{+}$ meson will consist of a charm and an anti-down quark. The neutral pentaquark will later decay into a neutron and D$^{0 }$meson. Because the vortex theory also reveals that the strong force couples a proton to a neutron, the neutron that was coupled to the proton in the nucleus will also be found amid the debris particles. References: 1. R.G. Moon, V.V. Vasiliev, Book of abstracts NUCLEUS-2003, October 7-10, Moscow, St.-Petersburg, Russia, p.251. 2. R.G. Moon, V.V. Vasiliev, Book of abstracts NUCLEUS-2004, June 22-25, Belgorod, Russia, p.259. 3. R.G. Moon, V.V. Vasiliev, Frontiers in the Physics of Nucleus, June 28-July 1, 2005, St-Petersburg, Russia, p. 34.7   

Partial Phonon Densities of States at the Fe-Sites in several Fe-Sn Intermetallic Compounds under Pressure

We studied the nuclear resonant inelastic x-ray scattering (NRIXS) of Fe-57 in several Fe-Sn intermetallic compounds up to 30 GPa. The samples (FeSn$_{2}$, FeSn, Fe$_{5}$Sn$_{3}$, and Fe$_{3}$Sn) were prepared in the HiPSEC laboratory with 95{\%} enriched Fe-57 and 93{\%} enriched Sn-119. NRIXS spectra were collected at HPCAT beamline 16-ID-D at the Advanced Photon Source. High-pressure experiments were conducted using a Paderborn-type DAC [1]. The pressure dependence of the partial phonon DOS spectra will be discussed in terms of elastic and thermodynamic parameters, such as the Lamb-M\"{o}ssbauer factor, mean force constant, free energy, and Gr\"{u}neisen parameters, using results from recent x-ray powder diffraction experiments of the Fe-Sn intermetallics under pressure. [1] H. Giefers et al., High Pressure Research 22 (2002) 501. [2] H. Giefers et al., J. Alloys Compd. accepted.   

High-Pressure Synchrotron RadiationX-Ray Diffraction and Vibrational Spectroscopic Studies of 3,3,7,7-tetrakis(difluoramino)octahydro-1,5-dinitro-1,5-diazocine (HNFX)

HNFX, 3,3,7,7-tetrakis(difluoramino)octahydro-1,5-dinitro-1,5-diazocine, belongs to a new class of compounds predicted to be potentially superior explosives or solid propellant oxidizers, the \textit{gem}-bis(difluoramino)-substituted heterocyclic nitramines [1,2]. HNFX has been studied in diamond anvil cell up to 30 GPa by synchrotron radiation x-ray diffraction, Raman and infrared spectroscopy at ambient temperature. The pressure-induced alterations in the profiles of the peaks, including their positions, widths and intensities are presented. No phase transition was observed on this range of pressure. The bulk modulus and its pressure derivative were estimated by the third-order Birch-Murnaghan isothermal equation of state up to 6.6 GPa. To the best of our knowledge, this work presents the first high-pressure investigations of HNFX and more generally of a \textit{gem}-bis(difluoramino)-substituted heterocyclic nitramine. [1] R. S. Miller;\textit{ Chemistry of Energetic Materials}; Ed. G. A. Olah and D. R. Squire; Chapt4; p77; (\textbf{1996}). [2] R. D. Chapman, R. D .Gilardi, M. F. Welker, C. B. Kreutzberger; \textit{J. Org. Chem.}; 64; p960; (\textbf{1999}).   

Structure phases of MgSiO$_{3}$ in Earth's lower Mantle: ab initio calculations

The Earth's mantle is divided into five layers with four major phase transitions at 410, 520, 660 and $\sim $2600-km depths, due to a structural and chemical changes of its main constituent minerals. MgSiO$_ {3}$ is believed to be a predominant mineral, at least in the upper part of the lower mantle. MgSiO$_{3}$ has a structural sequence from corundum-type ilmenite to CaIrO$_{3}$-type structure (\textit{Cmcm} symmetry). First-principles calculations have been performed within the full-potential linearised augmented plane-wave method (FP-LAPW). We calculated equilibrium lattice parameters at different pressures up to 150 GPa. Four crystal structures relevant to MgSiO$_{3}$ were considered and they can grouped in two pairs of distinct types:\textbf{ (a)} corundum-like and Rh$_{2}$O$_{3} $(II) structure-like, and \textbf{(b)} perovskite and CaIrO$_{3}$ structure-like. To investigate the structural phase transitions in MgSiO$_{3}$, we calculated the enthalpy (E+PV) of the four-structures as a function of pressure from 0 to 150 GPa. The intersection of the curves indicates which structure is more stable and gives the transition pressure.   

High-pressure X-ray diffraction study of the giant dielectric constant material CaCu$_3$Ti$_4$O$_{12}$: evidence of stiff grain surface

We measured the high-pressure X-ray diffraction of the giant dielectric constant material CaCu$_3$Ti$_4$O$_{12}$ (CCTO) under both hydrostatic and uniaxial compressions. We found that the cubic structure of CCTO is stable up to 57 GPa. Nevertheless we observed CCTO has unusual compression behaviors under hydrostatic pressure. Specifically, the volume reduction is less than that under uniaxial compression below 25 GPa, above it the volume reduction starts to approach and finally reach the same value as that under the uniaxial compression at about 30 GPa. We explained these remarkable phenomena by using the model that the samples are composed of grains that have shells stiffer than the cores.   

Dynamic Ellipsometry Under Extreme Conditions

Accurate temperature measurements at high temperatures and pressures require knowledge of emissivities at similar conditions. Presently, many temperature measurements at extreme conditions assume a constant ambient-condition emissivity. Development of a miniaturized ellipsometer has made it possible to measure optical properties, and thus emissivities, of materials under extreme shock conditions. We will present here past development and future directions of the miniaturized ellipsometer for use in high pressure- high temperature experiments. We will also discuss applications of the technique including phase transitions under dynamic compression, observed changes in the complex dielectric constants of various solids and water. Since emissivity can be determined from the complex dielectric constants, we expect emissivity to change upon phase transition. We will present here results on various solid-solid, solid-liquid and liquid-solid phase transitions. In particular, we will focus on the change in emissivity at the ($\alpha,\epsilon$) phase boundary in Fe and its implications for the Fe phase diagram.   

Ab Initio Studies of MgSiO$_{3}$ Post-Perovskite

Recent discovery of a post-perovsike phase of MgSiO$_{3}$ was considered as a new era in the studies of the Earth's deepest mantle. This discovery provides some explanations for many puzzling problems of the Earth's D" layer (pressures $\sim $120-136 GPa, temperature $\sim $2000-4000 K), such as the large seismic anisotropy, heterogeneity, variable depth, strong shear wave velocity discontinuity at the top, and anticorrelation between shear and bulk sound velocities. High pressures and temperatures existing in planetary interiors dramatically alter the structure and properties of materials, but are difficult to reproduce in the laboratory. Present-day quantum-mechanical simulations often give accuracy sufficient for geophysical or planetological purposes. The key roles of the MgSiO$_{3}$ post-perovsike phase transition will be addressed by further theoretical studies, including elasticity and anelasticity, electrical conductivity, radiative conductivity, energetics of stacking faults, effects of impurities on stability and properties of post-perovskite.   

Supersolid $^4$He Likely Has Nearly Isotropic Superflow

We extend previous calculations of the zero temperature superfluid fraction $f_s$ (SFF) {\it vs} localization, from the fcc lattice to the experimentally realized (for solid $^4$He) hcp and bcc lattices. The superfluid velocity is assumed to be a one-body function, and dependent only on the local density, taken to be a sum over sites of gaussians of width $\sigma$. Localization is defined as $\sigma/d$, with $d$ the nearest-neighbor distance. As expected, for fcc and bcc lattices the superfluid density tensor is proportional to the unit tensor. To numerical accuracy of three-places (but no more), the hcp superfluid density tensor is proportional to the unit tensor. This implies that a larger spread in data on $f_s$, if measured on pure crystals, is unlikely to be due to crystal orientation. In addition, to three decimal places (but no more) the curves of $f_s$ {\it vs} $\sigma/d$ are the same for both the hcp and fcc cases. An expected value for the localization gives an $f_{s}$ in reasonable agreement with experiment. The bcc lattice has a similar curve of $f_s$ {\it vs} $\sigma/d$, but is generally smaller because the lattice is more dilute.   

2D Imaging of Anti-relaxation Coating Quality Using Regionally Specific Hyperfine Polarization

We show that the hyperfine polarization in the vicinity ($\sim 10^{-5}\,\rm cm$) of cell surfaces can be used as a novel way to quantify and map the regional surface property inside optical pumping cells. In silicone-coated cells our method allows us to image the regional quality of the coatings, revealing for the first time the existence of areas that have strikingly different qualities.   

Improved Guillemin-Zener wave function useful in molecular and chemical physics.

Classical Guillemin-Zener[1] wave function for primary diatomic molecular system -- molecular hydrogen ion have shown high accuracy and quality of exponential functions basis. We want to submit improved compact wave function useful in study quantum mechanic. Compact electronic wave function for ground state of molecular hydrogen ion is \textbf{$\Psi ={\rm N}*$(exp(-a1$*\xi )$+c*exp(-a2$*\xi )$ )$*$( cosh(-b1*$\eta )$+d*cosh(-b2*$\eta ))$ } where N - normalization factor, $\xi $ and $\eta $ are spheroidal coordinates of electron, a1,a2,b1,b2 -- nonlinear variational parameters, c and d - linear variational parameters. By optimization of energy functional we found optimal values of this parameters for any internuclear distance R. For example, at R=2 a.u.: a1= 1.4345, a2= 1.9753, b1= 0.5399, b2= 1.3001, c= -0.332876, d= 0.592279 . Electronic energy of molecular hydrogen ion with this 4-term wave function equals -1.10263418 a. u. and very close to the ``exact'' value -- 1.10263422 a.u. The comparison was shown that our improved wave function equivalent to about 25- term standard mathematical expansion in spheroidal coordinates. This function can be used in educational physics as well as for molecular theory, chemical physics, theory of few particle coulomb systems, etc. \begin{enumerate} \item V.Guillemin, C.Zener. Proc. Natl.Acad. Sci. U.S., 15, 314, (1929) \item T.K.Rebane and O.N.Yusupov. Opt. and spectr . 72, ¹6, 1289 (1992) \end{enumerate}   

Breathing modes and stability of trapped two-component ultracold atoms

The breathing modes and stability of trapped two-component ultracold atoms are studied using a variational method. We consider a boson-boson, a boson-fermion, and a fermion-fermion mixture in a 3D isotropic harmonic trap and in a 1D optical lattice. When the two components are miscible, the corresponding in-phase and out-of-phase breathing mode frequencies are calculated against the value of the inter-component interaction. The stability of the two-component system is shown to have strong correlation with the behaviors of breathing modes.   

Semiclassical time-independent description of rapidly oscillating fields on a lattice in the Kapitza approximation.

We investigate a semiclassical dynamics driven by a high-frequency ($\omega )$ field, plus a static arbitrary potential on a one-dimensional tight-binding lattice. We find -in the spirit of the Kapitza pendulum- an effective, time-independent potential $V_{eff} (x)$ that describes the average motion to order $\omega ^{-2}$. This effective potential depends on the static external potential $V(x)$, on the lattice constant ``$a$'' and on the applied high-frequency field $f(x,t)$. One obtains that \[ \frac{V_{eff} (x)}{m}=\frac{a^2}{2}V^2(x)-a^2EV(x)+a^4\int {dx} (V(x)-E)^2\frac{\partial }{\partial x}\left[ {\sum\limits_{n=1}^\infty {\frac{f_n ^2(x)}{\omega ^2n^2}} } \right]. \] Where ``$m$'' and ``$E$'' are, respectively, the effective mass and unperturbed energy of the particle's average motion, and $f_n (x)$ is the n-th Fourier component of the driving field. Where appropriate, our results should be suitable for the description of semiclassical electronic motion in a crystal lattice and/or atomic motion in an optical one.   

Role of Broken Gauge Symmetry in Transport Phenomena Involving Neutral and Charged Particles in Finite Lattices

As opposed to the conventional, approximate theory of electrical conduction in solids, which is based on energy band, “quasi-particle” states in infinite lattices, a rigorous theory exists that can be used to explain transport phenomena, in finite lattices, at reduced temperature, through the effects of a broken gauge symmetry: The loss of translational invariance with respect to Galilean transformations that maintain particle-particle separation. Implications of this result in areas related to the transport of hydrogen (H) and its isotopes in nano-crystalline structures of palladium (Pd) and of neutral, coherent atomic waves in finite, optical lattices are presented. These include: 1. The prediction of a novel variant of a known, phenomenon, Zener/Electronic Breakdown in insulators, in which ions in nano-scale palladium-deuteride (PdD) crystals (as opposed to electrons in insulating crystals) that initially, effectively, are confined to particular regions of space, begin to move, spread-out, and conduct charge after they are subjected to an applied, external (constant) electric field for a sufficiently long period of time; and 2. A rigorous treatment of scattering at low temperatures that can be used to identify critical time- and length- scales for problems related to the transport of neutral atomic matter waves in finite, optical lattices, in the presence of gravitational fields.   

Quantum-degenerate Bose-Fermi mixtures on one-dimensional optical lattices

Using a quantum Monte Carlo method, mean field theory, and a strong-coupling expansion, we map out the ground state phase diagram of a mixture of ultracold bosons and spin-polarized fermions in a one-dimensional optical lattice. The properties of the uniform phases and their stability toward phase separation into pure boson and pure fermion phases are studied in detail over a wide range of chemical potentials for boson-boson and boson-fermion interactions ranging from weak to strong coupling limits. The nature of different phases is characterized by computing the charge stiffness, bosonic superfluid stiffness, and density-density correlation functions.   

Computer Simulations of Quantum Computational Operations Performed on the Vibrational Energy Levels of Small Molecules

The vibrational energy levels of a molecule can be regarded as a register for quantum information. An applied electric field (provided by a shaped laser pulse) will transform the state vector of the molecule in a manner that can be regarded as a quantum computational operation. Optimization algorithms have been developed and computer simulations have been run to determine the optimum laser pulses to act as specific operators. Simulations of 1-, 2-, 3-, and 4-qubit gates have been run using 2-, 4-, 8-, and 16-level tracts of vibrational energy levels, respectively. Simulations have been generated to model a realistic experiment using available technology. In the case of thiophosgene (SCCl$_{2})$, vibrational levels on the ground electronic state may be coupled via one or several energy levels on an excited electronic state. The coupling is provided by a single femtosecond laser pulse, which is shaped according to parameters consistent with available pulse shapers. Within these parameters, pulses have been simulated that perform gates such as the CNOT (4 levels) and Hadamard (2 levels) with fidelities over 95 {\%}. The limiting factors for efficacy of the shaped pulse in performing the gate---such as the number of ground state vibrational levels involved---have been explored.   

Autler-Townes splitting in a sodium molecular-ladder scheme

We report results from studies of the Autler-Townes (AT) splitting observed in sodium dimer using optical-optical double resonance spectroscopy. In our experiment, a single line Ar$^+$ laser at 4727 $\AA$ pumps the thermally populated ground state level $X\,^1\Sigma_g^+(v''=1,\,J''=37)$ of Na$_2$ to the intermediate $B\,^1\Pi_u(v'=9,\,J'=38)$ level. Then, a single mode cw Ti:sappher or dye laser couples to the higher Rydberg states. The signals are detected by monitoring the UV fluorescence from the triplet \emph{gerade} states back to the $a\,^3\Sigma^+_u$ state via collision energy transfer. Unlike all the other intermediate levels pumped by different Ar$^+$ laser lines, this strong transition populated the $B\,^1\Pi_u (v'=9,\,J'=38)$ level leads to an AT splitting on all the levels observed in the higher Rydberg states, for example, 3, 5, 6, 7\,$^1\Pi_g$ states and 3, 5, 6\,$^1\Delta_g$, states.   

Sensitive detection of lead monofluoride

The ground-state of lead monofluoride maybe uniquely sensitive to an electron electric dipole moment. Here we report on a radical-beam source of PbF and sensitive resonance-enhanced-multiphoton ionization detection of the molecule.   

Simple Analytic Theory of Cold Atom Feshbach Resonance Scattering

Magnetically tunable Feshbach resonances have been used very successfully in cold atomic gases to study a variety of condensed matter phenomena. We describe a simple analytic theory, in excellent agreement with full quantum scattering calculations, for the near-threshold resonant scattering 2-body T-matrix for magnetically tunable Feshbach resonances in ultracold atomic collisions. The theory is based on the analytic properties of the exact solutions to the Schrodinger equation for the van der Waals potential, and is characterized by 5 parameters: the scattering length, van der Waals coefficient, and reduced mass of the background entrance channel, the coupling width of the resonance, and the difference in magnetic moments between the separated atoms and the resonance level. The resonance scattering phase shift is completely characterized by two functions, an energy-dependent width and an energy-dependent shift, which are analytic functions of the background van der Waals potential. The theory permits a simple classification of resonances as open- or closed-channel dominated, and gives insight into the nature of atom pairing in scattering states. The excellent quality of the theory is illustrated by calculations of above-threshold scattering for the fermionic isotopes K-40, and Li-6 and for bosonic Rb-85.   

Role of the van der Waals interaction in atom-diatom reaction dynamics at low temperatures

Quantum-mechanical scattering calculations are reported for the $\mbox{O}(^3P)+\mbox{H}_2$ collision at energies close to the reaction threshold with emphasis on the sensitivity of the reaction dynamics to the van der Waals interaction. The dynamics has been investigated using the lowest $^3 A''$ GLDP potential energy surface developed by Rogers et al. [J. Phys. Chem. A {\bf 104}, 2308 (2000)] and its recent BMS1 and BMS2 extensions by Brand\~{a}o et al. [J. Chem. Phys. {\bf 121}, 8861 (2004)] which explicitly include the van der Waals interaction. Quasiclassical trajectory calculations on all three potential energy surfaces are also reported to explore the validity of this method near the reaction threshold and to assess the importance of quantum effects at low temperatures.   

Liouville-Space Descriptions for Intense-Field Coherent Electromagnetic Interactions

Liouville-space (reduced-density-operator) descriptions are developed for coherent electromagnetic interactions of quantized electronic systems, taking into account environmental decoherence and relaxation phenomena. Applications of interest include many-electron atomic systems and semiconductor nanostructures. 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. Intense-field electromagnetic interactions are treated by an alternative reduced-density-operator approach based on the Liouville-space Floquet-Fourier representation.   

High Intensity Ultrashort Pulse Interactions with Dielectric Filters

Modern ultrafast laser systems are capable of generating optical pulses of sufficient intensity to induce nonlinear effects in several of the materials used in thin-film dielectric optical filters. Such effects degrade the performance of these devices at high intensities. In this paper we describe computer simulations used to investigate the interaction of high intensity ultrashort laser pulses with three dielectric filters: a quarter- wave dielectric stack, a rugate filter, and an idealized rugate filter, with a continuously varying refractive index. Using the finite-difference time-domain (FDTD) technique, we simulated the reflection of optical pulses (with pulse widths in the range 5--100 fs and peak intensities up to $\sim10$ TW/cm$^{2}$) from all three devices. The simulated low intensity reflective properties were found to be in excellent agreement with calculations using the characteristic matrix method. For high intensity pulses a decrease of the filter reflectance and a distortion of the reflected pulse was observed.   

Study of ultrafast nonlinear pulse shaping effect of strained saturable Bragg reflector (SSBR) with coherent quantum control technique

In this work, a strained saturable Bragg reflector (SSBR) for passive mode-locking of Ti:sapphire lasers was investigated by pulse shaping technique. Incident pulses of several wavelengths and with zero, positive, or negative chirp were employed. A Kramers-Kronig relation like behavior of pulse shaping factor in the strained quantum well was observed. The phenomenon is attributed to anomalous dispersion and higher order nonlinearity.   

Nearly Perfect Squeezing of the Signal mode in Parametric Oscillation with Coherent and Squeezed pumping

A degenerate parametric oscillator operating above threshold is studied when the cavity is injected by a squeezed vacuum field at the second harmonic frequency. We presented a different method of finding the Wigner function for the intracavity modes. We use the solution of the quantum Langevin equation for the signal and pump modes to construct the steady-state Wigner function for the joint pump-signal mode. Then, the one-mode Wigner functions for the signal and pump modes are derived and the corresponding phase space is studied. These functions are used to determine the quantum fluctuations in the intracavity signal and pump mode field quadratures. The results have shown that nearly perfect suppression of quantum noise can be achieved in both the signal and pump modes. We have also studied the spectrum of the squeezing for cavity output signal mode; our result reveals nearly perfect squeezing can be achieved by controlling the relative cavity damping rates between the signal and pump modes. The second-order intensity correlation for zero time delay also studied for each mode and it is found that intracavity photons are always bunched, independent of the relative phase of the squeezed vacuum field with respect to the driving coherent field.   

Time-dependent phenomena of electron-electron and electron-hole pairs confined in quantum dots

We report time-dependent (TD) phenomena of electron-electron (ee) and electron-hole (eh) pairs confined in 2D quantum dots (QDs). We solved TD Schrodinger equation numerically, under the assumption of the Hartree-type wave function with the singlet spin state for both pairs of ee and eh. The individual (e's and h's) QD orbitals are expanded by the basis of the real spatial mesh. We further combined Poisson's equation to determine the pair's coulomb interaction in QDs. The stationary ground states are first obtained by the scf-calculation. In accordance with the QD size, both of ee and eh pairs are characterized by confined, intermediate and free states, and the difference among them is more distinctive in ee pairs than in eh pairs. The coulomb repulsion calculated by Poisson equation extends the confined region, because of its logarithmic distance-dependence due to 2D QD. The TD change in the coulomb interaction induces the non-resonative oscillations among the individual eigen-states. The projection of the calculated pair's wave function into the QD single electron stationary states reveals that the resulting frequency is determined with the competition between the coulomb interaction and the size of QDs.   

Physical activities to enhance an understanding of acceleration

On the basis of their everyday experiences, students have developed an understanding of many of the concepts of mechanics by the time they take their first physics course. However, an accurate understanding of acceleration remains elusive. Many students have difficulties distinguishing between velocity and acceleration. In this report, a set of physical activities to highlight the differences between acceleration and velocity are described. These activities involve running and walking on sand (such as an outdoor volleyball court).   

Motion Sensors in Introductory Physics Laboratory and beyond.

Motion sensors are commonly used to collect data in a typical computer-based introductory physics laboratory. It is part of the group of sensors that comes with an initial purchase of an interface. It is an important piece of equipment for the first year physics curriculum. It is used in position, velocity, and acceleration experiments as well as in momentum conservation, impulse-momentum theorem, and simple harmonic motion. In this paper its use will be expanded to few more experiments and lecture demonstrations pointing out its limitations and some of the pitfalls. A review of the websites that deal with motion sensors and computer assisted experiments will also be presented. In addition, their use in alarm and automation systems and surveillance technology will be introduced as an extrapolation to real world applications.   

Using the Open Source Physics Library to Teach Statistical and Thermal Physics

Statistical and thermal physics is becoming an increasingly important part of the undergraduate and graduate curriculum due in part to the the availability of inexpensive and powerful computational resources and the importance of statistical concepts in much of current research in physics and related areas. We will show examples of curriculum materials for teaching topics in statistical and thermal physics with a focus on Java programs written using the Open Source Physics library. Examples of simulations will be demonstrated.   

Propagation of a pulse trough a Lorentz dispersive thin film using the Finite Difference Time Domain

We consider the evolution of the main signal in a Lorentz dispersive thin film. The signal is a Gaussian-sine modulated pulse that is excited outside the thin film. We illustrate the change of the group velocity as the pulse changes its central frequency and we discuss the conditions to have a superluminal behavior. The results obtained are illustrated with animated graphics that obtained with our implementation of the Finite Difference Time Domain.   

Including Nanoscale Investigations in a General Introductory Physics Course

Modules were developed to introduce atomic force microscope (AFM) applications into a general introductory physics course. The goal was to elucidate fundamental concepts in optics at the nanoscale that would complement existing investigations at the macroscale, and to expose students to advanced instrumentation at an early level. The nanoscale investigations were inserted where necessary and were used to help define and delineate the various topics in optics. In a lab on lenses, students first use the AFM to view smooth glass and rough glass at the microscopic level, illustrating the range of applicability of geometrical optics. In a physical optics lab, students measure patterns from diffraction gratings and then view the grating at the microscopic level, revealing groove spacings and a new feature, the blaze angle. Further investigation yields from the blaze angle a way to separate the effects of the single slit diffraction envelope from the multiple slit interference pattern. Another lab investigates the height of the bumps on a CD surface and exposes students to a direct application of destructive interference. These and other examples will be presented.   

Deposition and Solidification of PPV Precursor

Future high performance polymer films will be comprised of molecules oriented in the plane of the substrate. As a first step, a mixture of poly(p-xylene tetrahydrothiophenium chloride) in methanol was spun onto a sapphire substrate. These samples were fabricated over a range of spin-coating times, methanol solution concentrations, and number of drops of solution. Analysis with an ellipsometer revealed that layer thickness is independent of spinning period and applied solution volume, and decreases as the parts per volume of methanol increases. Furthermore, the samples were exposed to various wavelengths of UV-visible radiation to determine the optimal wavelength for driving these precursor molecules into a solid thin film, insoluble in methanol. Light from a mercury arc-lamp with a wavelength of 365 nm was found to solidify the precursor within five minutes of exposure most effectively.   

Complex Permittivity of Powder Metal Compacts by Cavity Perturbation Technique

We present measurements of the complex permittivity of powder metal compacts using microwave cavity perturbation techniques. Using a 2.45GHz, TM$_{010}$ microwave cavity operating in conjunction with a vector network analyzer, we have systematically measured the real ($\varepsilon $') and imaginary ($\varepsilon $'') parts of the effective complex permittivity of pure, powder metal, cylindrical samples (0.25in by 0.25in). By placing these in the electric-field antinode, the dielectric properties at microwave frequencies are obtained by comparison with the expected change in the cavity Q by perturbation theory. We have studied the effect of varying particle size and green density as means to understand the absorption and heating of powdered metals in microwave fields. We acknowledge the additional work of undergraduates Kelly Martin and Charles Smith, who assisted in the setup of experimental apparatus, sample preparation, and data acquisition.   

Microwave Heating and Pre-sintering of Copper Powder Metal Compacts in Separated Electric and Magnetic Fields

We present results of microwave heating and pre-sintering of pure copper metal powder compacts. Using a 2.45GHz, WR284 microwave system operating in TE102 single mode resonance, we have systematically studied the microwave heating and pre-sintering behavior of various copper powder metal compacts as a function of particle size and green density. Cylindrical samples (0.25in by 0.25in) were positioned in either the magnetic-- or electric--field antinode, allowing the study of the separate effects of the two fields. The results show significant differences in heating rates and sample microstructure (SEM) even when average sample temperatures are below half the melting point of bulk copper. Numerical simulations of the absorption and heating have been developed to check the consistency of the experimental results. We acknowledge the additional work of undergraduate students John Diehl, John Rea, Charles Smith, and Devin Spratt, who assisted in the setup of experimental apparatus, sample preparation, and data acquisition.   

Expected Properties and Experimental Signals of Bose-Einstein Condensates

We have studied the expected response of Bose Einstein Condensed (BEC) atoms subject to various experimental probes. In the first part of this project, we modeled BEC atoms passing through a double slit and predicted the interference patterns that are expected to form on the screen. We contrast these interference patterns with those from thermal atoms and compare the patterns from condensed atoms with varying levels of inter-atomic interactions. This gives an experimental signature of the presence of BEC in the system as well as the strength of interactions. The second part of our project considers the response of BEC atoms trapped in an optical lattice, subject to an external oscillatory magnetic field. This system is modeled as having two discrete hyperfine energy states and we find the expected transitions between these states for the BEC atoms. The transitions are found to be similar to the Rabi oscillations observed with non-BEC atoms, subject to the same probe, with some notable differences.   

Variational Wavefunction Monte Carlo on Two-Dimensional Lattices

We apply numerical methods to the study of many-electron systems, in particular the cuprates, by using variational wavefunction Monte Carlo methods to search for the groundstate wavefunction. We investigate some of the current proposed models and wavefunctions for these compounds, in particular, the t-J model and the t-J model with ring-exchange on two- dimensional square lattices. We compute the energy of the Spin Density Wave (SDW) groundstate wavefunctions for the half- filled Heisenberg antiferromagnetic using the computing algorithm outlined by Gros . This verifies the agreement of the results with the current literature on the Heisenberg model. With the addition of the ring-exchange terms, we compare the energies of the Resonating Valence Bond state, the Gutzwiller state, the superconducting state, and the SDW state. This allows us to study the relationship between the non-magnetic and antiferromagnetic phases of the system at half- filling. The introduction of holes on the lattice allows us to simulate the relationship between the superconducting and the insulating phases of the magnet.   

A Fluorescence Spectroscopic Study of Cytochromes P450 1A2 and 3A4.

Fluorescence spectroscopy was used to study cytochromes P450 1A2 and 3A4. Spectra of P450s were acquired in the presence and absence of acrylamide quencher. In both P450s, quenching revealed three distinguishable species of amino acid fluorescence, with maxima at 297, 323, and 345 nm. The 345 nm tryptophan fluorescence was quenched by low levels of acrylamide; the 297 nm tyrosine fluorescence was resistant to quenching. The 323 nm fluorescence was observed at intermediate concentrations of quencher. Stern-Volmer plots of P450 quenching were non-linear, but were well-fitted to a superposition of linear plots for each fluorophore species. The effect of P450 1A2 binding on pyrene fluorescence was also examined. Upon binding to P450 1A2, the intensity of the 383 nm pyrene vibronic band was decreased relative to the intensities of the 372 and 393 nm bands. Fluorescence quenching of pyrene and other ligands upon binding to P450s will be used to evaluate distances between ligands and the P450 heme moiety by fluorescence resonance energy transfer. Fluorescence quantum yields of ligands, overlap integrals, and F\"{o}rster distances of many ligand-heme donor-acceptor pairs were calculated. Steady-state spectra and time-resolved data of bound ligand will be used to calculate substrate-heme distances in the P450 enzymes.   

Ferroelectric switching of KDP (KH$_{2}$PO$_{4})$ and doped KDP:Cr simultaneously studied by the epr and Mertz methods

Preliminary experimental results on ferroelectric switching in KDP will be presented. Additionally, a modified closed cycle cryogenic system for epr will be presented. Our goal is to reconcile differences in (epr) spectroscopy and the Mertz method on the simple ferroelectric, KDP/KH$_{2}$PO$_{4}$. A closed cycle cryogenic system, capable of reaching temperatures well below the Curie point of KDP (123K) was modified to accommodate the two measurements. The epr measurements probe the bulk of the KDP and favors slow ferroelectric domain switching while the Mertz method probes the surface and favors fast switching. A compromise in switching time between epr and Mertz is necessary. Preliminary results show the expected splitting in the epr lines near 1.45 kG. The Mertz data show switching currents consistent with the nucleation and domain growth model. The switching times are on the order of 20 microseconds for temperatures of approximately 100K. In addition to doped KDP we plan to investigate irradiated KDP and DKDP.   

Quantum amplification in the presence of decoherence

A one-dimensional Ising chain irradiated by weak resonant transverse field is a simple analytically solvable model of quantum amplification [Phys. Rev. A \textbf{71}, 062338 (2005)]. Operation of any practical device would be affected by decoherence. Here we present the results of the effect of decoherence on amplification dynamics. The dynamics of a 100-spin chain perturbed by random phase kicks of individual spins has been studied numerically. Time evolution of the total polarization is monitored for different strengths of decoherence. It is shown that decoherence decreases the maximum coefficient of amplification in this model. In the limit of strong decoherence, quantum dynamics resembles classical diffusion.   

Quantum manipulation and simulation using Josephson junction arrays

We discuss the prospect of using quantum properties of large scale Josephson junction arrays for quantum manipulation and simulation. We study the collective vibrational quantum modes of a Josephson junction array and show that they provide a natural and practical method for realizing a high quality cavity for superconducting qubit based QED. We further demonstrate that by using Josephson junction arrays we can simulate a family of problems concerning spinless electron-phonon and electron-electron interactions. These protocols require no or few controls over the Josephson junction array and are thus relatively easy to realize given currently available technology.   

Effect of correlated noise on a fault-tolerant quantum error correction protocol

We consider the effect of correlated noise in the context of a fault-tolerant quantum error correction protocol. The noise is represented by a set of classical fluctuating fields with partial spatial and temporal correlations. We explicitly account for the propagation of errors in the implementation of quantum circuits for ancilla verification and syndrome extraction. Errors arising from single-bit and two-bit gates are considered separately. The performance of the error correction protocol is characterized by means of the probability for an uncorrected error to occur calculated from numerical simulations of the error propagation. For single-bit gates we find that in the limit of strong correlations the crash probability is enhanced be an order of magnitude. For two-bit gates we find that the effect of correlated noise can be minimized by choosing an appropriate sequence of operations which takes advantage of the correlations.   

Modeling fluid dynamics on type II quantum computers

A quantum algorithm is presented for modeling the time evolution of density and flow fields governed by classical equations, such as the diffusion equation, the nonlinear Burgers equation, and the damped wave equation. The algorithm is intended to run on a type-II quantum computer, a parallel quantum computer consisting of a lattice of small type I quantum computers undergoing unitary evolution and interacting via information interchanges represented by an orthogonal matrices. Information is effectively transferred between adjacent quantum computers over classical communications channels because of controlled state demolition following local quantum mechanical qubit-qubit interactions within each quantum computer. The type-II quantum algorithm presented in this paper describes a methodology for generating quantum logic operations as a generalization of classical operations associated with finite-point group symmetries. The quantum mechanical evolution of multiple qubits within each node is described. Presented is a proof that the parallel quantum system obeys a finite-difference quantum Boltzman equation at the mesoscopic scale, leading in turn to various classical linear and nonlinear effective field theories at the macroscopic scale depending on the details of the local qubit-qubit interactions.   

Optimization of a quantum teleportation protocol based on collective spontaneous emission

Recently a quantum teleportation protocol has been proposed by Chen, {\it et al.} in {\it New Journal of Physics} {\bf 7}, 172 (2005) which is based on the collective spontaneous emission of a photon from a pair of atoms. If one can successfully distinguish between the superradiant and subradiant emission channels then one can teleport the state of the second atom onto a cavity field mode with which the first atom has previously interacted. One can employ temporal resolution, spatial resolution, or both in combination, of the emitted photon in order to distinguish superradiant from subradiant emission on the basis of a single detected photon. The overall success probability of the teleportation protocol is calculated under all three detection strategies and is optimized with respect to the spacing of the two atoms.   

Measuring the parity of an $N$-qubit state

We present a scheme for a projective measurement of the parity operator $P_z=\prod_{i=1}^N \sigma_z^{(i)}$ of $N$-qubits. Our protocol uses a single ancillary qubit, or a probe qubit, and involves manipulations of the total spin of the $N$ qubits without requiring individual addressing. We illustrate our protocol in terms of an experimental implementation with atomic ions in a two-zone linear Paul trap, and further discuss its extensions to several more general cases.   

A Variation of the Classic Double-Slit Experiment in Quantum Mechanics

The classic double-slit experiment in quantum mechanics was adapted previously to show that a distinct type of physical interaction between an atom with the double-slit screen through which it passes is not necessary to obtain which-way information concerning the atom. This adaptation relied on entanglement between: 1) an atom's emitting a photon in one of two micromaser cavities as the atom passes through the cavity system and 2) the atom's subsequent passage through the fixed double-slit screen. If the development of this entanglement is interrupted in a suitable fashion, it appears one can obtain complete interference as if there were no micromaser cavity system or laser through which the atom traveled on its way to the double-slit screen. It appears one can alter the development of an atomic distribution through an action that does not involve direct physical interaction with atoms that become part of the distribution. Distinct atomic distributions can be created that can convey binary information, apparently without the velocity limitation of the special theory.   

Circular Polarization and Quantum Spin: A Unified Real-Space Picture of Photons and Electrons

It is generally believed that no simple real-space semiclassical picture can consistently explain both a quantum wave and its spin. However, it is shown here [1] that a rotating vector field carrying angular momentum leads directly to the Einstein-de Broglie relations (E = h$\nu $ and p = h/$\lambda $, the heart of quantum mechanics), assuming only quantization of spin and Lorentz invariance of the phase angle. For electromagnetism, such a circularly polarized wave packet defines the photon. A very similar picture of a massive rotating vector field maps onto a complex wavefunction obeying the time-dependent Schr\"{o}dinger equation for a particle such as the electron, with m$>$0 and v$<<$c. This real vector field rotates about the spin axis at mc$^{2}$/h $\sim $ 10$^{20}$ Hz. This suggests a unified picture whereby all fundamental quantum particles consist of such coherent wave packets of rotating spin fields, with composite particles deriving their quantum properties from the coherent rotations of their fundamental constituents. This represents a consistent interpretation of quantum mechanics, an alternative to the conventional statistical or Copenhagen interpretation. [1] A.M. Kadin, http://arxiv.org/abs/quant-ph/0508064   

Paradox in Wave-Particle Duality for Non-Perturbative Measurements

In a modified double-slit experiment we determined the complementary wave and particle aspect of light in the low flux regime beyond the limitations set by Bohr's Principle of Complementarity (BPC). We inferred the presence of sharp interference fringes, while we maintain, with very high probability, the information about the particular pinhole through which each individual photon had passed. This paradoxical result poses interesting questions on the validity of BPC in cases, which employ non-perturbative measurement techniques that evade Heisenberg's uncertainty principle and/or quantum entanglement.   

Interconvertibility of single-rail optical qubits

We show how to convert between partially coherent superpositions of a single photon with the vacuum using linear optics and postselection based on homodyne measurements. We introduce a generalized quantum efficiency for such states and show that any conversion that decreases this quantity is possible. We also prove that our scheme is optimal by showing that no linear optical scheme with generalized conditional measurements, and with one single-rail qubit input can improve the generalized efficiency.   

Magnetic Nanoparticles \textit{in-vivo} Detection of Transplant Rejection

Superparamagnetic nanoparticles are being used to develop methodology for the \textit{in-vivo} detection and imaging of immune system attacks on transplanted organs. The signature for impending rejection of a transplant is enhanced presence of T-cells. Magnetic nanoparticles coated with specific antibodies (CD-2 and CD-3) will target and attach to these T-cells. Approximately 3 $\cdot $10$^{5}$ nanoparticles can attach to each cell. When a pulsed external magnetic field is applied to the decorated cells for a fraction of a second, magnetic moments of the nanoparticles aligned with the field. After the pulse is switched off, the net magnetic moment decays over several seconds by the N\`{e}el mechanism. The resulting magnetic remanence field (typically 10$^{-11}$ T) is measured using a multi-channel SQUID array. We present the data from live T-cells placed in realistic kidney phantom. The detection sensitivity was $\sim $2$\cdot $10$^{3}$ T-cells - a small fraction of the number actually invading the rejected transplant. The 7-channel SQUID array allows us to image the cell clusters with a few millimeters resolution.   

The silicon di-vacancy investigated with density functional theory

As part of a larger effort to quantitatively model the behavior of point defects in silicon, we have investigated the silicon di-vacancy using density functional theory with three different exchange-correlation functionals and super-cells containing as many as 1000 atoms. The literature on this defect contains two proposed atomic structures and disagreement on the relative stability of the configurations. Our study indicates that the determination of the ground state configuration is dependant on the particular choice for exchange and correlation. We will report on the atomic and electronic structures of this defect as well as formation.   

The Effect of Nanoparticles on Selective Gas Permeability of Thin Film Membranes Using Supercritical Fluid

The addition of inorganic nanoparticles to polymer thin films can be highly beneficial to their lubrication, strength, and UV resistance. Since nanoparticles are rigid, they do not conform to the molecular order of the film, producing nano-scale voids. Consequently, the introduction of nanoparticles into polymer films alters their porosity and this phenomenon may be exploited to engineer selectively permeable membranes. Porosity can also be introduced by swelling the films in supercritical fluids and then rapidly removing the solvent by decreasing the pressure which converts the solvent to the gas state. In order to test this hypothesis, solutions were made of nanoparticles, clay or gold, with polystyrene or poly (methyl methacrylate) polymer to observe the change in porosity and to analyze its dependence on polymer structure and on polymer nanoparticles interactions. Samples were exposed to ScCO$_{2}$ at two different temperatures and pressures of 36\r{ }C, 1200psi and 50\r{ }C, 1450psi. We then measured the permeability of O$_{2}$ and CO$_{2}$ gas. We found that exposure to supercritical CO$_{2}$ greatly increased the permeability to both O2 and CO2 gas. The degree of change in permeability in the nano-composite film depended on the particle aspect ratio and intensity in the matrix.   

Accurate Determination of Pyridine -- Poly (Amidoamine) Dendrimer Absolute Binding Constants with the OPLS-AA Force Field and Direct Integration of Radial Distribution Functions

OPLS-AA force field and direct integration of intermolecular radial distribution functions (RDF) were employed to calculate absolute binding constants of pyridine molecules to NH$_{2}$ and amide group hydrogen atoms in 0th and 1st generation poly (amidoamine) dendrimers in chloroform. The average errors in the absolute and relative association constants, as predicted with the calculations, are 14.1{\%} and 10.8{\%}, respectively, which translate into ca. 0.08 kcal/mol and 0.06 kcal/mol errors in the absolute and relative binding free energies. We believe that this level of accuracy proves the applicability of the OPLS-AA, force field, in combination with the direct RDF integration, to reproducing and predicting absolute intermolecular association constants of low magnitudes (ca. 0.2 -- 2.0 range).   

Cell-fibril interaction in peptide based Hydrogels relative to hydrogel stiffness

Peptide hydrogels are potentially ideal scaffolds for tissue repair and regeneration due to their ability to mimic natural extra cellular matrix. The 20 amino acid peptide MAX1 has been shown to fold and self-assemble into a rigid hydrogel based on environmental cues such as pH, salt, and temperature. The hydrogel is composed of network of short fibrils that are 3nm wide and up to several hundred nm long. In addition, slight design variations in the arms of the MAX1 sequence allow for tunability of the self-assembly/hydrogelation kinetics. In turn, by controlling hydrogel self-assembly kinetics, one dictates the ultimate stiffness of the resultant network. The cell-material interaction in normal and pathological conditions is investigated by 2D and 3D cell culture. As shown by optical and laser scanning confocal microscopy, cells are viable for 3 weeks and grow in clonogenic spheroids. Characterization of the proliferation, differentiation and constitutive expression of various osteoblastic markers is performed relative to hydrogel stiffness using spectrophotometeric methods. The well-defined, fibrillar nanostructure of the hydrogel directs the attachment and growth of osteoblast cells and dictates the mineralization of hydroxyapatite in a manner similar to bone. This study will enable control over the interaction of cellular systems with the peptide hydrogel for biomedical applications.   

Electronic structure calculations of the CePt3Si.

In a recent paper, Bauer \textit{et al}. [1] synthesized CePt3Si, a heavy fermion material with both transitions magnetic and superconducting, without inversion center in the crystal structure. This compound is very interesting from many points of view. In the present work, we report the analysis of the electronic structure of the CePt3Si in normal state. The calculation was performed with the program package WIEN2k, which use density functional theory (DFT) based on the full-potential linearized augmented plane-wave (LAPW) plus local orbital (lo) method [2]. We report band structure, total and partial densities of states, 3D density of charge and Fermi surface. We analyze the possible implications of our results in the superconductivity of this material. [1] Phys. Rev. Letter \textbf{92}, 027003 (2004). [2] http://www.wien2k.at   

On the origin of the peak effect in the classical superconductor Nb$_{3}$Sn: the role of critical fluctuations

We found a sharp pronounced peak effect in the magnetization and the isothermal magnetocaloric effect of a single crystal of the classical superconductor Nb$_{3}$Sn. The construction of a new type of isothermal calorimeter allowed us to study the underlying thermodynamics with an extremely high resolution. A small lambda-type anomaly could be resolved superimposed on the specific heat jump at $T_{c}$. It follows scaling laws as expected for the presence of critical fluctuations. We interpret this feature as the first clear observation of the regime of critical fluctuations in a classical superconductor. Simultaneous measurements of the specific heat and the isothermal magnetocaloric effect reveal that the peak effect occurs at the onset of fluctuations below the $H_{c2}$ line. No indications of a thermodynamic transition as the origin of the peak effect could be found. Our investigation favours thus rather a scenario in which a vortex lattice looses its topological order more continously due to the increasing strength of thermal fluctuations upon approaching the $H_{c2}$ line.   

Measurement of Local Reactive and Resistive Photoresponse of a Superconducting Microwave Device

Despite the voluminous work on the nature of nonlinear effects in high-temperature superconductors (HTS), the causes are not completely clear and remain under debate. The Laser Scanning Microscope (LSM) is a spatially-resolved method that can simultaneously measure optical and high frequency properties of HTS devices. Earlier results showed high resolution images of non-uniform microwave current distributions near the edge of a patterned transmission line structure [A. P. Zhuravel, A. V. Ustinov, K. S. Harshavardhan, and S. M. Anlage, Appl. Phys. Lett. \textbf{81}, 4979 (2002)]. We have developed a new operational mode in which the microscope separately images the resistive and inductive components of the bolometric photoresponse. The two images show interesting and dramatic differences, leading to new insights about linear and nonlinear properties of HTS microwave devices.   

$^{11}$B NMR Measurements of TbB$_4$

$^{11}$B pulsed NMR measurements have been performed to investigate local electronic structure and 4f spin dynamics for TbB$_4$ single crystals. $^{11}$B NMR spectrum shows a broad peak due to the 4f local moment. $^{11}$B NMR shift and linewidth are huge and strongly temperature-dependent. In addition, both are proportional to magnetic susceptibility, indicating that the hyperfine field at the boron site originates from the 4f spins of Tb. The shift and the spin- lattice relaxation rate show high anisotropy for field parallel and perpendicular to the c-axis. Anisotropy of the shift and the relaxation rates are analyzed with the results of magnetization to understand the microscopic details of anisotropic spin dynamics.   

$^{27}$Al NMR Measurements of YMn$_4$Al$_8$

We have performed $^{27}$Al nuclear magnetic resonance (NMR) measurements on the single crystals of YMn$_4$Al$_3$. $^{27}$Al NMR spectrum, Knight shift, spin-lattice and transverse relaxation rates, $1/T_1$ and $1/T_2$, were measured down to 4 K at 8 T. Experimental results of the Knight shift and the nuclear spin lattice relaxation rate as well as magneto-resistance and susceptibility are analyzed and discussed in conjunction with the pseudogap behavior in the spin excitation spectrum.   

On a partial differential equation for determining the free energy of ternary mixtures from light scattering data

With use of an approximation appropriate for liquid mixtures, we formulate a well- established relation for light scattering from ternary mixtures as a second-order nonlinear partial differential equation, which relates the inverse Hessian of the intensive free energy to the efficiency of light scattered near the forward direction. We examine the mathematical conditions under which light scattering data can be a suitable input for solving this equation, thereby to determine the free energy in a nearly model-independent fashion. We find that within the thermodynamically stable and metastable regions of the phase diagram, composition curves that are nearly perpendicular to the gradient of the dielectric constant are also not what are termed characteristic for the light scattering partial differential equation. This criterion, in turn, implies that free energy solutions in the neighborhood of a such a composition curve can be self-consistently constructed by combining a known free energy gradient along the curve with light scattering data in the surrounding region. These considerations suggest that an experimental method comprising light scattering in the ternary composition triangle, combined with other means of finding the needed boundary conditions along a suitable curve, should in principle be capable of determining ternary mixture free energies without adopting specific free energy models in advance.   

Direct microscopic observations of fluctuations near critical point

A series of experiments were performed on in orbit to study boiling, phase separation and fluctuations taking place in pure fluids (SF6) near critical point. A specially designed apparatus (ALICE 2) allowed a precise control of fluid's temperature. Local density fluctuations were observed by illuminating a cylindrical cell filled with the pure fluid near its liquid-gas critical point using a microscope and a video recorder. The apparatus was placed in orbit where there is no gravitational limitation in the size of the fluctuations. Using image processing techniques, we were able to estimate properties of the fluid from the recorded images showing fluctuations of the transmitted and scattered light. We found that the histogram of the image can be fitted by a Gaussian and by determining its width we were able to estimate these properties. We also estimated the wave number corresponding to the maximum of the radial average of the power spectrum and the corresponding characteristic length of the fluctuations.   

Quantum Phase Transitions and Entanglement in the Detuned Dicke Model

The quantum phase transition of mono-mode Dicke models with two atoms is discussed and analytical results are presented. For arbitrary detuning, the exact spectrum and the concurrence of entanglement are calculated for the two-atom system. It is shown that sequential quantum phase transitions occur in this system. Evidences have been found to support that quantum phase transition and concurrence are uncorrelated in this particular model. Furthermore, the results of quantum phase transition with extra mode are also discussed.   

Disorder and size effects in Kondo lattice nanoparticles

The evolution of Kondo interactions and magnetic order with size variation was revealed by the specific heat and magnetic susceptibility in a series of CePt$_{2}$ Kondo lattice nanoparticles with various sizes. In addition to the magnetic order suppression with size reduction, two regimes of size dependence of Kondo temperature T$_{K}$ was observed. As size d decreases from the bulk down to 22 nm firstly T$_{K}$ slightly decreases from 4.6 K to 3.7 K, as d further decreases, inversely T$_{K}$ increases up to 100 K for size d=3.8 nm. It is confirmed that the size-induced lattice disorders are responsible for the result for d $\ge $ 22 nm, while the rapid increase of T$_{K}$ for d =3.8 nm is attributed to the lattice contraction and quantum size effects.   

Potential use of combining the diffusion equation with the free Shr\"{o}dinger equation to improve the Optical Coherence Tomography image analysis

Optical coherence tomography (OCT) is a rapidly emerging medical imaging technology. In ophthalmology, OCT is a powerful tool because it enables visualization of the cross sectional structure of the retina and anterior eye with higher resolutions than any other non-invasive imaging modality. Furthermore, OCT image information can be quantitatively analyzed, enabling objective assessment of features such as macular edema and diabetes retinopathy. We present specific improvements in the quantitative analysis of the OCT system, by combining the diffusion equation with the free Shr\"{o}dinger equation. In such formulation, important features of the image can be extracted by extending the analysis from the real axis to the complex domain. Experimental results indicate that our proposed novel approach has good performance in speckle noise removal, enhancement and segmentation of the various cellular layers of the retina using the OCT system.   

Simulation of Xe$_{n}$Ar$_{m}$ Cluster Formation in a Molecular Beam: Comparison with Photoelectron Spectroscopy

We perform direct MD simulations of the formation of mixed Xe$_{n}$Ar$_{m}$ clusters (500$<$n+m$<$3000) in a supersonic beam as a function of initial beam conditions. We then model the 4d$_{5/2}$ (Xe) and 2p$_{3/2} $ (Ar) core hole photoelectron spectra of these clusters and compare them to the experimental spectra of Tchaplyguine \textit{et al}[1]. The predicted spectra are calculated as the sum of final state energy shifts of the ionized atoms (within the cluster) relative to the isolated gas phase ion using a self-consistent polarization formalism. We use the results of our earlier calculations on pure argon and xenon clusters [2] to determine the appropriate inelastic mean free path value for the signal electrons leaving the mixed clusters. These results allow us to gain a refined understanding of the size, stoichiometry, and core/shell structure of these mixed clusters. [1] M. Tchaplyguine, \textit{et al}, Phys. Rev A \textbf{69}, 031201 (2004); [2] F. Amar, \textit{et al}, JCP \textbf{122}, 244717 (2005).   

Helium Nanodroplets: spectroscopy and density-functional calculations

Helium nanodroplet spectroscopy has shown to be an excellent tool for studying superfluid helium in nano-scale geometries. Experimental signatures of superfluidity are the free rotation of many molecules in helium nanodroplets, and the droplet phonon spectra observed in electronic transitions of molecular dopants. We present new results and interpretations from a study of helium nanodroplets using time- dependent density-functional theory and its normal-mode analysis.   

Chaotic Dynamics of a Harmonic Oscillator Interacting Linearly with a Free Particle

We study the closed Hamiltonian dynamics of a free particle moving on a ring, over one section of which it interacts linearly with a single harmonic oscillator. On the basis of numerical and analytical evidence, we conjecture that at small positive energies the phase space of our model is completely chaotic except for a single region of complete integrability with a smooth sharp boundary showing no KAM-type structures of any kind. This results in the cleanest mixed phase space structure possible, in which motions in the integrable region and in the chaotic region are clearly separated and independent of one another. For certain system parameters, this mixed phase space structure can be tuned to make either of the two components disappear, leaving a completely integrable or completely chaotic phase space. For other values of the system parameters, additional structures appear, such as KAM-like elliptic islands, and one parameter families of parabolic periodic orbits embedded in the chaotic sea. The latter are analogous to bouncing ball orbits seen in the stadium billiard. The analytical part of our study proceeds from a geometric description of the dynamics, and shows it to be equivalent to a linked twist map on the union of two intersecting disks.   

Dielectric fluctuations and the origins of non-contact friction

Dielectric fluctuations underlie a wide variety of physical phenomena, from ion mobility in electrolyte solutions and decoherence in quantum systems, to dynamics in glass-forming materials and conformational changes in proteins. Here we show that dielectric fluctuations also lead to non-contact friction. A detailed understanding of non-contact friction is essential to micromechanical systems and the continued success of high sensitivity scanned probe microcopies such as magnetic resonance force microscopy. We study non-contact friction by using high sensitivity, custom fabricated, single crystal silicon cantilevers to measure energy losses over poly(methyl methacrylate), poly(vinyl acetate), and polystyrene thin films. We present a new theoretical analysis, relating non-contact friction to the dielectric response of the film, which is consistent with our experimental observations. We believe this constitutes the first direct, mechanical detection of friction due to dielectric fluctuations, and establishes a new route to the measurement of these fluctuations at the nanoscale.   

Conductance Behavior of Molecularly Linked Gold Nanoparticle Films Near the Metal-Insulator Transition

Self-assembled molecularly-linked metal nanoparticle films exhibit interesting electronic behavior that can range from insulating to metallic. In particular, the electronic behavior of 1,4-butanedithiol-linked gold nanoparticle films can be tuned by varying film thickness. We have investigated the temperature (T) dependence of the differential conductance (g) of 1,4-butane dithiol linked Au nanoparticle films. On the insulating side of the transition, the conductance behaves as$g=g_0 \exp \left[ {-\left( {\frac{T_0 }{T}} \right)^{\raise0.7ex\hbox{$1$} \!\mathord{\left/ {\vphantom {1 2}}\right.\kern-\nulldelimiterspace}\!\lower0.7ex\hbox{$2$}}} \right]$. Qualitatively, this is consistent with an Efros- Shklovskii ``variable range hopping'' model based on a competition between Coulombic and intercluster tunnelling processes. However, we find that hopping distances are too large (62 nm to 720 nm at 100 K) to be consistent with tunneling between clusters, and tend to scale with cluster size. We propose a modified ``quasilocalized hopping'' model based on competition between single-electron cluster charging and intracluster electron backscattering to explain this temperature dependence.   

Spin relaxation in a GaAs quantum dot embedded inside a suspended phonon cavity

The phonon-induced spin relaxation in a two-dimensional quantum dot embedded inside a semiconductor slab is investigated theoretically. An enhanced relaxation rate is found due to the phonon van Hove singularities. Oppositely, a vanishing deformation potential may also result in a suppression of the spin relaxation rate. For larger quantum dots, the interplay between the spin orbit interaction and Zeeman levels causes the suppression of the relaxation at several points. Furthermore, a crossover from confined to bulk-like systems is obtained by varying the width of the slab.   

Optical properties of semiconductor nanocrystals in the tight-binding (TB) approximation

Tight-binding (TB) approach is very attractive for computing electronic properties of semiconductor nanocrystals since it can handle large numbers of atoms, and since nanocrystals of arbitrary shape can be studied. TB approach provides realistic boundary conditions at the surface, which are often defined with uncertainty in the effective mass approximation. This is especially important in small nanocrystals, where a large fraction of atoms belongs to the surface. For example, in a 1000-atom Si nanocrystal about 2/3 of all atoms have less than four nearest neighbors. Successful TB approaches, such as NRL-TB method, exist to compute electronic structure of various bulk materials. We use NRL-TB to compute electronic transitions in nanocrystals of arbitrary shape. It appears, that a given TB parameterization that yields adequate electronic structure is incomplete in terms of predicting oscillator strengths of the transitions, in contrast to a common belief. Such TB parameterization must be augmented with additional quantities that contribute to the transition dipole matrix elements between localized orbitals. We thus resolve the existing controversy, despite the arguments that these extra contributions are problematic and should not be incorporated into TB. We also discuss practical ways to calculate the extra quantities, which are not supplied by the conventional TB parameterizations.   

Impurity mode techniques applied to photonic crystal slabs

A theoretical treatment is given of the modes of a photonic crystal slab containing an impurity. The photonic crystal slab is formed by embedding an hexagonal lattice array of dielectric cylinders in a background slab of uniform dielectric medium. This creates a free standing slab with a periodic patterning. The solution of Maxwell's equations for the modes of the slab are given as an expansion in the waveguide and radiative modes of a uniform slab of dielectric medium. The density of modes in frequency space is presented for the photonic crystal slab in the absence of an impurity. The impurity modes for a single site impurity introduced into the center of the photonic crystal patterning are computed using Green's function methods. Bound state impurities and in band resonances are studied and used to explain recent experimental results on photonic crystal light sources.   

Fabrication of nanoscale magnet-tipped cantilevers for magnetic resonance force microscopy

Magnetic resonance force microscopy(MRFM) is a promising new technique for acquiring magnetic resonance images of a single molecule; to date we have demonstrated a sensitivity of approximately 10,000 proton spins. In MRFM the force exerted on the cantilever, per spin, is proportional to the field gradient from the cantilever's magnetic tip. To increase the force requires shrinking the magnet size. Achieving the attonewton force sensitivity necessary to image single spins requires mitigating surface induced dissipation. We choose to meet both of these conditions by creating nanoscale magnets extending from the tips of silicon cantilevers. We will present a 50-nm wide overhanging cobalt magnet fabricated by a process involving electron beam lithography and anisotropic KOH etching. This process can be integrated into a fabrication protocol for ultrasensitive silicon cantilevers. With these cantilevers we expect a sensitivity of better than 1000 protons.   

Electrically Controlled Bandgap in a Chiral Material

The propagation of light along the axis of nonhomogeneity of a structurally chiral medium (SCM) under the influence of a low-- frequency (dc) electric field aligned along the same axis is investigated. The Pockels effect is assumed to occur, and the SCM is taken to possess locally a $\bar{4}2m$ point group symmetry. The frequency--domain Maxwell curl equations are cast in a 4$\times $4 matrix representation, and the Oseen transformation is invoked. The band structure is analyzed, as are also the eigenvectors as well as the transfer matrix. Finally, the reflection and transmission of a plane wave due to a SCM of finite thickness is considered. The low--frequency electric field is shown to control the bandgap.   

Organic Solar Cell with Carbon Nanotubes as Anode

Organic, polymer based solar cells present a low cost more versatile alternative to the current inorganic silicon based solar cells. In this research, carbon nanotubes have been used to replace the conventional anode used, indium tin oxide. Carbon nanotubes exhibit electronic, optical and mechanical properties desirable for polymeric based organic solar cells. ~ In this study, an oriented muliwall carbon nanotube sheet is used as the hole collecting electrode with RR-P3HT as the donor material and PCBM as the acceptor material. ~ An open circuit voltage of 0.57V, a short circuit current of 5.53mA/cm2, a fill factor of 0.37, and an efficiency of 1.16{\%} has been obtained.~ Performance dependence on incident light intensity and spectral studies along with other various investigations are presented.   

Theorems Determining Reciprocal and Nonreciprocal Properties of Negative Refractive Bi-Crystal Heterostructures

One of the most interesting questions in regard to the negative refractive bicrystal heterostructure is what multi-terminal electronic characteristics it may possess. The heterostructure possesses a symmetry breaking operation creating asymmetric fields associated with the SO(2) rotation group [1]. Two terminal behavior may be addressed by examining generalized theorems for complex anisotropic media [2]. Applying the anisotropic reaction theorem to subregions of the heterostructure, and then to the composite structure, we find that passive nonreciprocity [3] is not possible while active nonreciprocity is possible as mentioned in [4]. Possible device applications include beam steering, and isolation devices requiring direction sensing control. Simulations show what happens to the asymmetry when making the heterostructure macroscopically using LINbO3 crystals versus using nanostructure technology which would work with ferroelectric films deposited by MBE. A fabrication effort used to build the structure, with metal patterning to test a derivative property of the asymmetry, namely terminal reciprocity/nonreciprocity, will be discussed too [5]. [1] C. M. Krowne, PRL 93, 053902, 30 July 2004. [2] C. M. Krowne, IEEE Trans. Ant. Propag. 32, 1224 Nov. 1984, [3] C. M. Krowne, IEEE Trans. MTT 53, 1345, 2005, [4] C. M. Krowne, arXiv.org/abs/cond-mat/0408369, 17 Aug. 2004. [5] Private commun. S. W. Kirchoefer, J. A. Bellotti, and J. M. Pond, 2005.   

Energetic Material Simulations Framework

We have created a computational framework for the easy implementation of the modeling needs of the advanced energetic materials (AEM) community, with rapid turnaround. The framework constructs simulations described in eXtensible Markup Language (XML) from a set of components in shared object libraries. The complete set of standard molecular simulation components can be combined in any fashion creating typical simulations and providing unanticipated functionality. The framework includes multi-million atom molecular dynamics in a several thermodynamic ensembles, including the recently developed uniaxial Hugoniostat method. This constrains the system to states that correspond to points on the shock Hugoniot curve. It also includes ab initio crystal prediction capability. This procedure predicts the crystal structure and density of a solid using only the molecular structure of a single molecule. This predictive capability is considered crucial to the design and development process of AEM, since one of the fundamental properties required for the initial screening of a candidate energetic material is its crystalline density.   

The radiation equilibrium is classical

By definition, standing wave modes cannot interact among themselves, plus non-zero temperature means real walls must vibrate. Modal interaction by Doppler shifts at the walls is thus an unavoidable premise in cavity equilibrium, but it also suffices to yield Planck's law \textit{classically}, as follows. $\lambda/2$-intervals of the modes are immutable energy-bearing entities under these thermalizing interactions, like molecules in the kinetic theory, as Doppler shifts preserve phase and amplitude, thus only compressing or dilating the $\lambda/2$-intervals in time. The premise is really a conceptual aid, as energy immutability is guaranteed by definition: $ E(\lambda/2) = \int_0^{\lambda/2} | a \sin (2 \pi x/\lambda)|^2 \, d x = \int_0^{\pi} a^2 \sin^2 \theta \, d\theta $, independent of $\lambda$. Therefore, Boltzmann equipartition must be applied to these intervals, instead of to full modes as in Rayleigh-Jeans theory, and it dictates a common mean energy $u$, not $u \ne k_B T/2$ -- modes are mere Fourier components, not full particulate entities. The intervals yield $E = h \nu$ with $h \equiv u$ as the number of such intervals in a mode is proportional to its frequency. Mode closure sets under the thermalizing interactions form Planck's harmonic oscillators -- as the intervals are immutable, interacting modes can only differ by whole number of intervals, hence must be harmonically related, and \textit{replace} each other by exchanging intervals. The energy expectation at any $\lambda$ is then an average over any such mode closure set, yielding Planck's law \textit{without assumptions} and \textit{without breaking from classical physics}. The result identifies $h \equiv u $ as the analogue of $k_B$ for the frequency domain.   

Experimental one-way quantum computation using linear optics

Standard quantum computation is based on a universal set of unitary quantum logic gates which process qubits. In contrast to the standard quantum model, Raussendorf and Briegel proposed the one-way quantum computer, based on a highly-entangled cluster state, which is entirely different. We have experimentally realized four-qubit cluster states encoded into the polarization state of four photons. We fully characterize the quantum state by implementing the first experimental four-qubit quantum state tomography. Using this cluster state we demonstrate the feasibility of one-way quantum computing through a universal set of one- and two-qubit operations. Finally, our implementation of Grover's search algorithm demonstrates that one-way quantum computation is ideally suited for such tasks.   

Direct current bias effect on the magnetoresistance of submicron spin-valve ellipses

Submicron spin-valve devices with nonmagnetic I/V leads have been successfully fabricated using E-beam lithography in conjunction with an ion beam etching.. The layer structures are SiO2/Ta(5nm)/ NiFe(2nm)/ MnIr(10nm)/CoFe(2nm)/Cu(2.5nm)/CoFe(2nm)/NiFe(3nm)/Cu(1nm)/Ta-O(3nm) prepared by DC sputtering. Low field ac MR measurements were carried out with an external magnetic field applied parallel to the biasing/current direction. A significant shift on minor loop, with respect to that of the non-patterned film, is associated with an antiferromagnetic coupling resulted from the pinned layer's stray field after patterning. In addition, a direct current bias was superimposed to the low field ac for the studies of joule heating and possible spin transfer torque effect. In comparison to the MR measured at various ambient temperatures Joule heating effect was identified from the resistance background change. Extra features were developed during the magnetization reversal that may be associated with spin torque effect. Details of the MR minor loops regarding to the magnetization evolution and the switching mechanism will be presented.   

Interface Mott Transition and Colossal Electroresistance

Colossal electroresistance -- devices with huge nonlinear $I-V $with hysteresis -- found in strongly correlated materials is now receiving wide interest as a candidate of a novel non-volatile memory. In order to design such devices, it is important to understand how phase transition takes place at the interface between SCES and an electrode. We propose a simple theoretical scheme to calculate the electronic state at such interfaces. A density matrix renormalization group (DMRG) calculation of a 1D tight binding model is performed, where we find that the Mott transition takes place, and by altering the external bias, we can change the transport property drastically from insulating to Ohmic (metallic). Next, we propose a mechanism of colossal electroresistance based on first order metal-insulator transition. In this mechanism, the switching between on and off states is realized by a hysteresis loop of bias voltage crossing the phase coexistence regime near the transition. (cond-mat/0509050, to appear in Phys. Rev. Lett.)   

Magnetic field effects in double-walled carbon nanotubes

Double-wall carbon nanotubes (DWCNs) are coaxial two-tube systems which are now considered very important from their mechanical and electronic properties. They may be achieved, for instance, by peapod-derived methods or synthetized by pulsed arc dicharge processes. The distance between both cilinders are quite the same of the one found between carbon planes in graphite structures, and the correspondent intertube electronic interactions (van der Waals energies) are also much inferior than the electronic correlation within each single tube. Here we present a theoretical discussion of electronic and transport properties of a particular family of DWCNs named commensurate structures of the armchair (n,n)@(2n,2n) and zigzag (n,o)@(2n,0) types. Emphasis is given on the role played by the geometrical aspects of the tubes and the relative atomic positions on the local density of states and conductance of the systems. We investigate the origin of the conductance suppression and the possibility of founding Aharonov-Bohm effects in the double walled carbon when applying a magnetic field along the axial direction. The field is theoretically described by following the Peierls approximation into the hopping energies and by adding a Zeeman energy in the diagonal term of the tight-binding Hamiltonian. This should be interesting not only for the basic understanding of DWCNs but also to allow a characterization upon their responses under external fields.   

Simple, Rapid Chirality Characterization of Single Walled Carbon Nanotubes

Raman scattering and optical absorption spectroscopy are used for the chirality characterization of single wall carbon nanotubes (SWNTs) dispersed in various solvent systems. Radial breathing mode (RBM) Raman peaks for semiconducting and metallic SWNTs are identified by directly comparing the Raman spectra with the Kataura plot. The SWNT diameters are then calculated from these resonant peak positions. Next, a list of (n, m) pairs, yielding the SWNT diameters within a few percent of that obtained from each resonant peak position, is established. The interband transition energies for the list of SWNT (n, m) pairs are calculated based on the tight binding energy expression for each list of the (n, m) pairs, and the pairs yielding the closest values to the corresponding experimental optical absorption peaks are selected. The results reveal the most probable chiralities for the semiconducting nanotubes and for the metallic nanotubes present in a batch of SWNTs. Directly incorporating the Raman scattering data into the optical absorption spectra, the present method is considered the simplest practical technique currently available.   

Reentrant Disordering of Colloidal Molecular Crystals on 2D Periodic Substrates

We study colloidal ordering and disordering on two-dimensional periodic substrates where the number of colloids per substrate minima is two or three. The colloids form dimer or trimer states with orientational ordering, referred to as colloidal molecular crystals. At a fixed temperature such that, in the absence of a substrate, the colloids are in a triangular floating solid state, upon increasing the substrate strength we find a transition to an ordered colloidal molecular crystal state, followed by a transition to a disordered state where the colloids still form dimers or trimers but the orientational order is lost. These results are in agreement with recent experiments.   

Behavior of (Pb$_{0.9}$Ba$_{0.05}$Sr$_{0.05})$(Zr$_{1-x}$Ti$_{x})$O$_{3}$ solid solutions in the vicinity of morphotropic boundary

(Pb$_{0.9}$Ba$_{0.05}$Sr$_{0.05})$(Zr$_{1-x}$Ti$_{x})$O$_{3}$ series of solid solutions possess rather narrow width of the morphotropic region that does not exceed 5{\%} (determined by X-ray diffraction studies). Using a modified ceramic technique, we obtained samples with a grain size of the order of 20-25 $\mu $m. We performed detailed investigations of piezoelectric and dielectric properties of these samples. It is shown that the temperature interval of the diffuse phase transition depends on the position of the given solid solution in the ``Ti concentration -- Temperature'' phase diagram. The maximum interval is observed in the middle of the morphotropic region. There exists a temperature interval of the order of several decades of degrees above the Curie point where piezoelectric resonance can be observed. The width of this temperature interval reduces considerably when solid solution composition moves from morphotropic region.   

Electroabsorption of wide bandgap semiconductors GaN and ZnO: the theory and experiments

Dow and Redfield developed a theoretical model in 1970's to describe the change of the broadening of exciton absorption peak and the tilting of band edge in semiconductor under an electrical field. In this work, we apply this generic model to different kinds of wide bandgap semiconductors, such as GaN and ZnO. The exciton electroabsorption problem can be treated non-perturbatively in the center-of-mass coordinate as one electron in a Coulomb potential and an electrical field. The corresponding three-dimensional Schrödinger equation was solved for the electron-hole envelope wavefunction at the origin and the density of states per unit energy by introducing parabolic coordinates. Elliot formula was then used to calculate the actual optical electroabsorption coefficient. Thermal broadening of the exciton absorption peak was considered by convoluting a Guassian function, whose line width is related to the temperature. Experimental temperature dependent absorption data of GaN and ZnO were measured from 77K to 300K. The electroabsorption spectra of GaN in an optical modulator structure have also been examined. These experiment results were used to verify the theoretical model of electroabsorption of wide bandgap semiconductors.   

Satellite bands in the rotational spectrum of doped helium clusters

Recent measurements of infrared spectra of Carbon dioxide molecules embedded in Helium droplets reveal a weak satellite band (SB) accompanying a sharp R(0) rovibrational line. By re-analyzing previous quantum Monte Carlo calculations of the rotational dynamics of various dopant molecules in Helium clusters, we find indeed evidence for rotational excitations with energies and spectral weights close to the measured values of the SB. We present new simulations, in which specifically devised imaginary-time correlation functions are introduced for an explicit theoretical characterization of individual excitations, as well as for enhanced computational efficiency in the calculation of weak spectral features. The proposed assigment of the SB to a coupled rotational state of the molecule and a ring of Helium atoms is discussed.   

Surface Studies of Enhanced Metal/Carbene Contacts

Scanning tunneling microscopy and spectroscopy (STM and STS) have proven invaluable in understanding and manipulating the physical and electronic properties of molecular-scale systems. Such systems show great promise in current and future electronic applications. Indeed, self-assembled monolayers (SAMs) composed of alkanethiols have been extensively explored as a possible route to formation of metal-organic semiconductor contacts. While S is effective in this function, it behaves more as a capacitor than a conductor at the metal-adsorbate interface. This presentation will focus on investigations of the formation of metal-carbene complexes. For molecular electronics applications, conjugated metal-carbenes are expected to exhibit significantly greater conductance in transistor applications than corresponding thiols. A particular emphasis of this presentation is the structural and electronic information yielded by ultra-high vacuum (UHV) STM/STS studies of the formation of a Ru-carbene from diazoalkane precursors. Early theoretical predictions and UHV-STS indicate a strong coupling of the HOMO for bromophenyl diazomethane to the Ru d-band.   

Single Photon Transport In One-Dimensional Circuit Quantum Electrodynamics Systems

Interesting transport properties of a single microwave photon emerge when a superconducting quantum bit in a cavity is coupled to a one-dimensional waveguide. Here we adopt a real-space model Hamiltonian to give a unified approach which accounts for the recent experimental results, and make new predictions on the properties of single photon transport, such as general Fano lineshape, symmetric vacuum Rabi splitting for leaky cavity at resonance, and one-photon switching capability. We further exploit the large tunability of the qubits to show that further interesting one-photon transport properties can emerge in multiple-qubit system, especially when the transition frequencies are dynamically controlled. In particular, having two qubits coupled to the waveguide gives rise to a transmission lineshape for electromagnetic wave that is analogous to the electromagneically induced transparency (EIT) in atomic system. Furthermore, by cascading these double-qubit structure together to form an array, and by dynamically tuning the transition frequency of the qubits, we show that a single photon pulse can be stopped, stored, and time-reversed in the system, leading to highly non-trivial information processing capabilities on chip. Moreover, with properly designed array, two photons can be stopped and stored in the system at the same time. Finally, the unit cell of the array can be designed to be of deep sub-wavelength scale, thereby realizes the miniaturization of the circuit.   

Electric Characterization of FeSi$_{2}$ Nanowires by Conductive-AFM

Since continuing miniaturization of silicon electronics encounters the limits of lithography, silicide nanowires are proposed to act as both devices and the wires that access them in bottom-up fabrication scheme. Iron silicides nanowires can be metallic, magnetic and semiconducting depending on different phases it takes, which make it promising for nanodevices design. Here we characterized Schottky Barrier Height (SBH) of iron silicide nanowires on n-type silicon, which is not uniform ranging from 0.35 to 0.8eV with diamond coated tips. To measure electric transport properties within a single nanowire, we deposited a gold pad to partially cover one nanowire, and use diamond coated tip as another terminal. Nanowires with dimension of 5nm high by 10nm wide by several micrometer long have resistance about 20k ohm at room temperature, this is about 4 times larger than bulk iron silicide resistivity. We tried different metals coated tips and tips with different force constants. Diamond coated tips are most wear-resistant, but have highest contact resistance, which is estimated to be 20k ohm. Other metal coated tips, such as PtIr, PtCr coated tips, have lower contact resistance, less than 1k ohm, but these tips apex are easily scanned off. Suitable contact force were also calibrated to be around 200nN, too strong force will damage nanowire and tip coating layer, while too weak contact lead to high contact resistance.   

Novel commensurability effects in superconducting films with antidot arrays

Vortex pinning by arrays of microholes is already a well established tool for enhancing the critical current in superconducting films. The pronounced peaks in j$_{c}$(H) characteristics at so-called matching fields are generally attributed to the collective locking of vortices to the pinning sites. However, the issues of symmetry and composition of the resulting vortex lattice are often oversimplified. We investigated in detail the vortex configurations in superconducting films with regular antidot-arrays within the non-linear Ginzburg-Landau theory, where demagnetization effects and overlapping vortex cores are fully taken into account (contrary to the London approach). In addition to the well-known matching phenomena, we predict: (i) the nucleation of giant-vortex states at interstitial sites; (ii) the combination of giant- and multi-vortices at rational matching fields; and (iii) for particular interstitial vorticity, the symmetry imposed creation of vortex-antivortex configurations.   

Threefold onset of vortex loops in superconductors with a magnetic core

Superconductivity and magnetism are known to coexist inside several compounds, such as RuSr$_{2}$LnCu$_{2}$O and ErNi$_{2}$B$_{2}$C. Recently, artificially nano-engineered superconductors with magnetic inclusions have been realized experimentally. In mesoscopic samples, where the ratio volume to area is small, the vortex patterns vary according to the sample symmetry. On the other hand, magnetic inclusions give rise to vortex loops inside the superconductor. To capture these aspects, we consider theoretically a sub-micron superconductor (e.g. sphere) with a static magnetic moment in its center in order to observe these confined vortex states. The 3D Ginzburg-Landau theory is applied and solved in the finite difference scheme, from which we obtain the very complex 3D vortex configurations. For large samples, when the influence of the boundary diminishes, we found that vortex loops always nucleate in threes. The final superconducting state is characterized by the number of created vortex loops and the number of vortex-antivortex pairs that spring to the surface.   

The coexistence of type-I and type-II superconductivity in mesoscopic single crystals

We show that the well established criteria for type-I to type-II transitions in bulk materials do not apply to mesoscopic superconductors, as the dual point depends both on the material properties and on temperature. In electrochemically formed triangular Pb single crystals, we even found the coexistence of both types of superconductivity -- while retaining Meissner behavior with increasing magnetic field all the way through the superheated phase, the sample can still capture vortices in the metastable regime. When the field is swept back, in realistic type-I structures with intrinsic pinning, the vortices can survive in the sample even when the field changes polarity, contrary to conventional behavior. All findings are substantiated by state-of-the-art 3D Ginzburg-Landau simulations and $\mu $-Hall probe measurements.   

An Accurate Computation of the de Haas-van Alphen frequencies and effective masses of NbSe$_2$

NbSe$_2$ is a layered superconducting material with a co-existing nearly commensurate CDW. Since both the CDW and superconducting transitions are related to a Fermi Surface instability, an accurate calculation of the Fermi Surface is essential for a microscopic understanding of the physical behavior of this compound. Here we demonstrate a novel computational method for calculating the dHvA frequencies and electron effective masses of NbSe$_2$. Our computational approach employs a FLAPW calculation of the NbSe$_2$ Fermi Surfaces using ~ 40000 k-points. We extract the extremal cross sections (located in the Gamma-K-M and A-L-H planes) into a bitmap and then compute the dHvA frequencies from the number of pixels in the cross sections averaged over an extended Brilluoin zone. The effective masses are calculated by repeating this method with the Fermi energy shifted by 0.002 Ry. Our method satisfies Luttinger's theorem to within 0.1\% - a full order of magnitude more accurate than previously published calculations. We use this accuracy to predict the dHvA frequencies and effective masses of NbSe$_2$ under pressure.   

Bipolar surface conduction of BaTiO$_{3}$ in ultrahigh vacuum corresponding to electron and hole accumulation

The surface of the ferroelectric is unique due to the spontaneous polarization. One of such properties is a self-field effect [1,2]. We have reported the self-field effect of BaTiO$_{3}$ [2,3], which does not seem to be accepted, probably due to the suspicions about the formation of the oxygen vacancies or water absorption at the surface in ultrahigh vacuum. Here, oxygen vacancies in BaTiO$_{3}$ facilitate $n$-type conduction. In this talk, we present the enhancement of both $n$- and $p$-type surface conduction in 10$^{-10}$ --10$^{-11}$ torr. We observed this in both top-seed-solvent-growth (TSSG) and KF-flux grown samples that are nominally pure and transparent. Furthermore, we confirmed the evident reduction of surface conductance above Curie temperature, where it agreed with the conduction without a carrier surface layer. The significant reduction of the conduction by the exposition of the surface to the low vacuum indicates that the conduction occurs really at the very thin layer at the surface. Other several experiments verify the existence of the surface carrier layer by the ferroelectric self-field (depolarization field) effect. [1] Appl. Phys. Lett.66, 1770(1995), Phys. Rev. B57, 789(1998), [2]\textit{ Phys. Rev. Lett.} 86, 332(2001), [3] Focus http://focus.aps.org/story/v7/st1   

Directional sliding of histone octamers caused by DNA bending

Chromatin-remodeling complexes such as SWI/SNF and RSC of yeast can perturb the structure of nucleosomes in an ATP-dependent manner. Experimental results prove that this chromatin remodeling process involves DNA bending. We simulate the effect of DNA bending, caused by chromatin-remodeling complexes, on directional sliding of histone octamers by Brownian dynamics simulation. The simulation results show that, after a DNA loop being generated at the side of a nucleosome, the histone octamer slides towards this DNA loop until the loop disappears. The DNA loop size is an important factor affecting the process of directional sliding of the histone octamer. A model for directional sliding of histone octamers induced by chromatin-remodeling complexes is suggested. (This research was supported by National Natural Science Foundation of China, and the Innovation Project of the Chinese Academy of Sciences.) (Email: pywang@aphy.iphy.ac.cn)   

Oscillations of Cantilevers

Resonance vibration of micro and nano sized cantilevers can be used to measure elastic constants and mass, detect absorbed material, and form part of mechanically resonant filters in electronic devices. We have measured the harmonic content of cantilevers driven by electrostatic forces containing two harmonically related terms, as well as nonlinear terms and parametric terms. The results are compared to numerical simulations of the forces between the cantilever and the counter electrode and the resulting motion of the cantilever.   

Electrical Switching and Rectification in Carbon Nanotube Y\_Junctions

Recently, we have shown evidence for a dramatic electrical switching behavior in a Y-junction carbon-nanotube morphology [1]. The mutual interaction of the electron currents in the three branches of the Y-junction is shown to be the basis for a potentially new logic device. The Y-junction nanotubes that were prepared using our thermal CVD process [2] typically show the presence of a catalyst particle at the junction. The observed modulation of the current from an on- to an off state could be mediated by defects and the topology of the junction, or due to the presence of catalyst particle at the junction. We will compare the switching characteristics observed in a Y- junction nanotube that does not contain the catalyst particle to those reported in Ref. 1 with a view to elucidating the switching mechanism in branched nanotubes. (1) P. Bandaru et al., Nature Materials, vol. 4(9), 663-666, (2005) (2) N. Gothard, et al., Nanoletters 4, 213-217 (2004)   

Combined HRTEM and Auger spectroscopy evidence for the existence of grain boundary premelting in nickel-doped tungsten

A combined high resolution transmission electron microscopy (HRTEM) and Auger spectroscopy study has revealed the formation of 0.6 nanometer thick, nickel-enriched, "liquid-like" grain boundary (GB) films in Ni-doped W specimens at 95C below the bulk eutectic temperature where the bulk liquid phase is no longer stable [Appl. Phys. Lett. 87, 231902 (2005)]. The stabilization of subeutectic liquid-like grain boundary cores in this model two-component metallic alloy is phenomenologically analogous to the long-sought phenomenon of grain boundary premelting. Despite various macroscopic indications of GB structural transitions, this result, to our knowledge, is the first direct evidence for the existence of such disordered nanostructures at metallic GBs. This observation offers a new explanation to the long-standing mysterious solid-state activated sintering mechanism, where accelerated sintering is attributed to the enhanced diffusion in liquid-like GB layers. This discovery also provides insights to resolve several long-standing controversies, e.g., the mechanisms of Ni-induced abnormal grain growth and GB embrittlement.   

Enhanced near-field optical moir\'{e} effect.

We demonstrated an enhanced near-field optical Moir\'{e} effect by inserting a silver slab between two subwavelength gratings. The evanescent fields can be greatly enhanced by the surface plasmon excitation supported by the silver slab, which leads to a remarkable contrast improvement in the Moir\'{e} fringes. The crucial role of surface plasmon excitation was elucidated by numerical simulation. Experimental enhanced near-field optical Moir\'{e} effect, which agreed very well with the simulation, was also presented. This new effect can be used to extend the conventional Moir\'{e} techniques into usage of subwavelength gratings.   

Electroformation of Phospholipid Vesicles Monitored by Quartz Crystal Microbalance and Optical Microscopy

Quartz Crystal Microbalance with dissipation monitoring (QCM-D) and Optical Microscope were used in combination to investigate the electroformation of giant vesicles from 1,2-Dimyristoyl-sn-Glycero-3-Phosphocholine (DMPC). The gold-coated quartz crystal coated by lipid film and the ITO-coated glass slide were used as electrodes for electroformation. Immediately upon voltage application (1V, 10Hz) between the two electrodes increases in the frequency and decreases in the dissipation were observed indicating the loss of lipid from the QCM surface. Concurrently, we observed vesicles on the QCM electrode surface. By fitting the frequency shift (delta F), and dissipation change (delta D), to a viscoelastic model, we were able to find the time evolution of the lipid film thickness. By counting the vesicles in the active QCM area, and comparing it to the thickness of lost lipid, we are able to provide an upper bound for the average vesicle thickness.   

Hidden Variables and Placebo Effects

God's response to prayers and placebo leads to a question. How does He respond deterministically? He may be controlling at least one of the two variables of the uncertainty principle by extending His invisible soul to each body particle locally. Amazingly, many Vedic verses support this answer. One describes the size of the soul as arithmetically matching the size of the nucleons as if a particle is a soul. One gives a name meaning particle soul (anu-atma), consistent with particle's indeterministic behavior like that of (soulful) bird’s flying in any directions irrespective of the direction of throw. One describes souls as eternal consistent with the conservation of baryon number. One links the souls to the omnipresent (param- atma) like Einstein Rosen bridges link particles to normal spacetime. One claims eternal coexistence of matter and soul as is inflationary universe in physics/0210040 V2. The implicit scientific consistency of such verses makes the relationship of particle source of consciousness to the omnipresent Supreme analogous to the relationship of quantum source of gravitons in my gr-qc/0507130 to normal spacetime This frees us from the postulation of quantum wormholes and quantum foam. Dr. Hooft's view in ``Does God play dice,'' Physicsword, Dec 2005 seems consistent with my progressive conference presentations in Russia, Europe, India, and USA (Hindu University) in 2004/05. I see implications for nanoscience.   

Electrically Tunable Chiral Materials for Spatial Light Modulators

An electrically tunable chiral material is being used to develop a spatial light modulator in which the spectral wavelength is controlled by an electric field. We prepared a cholesteric liquid crystal cell by using a nematic host doped with chiral dopants. Upon applying an ac electric field parallel to the helical axis, different textures can be obtained. In addition to obtaining transitions among cholesteric textures by amplitude modulation, in this work we develop a scheme of switching the cholesteric liquid crystal among different textures and the reflected wavelength. We also analyzed the issue of the frequency and amplitude dependent behavior of cholesteric texture. With the combination of the amplitude and frequency modulation, reflected light with different wavelength is observed. This device can be operated in various spectrum regions as an optical filter. Furthermore, the device has good potential in display applications   

Polariton spectrum dependence on concentration of admixture layers in imperfect Si-superlattice.

Advances in technology allowing growth of ultrathin crystalline films and periodic structures with controlled characteristics by molecular beam epitaxy has led to an increasing interest for study of excitations in perfect crystalline lattice. Investigation of disorder effects in imperfect superlattice allowing modeling the crystal properties is still of a great interest. We consider a model of superlattice as a macroscopically homogeneous system with randomly included admixture layers. The virtual crystal approach which is the method to describe quasi-particle excitations in disorder media is used. Polariton spectrum of imperfect superlattice (which is one-dimensional Si-crystal with two elements-layers in the cell) is obtained. Peculiarities of the dependence of band gap width on admixture layers concentration have been studied for different polariton branch. The results are the evidence of substantial polariton spectrum reconstruction caused by presence of defect layers.   

Low Background and Close Proximity Sensors of Atmospheric Gamma Radiation Following Lightning

Atmospheric $\gamma $ radiation following lightning, 10-80{\%} more than normal background, in excess of radon progeny attached to precipitation, which decays with a half-life of about 50 min, has been observed in Japan and at the Lightning Research Lab in Starke, Florida. This excess $\gamma $ radiation may be from the decay of radioactive ejectiles from nuclear reactions initiated by protons or photons resulting from the high potential gradients during thunderstorms. The most likely candidates for 10-100 min $\gamma $ activity resulting from reactions on atmospheric elements are $^{39}$Cl or $^{38}$Cl and/or annihilation $\gamma $ rays from positron emitters. Using a high resolution Ge detector the 1.267 MeV peak from $^{39}$Cl and excesses of the 511 MeV annihilation peak, as well as anomalous increases in the ratio of $^{214}$Bi to $^{214}$Pb (radon progeny), have been observed following thunderstorms but all with poor signal to noise ratios. Delayed activity in condensates from 10 liters of rain water with extremely low background at the underground LLRL in Ogoya, Japan is underway and activity in shielded detectors from nearby triggered lightning at the Lightning Research Lab is being planned in order to improve statistics and signal to noise ratios.   

NMR Studies of Water and Methanol Transport in Highly Sulfonated Membranes for Fuel Cells.

Pulse gradient spin echo NMR was used to characterize the diffusion of water in highly sulfonated polyarylenethioethersulfone (SPTES) polymers. The proton NMR spectra as well as the diffusion rates were determined as a function of temperature. Comparison of electrical conductivity and diffusion activation energies indicate that H$^{+}$ and water transport are closely correlated. Several of the membranes were selected for further study for possible applications in direct methanol fuel cells. Both water and methanol diffusion coefficients were determined for membranes equilibrated in 2M aqueous methanol solutions. Water mobility is correlated with proton conductivity whereas methanol mobility is associated with undesirable crossover effects. The membranes based on SPTES polymers present a superior water/methanol diffusion ratio to that of the widely studied NAFION membrane. Finally, the effect of hydrostatic pressure up to 2.5 kbar on diffusion has been determined. Using pressure as the thermodynamic variable allows one to compute activation volumes for molecular transport. The results obtained for SPTES materials are similar to those of other sulfonated polymers, in that the activation volume is observed to decrease with increasing water content of the membrane.   

Effect of $\gamma $-radiation on CdTe/CdS solar cells

Heterostructures on the base of CdTe/CdS is widely used in modern photovoltaics. It is well known that during preparation of the CdTe/CdS heterostructures and annealing solid solutions CdTe$_{1-x}$S$_{x}$ are formed due to diffusion of Te atoms into CdS and S atoms into CdTe. This work present a study of the effect of $\gamma $-radiation on CdTe/CdS solar cells. The heterostructures are prepared on 1cm$^{2}$ molybdenum substrate. It is found that output parameters of the cells change non-monotonicaly upon monotonic increase of the intensity of the $\gamma $-radiation. At lower intensities, short-circuit current increases and open-circuit voltage decrease, while at higher intensities short-circuit current decrease and open-circuit voltage increase with increasing the intensity of $\gamma $-radiation. It is found that the radiation-induced increase of the short-circuit current is much faster than decrease of the open-circuit voltage. As result, efficiency slightly increases with increasing the radiation intensity at low intensities and decreases at higher intensities. The reason of the non-monotonic change of the output parameters is related to generation of the radiation defects, which increases the compensation degree of the base layer. This suggestion is tested by measuring the specific resistance as a function of the $\gamma $-radiation intensity. It is found that the specific resistance also changes non-monotonically upon monotonic increase of the $\gamma $-radiation intensity, thus confirming the above suggestion.   

Spooky Phenomena in Two-Photon Coherent Atomic Absorption

Physical processes on two-photon coherent atomic absorption of multiple laser beams were discussed more than twenty five years ago. These processes can be divided into two distinct groups. In the first group, laser beams are from a single source$^{1,2}$, and in the second group laser beams are from two different sources$^{3}$. Several experiments in the first group were carried out and have led to the 2005 Nobel Prize in physics. The second group is more interesting. Atoms are in random motion and two photons are from different sources. Classically, it is impossible for atoms to transit coherently in the absorption process, but quantum mechanically, such a transition is possible and that is one of the spooky phenomena in quantum mechanic. To assure the coherent transition, each photon as absorbed by the atom must have two possible paths of choices. If one photon has the choice and other one is not, then the atomic transitions cannot be coherent. The present talk will review various spooky phenomena associated with two-photon coherent atomic absorption, and will clarify some theoretical misunderstandings regarding these interesting transitions. Reference: \begin{enumerate} \item M. C. Li, \textit{Nuovo Cimento }\textbf{39B }(1977) 165. \item M. C. Li, \textit{Phys. Rev. A }\textbf{16 }(1977) 2480. \item M. C. Li, \textit{Phys. Rev. A }\textbf{22 }(1980) 1323. \end{enumerate}   

Modulation of Luttinger liquid exponents in multi-walled carbon nanotubes

We develop a theoretical framework that applies to the intermediate regime between the Coulomb blockade and the Luttinger liquid behavior in multi-walled carbon nanotubes [1]. We show that, in the crossover regime, the tunneling conductance follows a power-law behavior as a function of the temperature, with an exponent that oscillates with the gate voltage as observed in the experiments. We also evaluate the effects of a transverse magnetic field on the transport properties of the multi-walled nanotubes. For fields of the order of 4 T, we already find important changes in the band structure of the outer shells. We then predict the appearance of sensible modulations in the exponent of the conductance for higher magnetic fields, as the different subbands are shifted towards the formation of flat Landau levels. [1] S. Bellucci, J. Gonzalez and P. Onorato, Phys. Rev. Lett. 95, 186403 (2005).   

Superconductivity of multi-walled carbon nanotubes

\newcounter{bean} We investigate the superconductivity of multi-walled carbon nanotubes, paying attention to the balance between the Coulomb interaction and the effective {\em e-e} interaction mediated by phonon-exchange. Our main aim is to confront the recent observation of sharp transitions in the resistance of multi-walled nanotubes, in situations where most part of the shells are electrically active [1]. For this purpose we will adapt the mechanism already shown for the superconductivity of nanotube ropes, where the electrostatic coupling among a large number of nanotubes leads to a drastic reduction of the Coulomb interaction [2]. We will consider in particular the interaction and Cooper-pair tunneling among a large number of shells in the multi-walled nanotubes, drawing a connection between the superconductivity of these structures and the more conventional one in the intercalated compounds of graphite. \begin{list}{[\arabic{bean}]}{\usecounter{bean} } \item I. Takesue {\em et al.}, to appear in Phys. Rev. Lett. \item J. Gonz\'alez, Phys. Rev. Lett. {\bf 88}, 076403 (2002); J. V. Alvarez and J. Gonz\'alez, Phys. Rev. Lett. {\bf 91}, 076401 (2003). \end{list}   

Structural changes in Bi$_{2}$Te$_{3}$ under pressure.

Bi$_{2}$Te$_{3}$ based compounds continue to receive intense research activities due to the enhanced figure of merit observed in the super lattice structure with Sb$_{2}$Te$_{3}$ [1]. Synthesis of different chemical compositions with varying particle sizes and doping has gained importance subsequently. The thermo electric properties of the new compositions strongly depend on the structure and P-T phase diagram of the parent compound Bi$_{2}$Te$_{3}$. In order to understand the structural properties of Bi$_{2}$Te$_{3}$ in detail, we have performed pressure studies up to 30GPa using insitu angle dispersive and energy dispersive x-ray diffraction techniques using a diamond anvil cell with different pressure media. Bi$_{2}$Te$_{3}$ is found to undergo pressure induced structural transition around 7.8 GPa to a new high pressure phase from the ambient rhombohedral phase. This transition is found to be completely reversible with a large hysteresis observed during downloading. The details of the high pressure phase and the pressure medium dependence of the transition will be discussed further. [1]. Venkatasubramanian etal., Nature, \textbf{413}, 597 (2001)   

EPR, Biology, and Consciousness

It seems that Darwin, in his concluding remark (1859, p490) ruled out the possibility of cosmic connection to evolution based on the fixed law of gravity, known then. More recent Dirac’s Large Number Hypothesis as described in http://www.arXiv.org/pdf/physics/0210040 v1 raises a possibility that the universal constant of gravity is decreasing and all coupling constants are increasing with time, so reported by some observations. Deeper investigation of the connection between evolution and the variation of the universal constant of gravity seems worthwhile to see if it impacts the passage of time in a stronger (gravitational according to the spirit of the above archive) field and affects the aging process, and explains locality and causality in random evolutionary mutations. If there is no physical locality and causality consistent with the special theory of relativity, there must be some spiritual locality and causality at superluminal speeds to explain the implicit hidden variables. Then there is a question of how to test spiritual locality and causality. Psychic effects and dream signals look promising, if they exist and can be tested with space age technology. This is neither about religion nor about Einstein’s orthodoxy in light of the spirit of EPR. This is about frontiers of science of the new millennium: biology, and consciousness.   

Ion Size Effect in Glow Peak Temperature in Binary Mixed Crystals Doped with Divalente Europium

Thermoluminiscence measurements at room temperature of ``beta'' irradiated divalent Europium doped binary mixed alkali halides with KCl and KBr components at several concentrations x in molar fraction. The experiments have been carried out to identify the effect of composition of glow peaks. A typical glow peak has been distinguished for each composition. A linear dependence of its temperature on the composition x has been found. This is associated with the size change of ions Cl and Br. Initial comparative cathodoluminiscent measurement was carried out irradiating a single sample with electrons in an electron microscopy using a 30 KV voltage. With the present results is speculated the behavior of the mixed binary crystals with components KCl and RbCl, doped with divalent Europium.   

Applications of the Microchannel Plate in Accelerator Mass Spectrometry

A microchannel plate (MCP) detector with active area of 10 x 2 cm$^{2}$ is used as a position detector for mass spectrometry applications. The MCP detects electrons scattered by an MeV ion beam as it goes through a thin Carbon foil which is placed at a 45$^{o}$ angle with respect to the beam. The scattered electrons' transverse motion is constrained by application of uniform electric and magnetic fields parallel to the axis of the MCP. The amplified charge is deposited on a double-delay line anode. Differential timing and charge partitioning are used to determine the horizontal and vertical positions of the ion in a plane normal to the MCP axis.   

Evaporation Driven Microdroplet Pattern Formation in Surfactant X-77.

The evaporation of sessile solution droplets of size 300-1000 $\mu $m on two different types of surface have been investigated using digital video microscopy. It is found that when the surfactant X-77 droplet is left to evaporate on a glass surface, the droplet evaporates without reducing its diameter while the fluid within the droplet flows from the center to the pinned edge causing it to swell. The final thin solution film eventually breaks away from the edge and shrink toward the center; simultaneously the edge coalesces into smaller droplets, forming islands on once the perimeter of the droplet. When the same experiment is repeated on hydrophobic surface, the droplet is found to shrink while its edge swells then forms islands. Depending on the concentration of X-77, ring or even a smooth round patch in the center instead of islands are formed. Solution concentration between 0.5- 10 {\%} were used for the experiments. The dynamic transition from islands to ring to round drop again will be presented.   

Investigations of Wafer Scale Etching with Xenon Difluoride

A good and uniform bulk silicon wafer etching method can be applied to the wafer thinning process in MEMS and 3D applications. In this study, the use of a Xenon Difluoride (XeF2) gas-phase etching system, operating at room temperature, has been investigated for bulk silicon wafer thinning. We investigated the Si-wafer surface morphology and profile following each XeF2 etching process cycle. Theoretical results are used to compare with the experimental results as well. A clean wafer surface by proper surface treatments is significant to achieve a uniform surface profile and morphology for XeF2 etching. A proper design of etching cycle with nitrogen ambient during etching is necessary to achieve the fastest and uniform silicon etching rate. The silicon etching rate is reported as a function of etching pressure, nitrogen pressure, and etching duration.   

E/T-scaling of the spin fluctuations in the Zn-Mg-Ho magnetic quasicrystal

We report inelastic neutron-scattering results on the icosahedral Zn-Mg-RE (RE: rare-earth) magnetic quasicrystals. In the Zn-Mg-RE quasicrystals, slowing down of spin dynamics starts at considerably high temperatures (about 50~K) with developing antiferromagnetic correlations, nevertheless, the spins stay paramagnetic until low-temperature freezing transitions ($T_{\rm f} = 1.95$~K for RE = Ho), indicative of strong frustration effect. Neutron inelastic scattering further reveals that for RE = Ho, the scattering function $S(Q, \hbar \omega)$ is almost temperature independent for $\hbar \omega > 0$ in a wide temperature range up to 200~K. Corresponding dynamic susceptibility for $\hbar \omega < 1.5$~meV is scaled as ${\rm Im}\chi(\hbar\omega, T)T^{1/3} \propto (\hbar\omega/T)^{-1/3}Z(\hbar\omega/T)$, where $Z(\hbar\omega, T) = \tanh(\alpha\hbar\omega/T)$ is the scaling function. This $\hbar\omega/T$ (or $E/T$) scaling is identical to those frequently observed in systems near a quantum critical point, such as UCu$_4$Pd. In view of the frustration-reduced low freezing temperature of the Zn-Mg-Ho quasicrystal, the $E/T$-scaling may also be indicative of the quantum criticality in this magnetic quasicrystal.   

Perpendicular Hot Electron Transport in the Spin Valve Photo-Diode

The spin valve photo diode (SVPD) provides a new way to explore perpendicular hot electron transport in ferromagnetic multilayers using photoexcitation and internal photoemission. Since electrons are excited everywhere in the metallic multilayer, structure geometry is vital to optimize the magnetocurrent (MC). Initial theoretical models indicate that the MC should increase by increasing the thickness of a capping layer at the surface, but experiments demonstrate a nonmonotonic dependence resulting in an optimum capping layer thickness to maximize MC. The inconsistency between experiment and this theoretical model is also discussed, leading to a new proposal for hot electron transport in the SVPD.   

Wafer Bow Effect on Copper Wafer Bonding

A good bonding quality of bonded interconnects is the key factor to achieve successful three-dimensional (3D) integration applications. Prior to copper interconnect bonding in real 3D devices, fundamental researches about copper blanket film bonding should be studied. Since two wafers with large wafer bows may be difficult to contact during bonding, the bonding quality may be affected. In this study, wafer bows of different silicon wafers coated with copper and tantalum films were measured at different temperatures to simulate the wafer bow evaluation during bonding. We further investigated the bonding qualities of bonded wafers with different wafer bows after bonded at different temperatures. Dicing tests were performed to analyze the qualities of copper bonded wafers. Based on the results, a criterion of wafer bows for good copper bonding quality is suggested.   

Toward a model for pattern formation in thin film polymer blends

We outline a dynamic theory for simultaneous phase separation and dewetting in thin film polymer blends with free surfaces. Although an understanding of the processes of both dewetting and phase separation is well advanced, the coupling between the two has received little attention theoretically. We consider the coupling between surface driven instabilities and compositional instabilities in a thin film on a flat solid substrate with a free upper surface. We utilise a simple model, in which only fluctuations of composition within the plane parallel to the substrate are allowed, and neglect the possibility of fluctuations normal to the substrate. Such a model yields quantitative relationships for the stability in terms of the height of the film and the various thermodynamic parameters. From a dynamic viewpoint, the attraction of such a model is that it permits an analytical description of the early dynamic stages of an instability. Reduction of the problem from 3D to 2D is important when undertaking numerical studies on films that are nanometers thick, in which the typical lengthscales of dewetting and phase separation in the plane of the film are of the order of microns. As an example, the consequences of phase separation within a system which has already undergone considerable dewetting is highlighted by comparing with the process of phase separation when no dewetting occurs. With simultaneous dewetting, the phase separation becomes retarded and the co-continuous nature of the phases are absent.   

Nonequilibrium theory of polymer stretching based on the Master Equation

Single polymer pulling experiments such as protein unfolding measurements and dynamic force spectroscopy are increasingly done at high loading rates where equilibrium statistical mechanics is no longer applicable. I will present a theory that takes into account non-equilibrium effects using a Master Equation [PRE v. 72, 031805 (2005)]. If the molecular length is used as a stochastic variable, the transition probabilities have a simple analytic form. This theory predicts significant differences between experimental designs using controlled end-to-end positions and those that utilize a force-control mechanism. The most prominent non-equilibrium effect is a loading rate-dependent hysteresis in the force-extension curve. Molecular relaxation close to and far from equilibrium will also be discussed in the frame-work of the Master Equation theory.   

Mechanical and Tribological Properties of Amorphous Carbon Films

Incorporation of silicon into hydrogenated amorphous carbon films has been shown to reduce film stress, make the friction coefficient less sensitive to moisture, and increase thermal stability while maintaining high hardness, low wear, and small friction coefficient. Molecular dynamics simulations utilizing the reactive empirical bond-order potential (REBO) have been used to examine the temperature dependence of the material and tribological properties of hydrogenated amorphous carbon films containing silicon. The REBO potential is one of the few potential energy functions capable of modeling chemical reactions likely to accompany sliding. Existing models for the C-Si-H potential are based on the original REBO potential developed by Brenner in 1990. These models do not calculate elastic modulus accurately. A new parameterization for the inclusion of Si into the second generation REBO potential has been developed. Calculations of elastic modulus from the second generation Brenner potential are in much better agreement with experimental values. Initial studies have shown that the inclusion of Si in amorphous carbon films increases the amount of sp$^{3}$ bonding in the film while decreasing the graphitic sp$^{2}$ content, making the films more diamond-like.   

Neutron Diffraction study of Ru doping in Pr$_{0.5}$Ca$_{0.5}$Mn$_{1-x}$Ru$_{x}$O$_{3}$ (x $\le $ 0.10)

Small amount of Ru substitution ($<$10 {\%}) for Mn in charge-ordered manganites destroys charge-ordering (CO) and induces ferromagnetic metallic state. To probe the dramatic role played by Ru in preventing the CO state, we have carried out neutron diffraction studies of Pr0.5Ca0.5Mn1-xRuxO3 compounds (x = 0.0, 0.05 and 0.10) in the temperature range 300-10 K. Evolution of lattice parameters with temperature points out that lattice distortion which accompanies charge ordering disappears on Ru doping. Also Ru doping alters the MnO$_{6}$ octahedron shape from 4-long/2-short type to 2-long/4-short type that suppresses the antiferromagnetic ordering and hence induces ferromagnetism through double exchange interactions. The local structure of x = 0, 0.05 and 0.10 compositions were analyzed using Pair Distribution Function (PDF) at 295 K and 15 K.   

Pressure induced structural changes in AgPb18SbTe20

The cubic chalcogenide AgPbmSbTe2+m type compounds are an important class of thermoelectric materials for which figures of merit as high as 2.2 at 800 K have been reported recently [1]. The structures of these compounds have been investigated under pressures up to 30 GPa using a diamond-anvil cell and an imaging plate with synchrotron radiation at HPCAT. Various runs were performed with different pressure media such as 4:1 methanol-ethanol and silicone fluid. The results show pressure induced structural transitions in AgPb18SbTe20. The NaCl-type cubic phase transforms to an orthorhombic phase at 6.4 GPa and then to a CsCl-type cubic structure at 14 GPa. The high pressure CsCl-type phase is stable up to 30 GPa. The ambient structure is recovered on releasing the pressure from the cell. The results will be presented in detail. [1]. K.F. Hsu etal., Science, 303, 818 (2004)   

Metal-insulator transition in Hubbard-like models with random hopping

We study the combined effects of random hopping disorder and short-ranged interparticle interactions in half-filled Hubbard-like models of spinless and spinful fermions in $D \geq 2$ dimensions. In a given realization, the hopping disorder is chosen to break time-reversal invariance, but to preserve the special ``nesting'' symmetry responsible for the charge density wave instability of the clean, non-interacting Fermi surface. For spinless fermions, the hopping disorder may arise from the application of a random orbital magnetic field to the otherwise \emph{clean} Hubbard-like model. For the case of spin 1/2 fermions, in addition, the spin SU(2) rotational symmetry is assumed to be broken completely, e.g. by a homogeneous spin-orbit coupling. Using a perturbative renormalization group approach [1], we identify a new, disorder-driven metal-insulator transition in $D = 2+\epsilon$ dimensions, stabilized by the presence of the interactions. Without the ``nesting'' symmetry, these systems would be in the unitary symmetry class ``with magnetic impurities,'' where interaction effects are irrelevant [1]. [1] For a review, see e.g. D. Belitz and T. R. Kirkpatrick, Rev. Mod. Phys. 66, 261 (1994).   

Carrier mobility as a function of InGaAs quantum well to quantum wire transition on GaAs (311)A

Molecular beam epitaxy is used to study self-assembly of In$_{0.4}$Ga$_{0.6}$As quantum wires on GaAs (311)A. Using an InGaAs layer embedded in an AlGaAs/GaAs heterostructure, the correlation between surface morphology, photoluminescence, and electrical transport is investigated. In particular, we will present data and discuss carrier mobility as a function of InGaAs deposition and the resulting surface morphology. As the thickness of the InGaAs layer is increased, a compressive strain field develops, which is relieved at a critical thickness in the form of faceted quantum wires. Paralleling this dramatic change in morphology, the conductivity of the sample is observed to change as measured using Hall bar structures oriented both parallel and perpendicular to the quantum wires. A large anisotropy in conductivity is found to be directly correlated to changes in surface morphology and photoluminescence measurements. The surface morphology was revealed using atomic force measurements on an InGaAs capping layer that reproduced the quantum well or quantum wires embedded in the AlGaAs/GaAs heterostructure.   

Synthesis and Electrical Characterization of Polymer based SiC Nanofibers

Silicon carbide nanofibers have been synthesized from a precursor solution of Polyethyleneoxide (PEO) using the electrospinning technique. The morphology of the fibers was investigated using the scanning electron microscope (SEM) and Transmission electron microscope (TEM). The electrospun fibers were annealed at various high temperatures and their electrical conductivity was measured using the four point probe method. The effect of the annealing temperature on the electrical conductivity of the fibers was investigated.   

Highly Sensitive Micro-Hall Magnetometers Using InSb Quantum Wells

Delta doping in the barrier of InSb/Al$_{0.12}$In$_{0.88}$Sb heterostructures grown by molecular beam epitaxy are studied as micro-Hall sensors. By varying the doping level and the delta position in the barrier, full control of the two dimensional electron gas confined in InSb quantum well is achieved. For example, we demonstrate control of device parameters such as the absolute magnetic sensitivity, noise level, and device detection limit of a micro-Hall sensor. The 200$\mu $m x 35$\mu $m device demonstrates a detection limit of 25 nT and 11 nT at 300 K and 80 K, respectively. These limits were measured at the operating frequency of 10 kHz where 1/f noise is negligible. In addition, if spatial resolution is not needed pT detectivity can be achieved using larger sensors on the order of 1mm$^{2}$. The devices discussed here show a low thermal drift that is less than 2{\%}/$^{o}$K and attractive signal linearity up to 0.1 T for either polarity of the magnetic field at room temperature.   

Optical Properties of Hydrogenated Silicon Nanoclusters: First principles study

Silicon nanoclusters have significant interest due to their potential application to optoelectronic devices in visible range. Using first principles approach, we investigate the electronic and optical properties of hydrogenated silicon nanoclusters. The highest occupied molecular orbital (HOMO) -- lowest unoccupied molecular orbital (LUMO) gap dependence on the cluster size show the same trend by using any exchange-correlation functionals. However, a reasonable agreement to experimental absorption spectra peak values cannot be achieved from conventional LDA or GGA functional-based calculations. Using B3LYP hybrid functional within time-dependent density functional theory, we obtain excitonic energies in quantitatively good agreement to experimental data. The passivant effect on HOMO-LUMO gap and excitonic energies will be also presented.   

Spiral structure of a frustrated spinel CdCr$_{2}$O$_{4}$ studied by polarized neutron techniques

CdCr$_2$O$_4$ has the spinel structure, in which geometrical frustration exists between the Cr$^{3+}$ moments located at the corners of the tetrahedron. This compound shows a transition to a spiral magnetic structure with a characteristic wave vector of $Q=(0,\delta,1)$ ($\delta\sim0.09$) at 7.8 K, which is accompanied by a tetragonal structural distortion with the $c$ axis elongated. [1] We first performed a 1D polarized neutron analysis and determined that the Cr$^{3+}$ moments has a planer anisotropy in the $ac$ plane. Finally, spherical neutron polarimetry experiments were performed using a CRYOPAD on TAS-1 at JRR-3. It is found that the spiral structure is elliptical with the spin component $\sim$24\% elongated along the $c$ axis. [1] J.-H. Chung $et$ $al$., Phys. Rev. Lett. 95, 247204 (2005).   

Classical Nuclear Motion in Quantum Transport

An {\em ab initio} quantum-classical mixed scheme for the time evolution of electrode-device-electrode systems is introduced %and used to study nuclear dynamics in quantum transport. Two model systems are discussed to illustrate the method. Our results provide the first example of current-induced molecular desorption as obtained from a full time-dependent approach, and suggest the use of AC biases as a way to tailor electromigration.   

Production of Cold NO Molecules

Although the success of laser cooling and trapping has had a major impact on atomic physics, the requirements of simple internal structure have limited its use to a few atomic species. A modified Stark guide can be used as a source for cold polar molecules by filtering the cold fraction from a thermal source. We present experimental results for the enhancement of the lowest ro-vibrational states from an effusive source of nitric oxide by the Stark guide capturing those particles with the lowest transverse velocities. By inserting a sphere in our straight guide, we block line-of-sight trajectories between the source and the output of the guide. We have implemented the principle of velocity selection, only those molecules with sufficiently low energies to reflect from the guide will make it to the output. Using a field-stabilized Rydberg time of flight technique we measure the output speed distributions from this filtered source to be between 7-20 K, depending on strength of guide field.   

THz Differential Radar for Detection of Weak Molecular Absorption Lines in Bio-Aerosol

THz frequency range (300-3000 GHz) promises unique capabilities and advantages for detection of trace gases and biological aerosols immersed in the atmosphere. Techniques used for microwave atmospheric remote sensing can be used within the atmospheric transmission windows to carry out standoff detection of biological markers in real time. THz spectroscopy has been used as an important new tool in investigations of atmospheric molecular gases and a wide range of airborne biological materials. We have embarked upon development of field deployable THz differential radar. Bio-aerosols are the most difficult analytes to face due to their heterogeneity in size, toxicity, and bio/chemical composition. JPL has demonstrated monolithic solid-state THz sources with impressive output power. The sources are enabled by W-band power amplifiers and planar Schottky diode multipliers. These, together with room temperature detectors (based on the same technology), allow one to make a compact and robust transmitter/receiver with sufficient sensitivity and frequency agility to carry out detailed investigation of various molecular vapors and bio-aerosols at standard temperature and pressure. Current status of the THz differential radar technology development effort along with future trends will be presented.   

Metal--Insulator Transition in the Flux-Flow Resistivity of Optimally Doped YBa$_{2}$Cu$_{3}$O$_{6+x}$

We have made high resolution microwave measurements of the flux-flow resistivity of optimally doped YBa$_{2}$Cu$_{3}$O$_{6+x}$ in the mixed state at temperatures down to 1.2~K. We find that the effective resistivity of the vortex cores exhibits a metal--insulator transition, with a minimum at 13~K and a logarithmically growing form below 5~K, as has been seen in the low-temperature DC resistivity of under-doped cuprates in which superconductivity has been globally suppressed. Our work is the first report of a metal--insulator transition in optimally doped YBa$_{2}$Cu$_{3}$O$_{6+x}$, and the first to be seen in a system in which superconductivity has not been globally suppressed. The transition is seen in samples of the highest quality and in magnetic fields as low as 1~T.   

Glassy Dynamics in the Adaptive Immune Response Prevents Autoimmune Disease

The immune system normally protects the human host against death by infection. However, when an immune response is mistakenly directed at self antigens, autoimmune disease can occur. We describe a model of protein evolution to simulate the dynamics of the adaptive immune response to antigens. Computer simulations of the dynamics of antibody evolution show that different evolutionary mechanisms, namely gene segment swapping and point mutation, lead to different evolved antibody binding affinities. Although a combination of gene segment swapping and point mutation can yield a greater affinity to a specific antigen than point mutation alone, the antibodies so evolved are highly cross-reactive and would cause autoimmune disease, and this is not the chosen dynamics of the immune system. We suggest that in the immune system a balance has evolved between binding affinity and specificity in the mechanism for searching the amino acid sequence space of antibodies. Our model predicts that chronic infection may lead to autoimmune disease as well due to cross-reactivity and suggests a broad distribution for the time of onset of autoimmune disease due to chronic exposure. The slow search of antibody sequence space by point mutation leads to the broad of distribution times.   

Attractions Between Like-Charged Walls Via Local Molecular Field Theory

A simple model used to explore the interaction between like-charged macroions as mediated by intervening counterions is treated with local molecular field theory (LMF). LMF has recently been extended to general Coulombic systems by splitting the Coulomb potential $1/r$ into a short-ranged core that can be explicitly simulated and a long-ranged portion treated using a mean field approach; the potential separation is determined by a physically-relevant spacing parameter $\sigma$. Here we show that LMF can treat the two-wall model system surprisingly well using an analytical Poisson-Boltzmann type technique. Also, combining self-consistent solution of LMF with simulation of the short-ranged core particles using the minimum image convention yields even more accurate results without using costly and complex Lekner or Ewald sums.   

Coverage and Temperature Dependence of Bond Character of Thiophene on Ge(100)

We studied the adsorption and decomposition of thiophene (C4H4S) on Ge(100) using high-resolution core-level photoemission spectroscopy (HRPES) and scanning tunnelling microscopy (STM). Ge 3d, S 2p, and C 1s core level spectra show the existence of three different adsorption geometries, which are assigned to a weakly bound state, a chemisorbed bonding state ([4+2] cycloaddition reaction product), and a decomposed bonding state (desulfurization reaction product). Furthermore, we found that the ratio of the components induced by three adsorption geometries changes depending on the molecular coverage and the annealing temperature. Under 0.25 ML, the kinetically favorable weakly bound state is initially formed. As the molecular coverage is increased (over 0.25 ML), thermodynamically stable [4+2] cycloaddition reaction products are additionally produced. In addition, the temperature dependent behaviors show that the weakly bound state desorbs followed by the [4+2] cycloaddition reaction product as the molecular thiophene or may decompose to form metallocyclic compounds (C4H4Ge2) and sulfide (Ge2S). We elucidate systematically the change of the bonding states of adsorbed thiophene on Ge(100) according to the amounts of thiophene molecules and annealing temperature. Moreover, we performed density functional theory (DFT) calculations to observe the energetics of three bonding states of thiophene on Ge (100) system.   

Lattice Distortion Driven by Spin-Lattice Coupling

We analyzed lattice-coupled antiferromagnetic spin models on a variety of frustrated lattices. Inspired by the picture of a hexagonal spin cluster proposed for the paramagnetic ZnCrO$_4$ (S. H. Lee \textit{\ et al}., Nature(2002).), we considered hexagon contractions in the \emph{pyrochlore} lattice. Hexagon distortions give rise to mutually orthogonal arrangements of spins for nearby hexagons, and has an energy gain of $-alpha^2/2$ per spin, where $\alpha$ is the spin-lattice interaction strength. However, due to the local rotational symmetry of the $\langle S_i\cdot S_j \rangle$, mean-field theory predicts a lack of lattice displacement in the \emph {triangular} and \emph{kagom\'{e}}lattices. In contrast to the valence-bond-solid(VBS) state of the Affleck-Kennedy-Lieb- Tesaki type, we argue that a type of VBS order (partial VBS, PVBS) with only four of the six bonds of the triangular lattice being filled by singlets can be stabilized through spin-lattice interactions and lead to lattice deformations as in the compound YMnO$_3$ (Seongsoo Lee \textit{\ et al.}, PRB(2005)). The ground state is derived as the direct product of states, one of which represents the conventional long-range ordered spins, and the other given by the $\sqrt{3}\times\sqrt{3}$ modulation of the valence bond amplitudes, $|GS \rangle = |LRO \rangle \otimes |PVBS \rangle$. The excitation spectrum for the modulated valence bond state is worked out within the single- mode approximation. The spectrum offers a new collective mode, which is distinct from the spin wave excitations of the magnetically ordered ground state, and in principle, observable by neutron scattering.   

Photonic digitizing by laser crossing in thin-film semiconductors

During the last two years, we developed a straightforward laser modulation concept, which has the potential to be employed in similar ways as microelectromechanical systems (MEMS). The concept is realized by crossing two laser lines in a semiconducting thin-film on glass, i.e., by switching one of the beams, one achieves a clearly resolved (up to 30{\%}) modulation of the other beam. In case of thin-film GaAs response times in the picosecond range are possible. We investigated the transmission mode of the modulation switch and demonstrated that the GaAs/glass interface exhibits a more effective switch as the GaAs film itself. The experiments have been carried out at room temperature with moderate laser powers on the order of 10 mW. We studied also the influence of polarization on the modulation characteristics in transmission and reflection using birefringent CdS samples. We observed that polarization variations of the green laser varies the modulation amplitude of the red transmitted and reflected ray, paving the way for polarization sensitive all-optical cross connects (OXCs) operating in the THz regime.   

Electronic properties of carbon tori in external fields

Electronic states of achiral carbon tori in electric and magnetic fields are studied by the tight-binding model with the curvature effect. Electronic properties, such as electronic states, energy gaps, and density of states, are very sensitive to the changes in the direction and the magnitude of the external fields. The electric field can widen the $\pi $-electron energy width; furthermore, there are more low- and extreme-energy states. Energy gaps are drastically modulated by\textbf{ E. }The complete modulation of energy gap ($E_{g}\ne $0 to $E_{g}$=0) happens more frequently when\textbf{ E} deviates from the symmetric axis, or its magnitude is sufficiently large. The electric field could change the state degeneracy. Moreover, the modulation of electronic states is enhanced by the magnetic field.   

Electronic properties of multilayered nanographite ribbons in an electric field.

The low-energy electronic properties of the AB-stacked multilayered nanographite ribbons in an electric field are studied through the tight-binding model. They are strongly dependent on the geometric structures (the interlayer interactions, the ribbon edges, the ribbon width, and the ribbon number) and the field strength. The interlayer interactions significantly affect energy dispersions, energy gap (E$_{g})$, density of states (DOS), and free carriers. DOS exhibits many special structures, including plateau, discontinuities, and divergent peaks. The electric field leads to the shift of the Fermi level (E$_{F})$, the production of the new edge state, the change of the band gap, the alternation of the subband spacing, and the semiconductor-metal transitions. In gapless zigzag ribbons, the electric field not only lifts the degeneracy of flat bands at E$_{F}$ but also induces an energy gap. E$_{g}$ is dependent on the ribbon width and the field strength. The semiconductor-metal transitions occur in both armchair ribbons and zigzag ribbons in the increase of the electric field. The above-mentioned effects due to electric field is completely reflected in the features of DOS such as the generation of special structures, the shift of peak position, the change of peak height, and the alternation of band gap.   

Possible observations of microwave influences on quantum states and transition rates

A realistic interpretation of quantum phenomena leads to a conclusion that there might be a hidden mechanism guiding the transition of quantum objects. Thus, manipulations of such a mechanism may affect the regular pathways of quantum phenomena. A recently formulated hypothesis suggests that quantum phenomena on the mesoscopic scale can be influenced by microwave radiation of order 10\^{}11 Hz. This frequency seems to present a watershed between observable quantum effects and classical physics. Electromagnetic activities at higher frequency exhibit characteristic quantum mechanical behavior whereas at lower frequency they present typical Maxwell waves. In the range of 10\^{}11 Hz both kinds of electromagnetic activities coexist: millimeter microwaves and far-infrared quanta. The destruction of long-range quantum order in superconductors at about 10\^{}11 Hz is interpreted in terms of energy gap influences. The same effect could be also responsible for a small but importunate impact of the 10\^{}11 Hz radiation on biological objects. In this work, we propose to investigate the possible impacts of this radiation on quantum phenomena. This includes the study of the decay rate changes of certain nuclei when exposed to microwave radiation and electron tunneling between superconductors. The suspected mechanism could be recognized experimentally as it would imply a clear threshold effect at some point around the 10\^{}11 Hz frequency.   

Structural and chemical changes in ultra-high-molecular-weight polyethylene due to gamma radiation-induced crosslinking and annealing in air .

Ultra-High-Molecular-Weight-Polyethylene (UHMWPE) is used to fabricate one of the articulating surfaces used in many total joint replacements. Various sterilization and annealing methods affect the mechanical wear properties in ways still unknown at the microscopic and molecular levels. We have investigated crosslinking induced by gamma irradiation and annealing time (both done in air). TEM was used to probe the organization of crystalline lamellae in the polymer. Pyrolysis interfaced with GC-MS was used to quantify the extent of crosslinking by identifying peaks that correlate with crosslinking structures along its backbone. Changes in lamellar stacking and the extent of the crosslinking suggest two types of crosslinking occuring in the material. Lamellar mobility is enhanced during the transitions from one type of bonding to the other, and this appears to optimize near eight hours of annealing time. Results from decomposition experiments and percent crystallinity measuresments provide further support for this postulated theory.   

Antiferromagnetism in Superconducting Tl2223 well below Tc

We examine transverse field (TF; 5 kOe) Tl2223 muon-spin-rotation (muSR) data to search for antiferromagnetism (AF) near and in the vortex cores. [1] The vortex frequency signals are fitted by Gaussian and Lorentzian curves. For T $<$ 0.4Tc, Lorentzians fit much better, indicating AF. Since this is especially true for the high-field side, AF appears to exist near and in the vortex cores. [1] Zero field (ZF) muSR data of flux trapping in Tl2223 recorded at 10 K are also studied. We examine signals arising from the initial vortex core, critical field, and remnants of the initial mixed state at five kOe. Three signals at 1.5 MHz, 9-10 MHz (0.7 kOe; Bc1)and 24-25 MHz (1.8 kOe; a 2D-3D transition) are observed. The 110-Oe flux arises from magnetism initially present in the vortex state [1] before the flux trapping. Its ZF fit values are consistent with the AF-vortex TF-fit parameters. Both TF and ZF-muSR evidence suggests AF exists in the Tl2223 superconductor, supporting theories that predict a magnetic origin for cuprate superconductivity. [1] J. Lee et al, J Applied Physics 95 (2004) 6906; Virtual J Applications of Superconductivity, June 2004 V6 Issue 11; www.jyi.org/volumes/volume10/issue6/articles/prudchenko.html   

Interacting electrons in quantum-critical crossover regime in quantum dots

\begin{document} We present numerical study of the statistics of ground state \$S\_z\$ in the crossover in which spin-orbit scattering is tuned in 2D quantum dots {[}1{]}. The spin-orbit interaction introduces new symmetry limits of the single-particle Hamiltonian {[}2{]}. With the help of universal Hamiltonian for this case {[}1{]} we have created a framework for calculation of spin statistics in disordered quantum dots in the large Thouless number limit. In addition we have analyzed the spin excitation spectrum in the quantum critical regime dominated by collective critical fluctuations {[}3{]}. \begin{thebibliography}{1} \bibitem{key-1}Y. Alhassid and T. Rupp, Phys. Rev. Lett. 91, 056801 (2003). \bibitem{key-2}I.L. Aleiner and V.I. Fal'ko, Phys. Rev. Lett. 87, 256801 (2001). \bibitem{key-3}G. Murthy, Phys. Rev. B 70, 153304 (2004). \end{thebibliography} \end{document}   

Eriksen model: neural network dynamics analysis and simulations

We analyze a connectionist neural network model of the two-alternative forced choice Eriksen task, in which subjects must correctly identify a central stimulus and disregard flankers that may or may not be compatible with it. We have analyzed a linearized version of the connectionist model for the Eriksen two-alternative forced-choice task. We show that, provided solutions remain within the central domain of the logistic function in which it may be approximated by a linear function that matches its slope g at the bias point b, analytical solutions of a decoupled, linearized model modulated by a pre-determined attention signal provide reasonable estimates of critical times at which evidence in favor of the correct and incorrect alternatives cross over for incompatible trials. We then derive estimates of accuracy as a function of response time by interrogating a drift-diffusion (DD) process with variable drift rate, fitted to outputs from the perception layer of the fully nonlinear model. We compare our results with numerical simulations of the full nonlinear model and discuss the possibility to use information theory to optimize our network model.   

Generation of Soft Shockwaves by High-Speed Non-Rigid Liquid Spray

High-pressure, high-speed sprays are an essential technology in many applications, including fuel injection systems, thermal and plasma spray coating, and liquid-jet machining. Those liquid jets, often optically opaque due to the highly dense liquid droplets surrounding the sprays, can travel at supersonic speeds and generate shockwaves. It has been believed that the shock wave can be sustained through only continuous compression with dissipative and irreversible process. However, the characteristic of the shockwaves generated by the liquid jet is different from those generated by supersonic rigid objects. Unlike those created by supersonic rigid object, the soft shock wave reveals the reversible compression process followed by the decompression process behind of the shock wave front. We employed the time-resolved x-radiograph using synchrotron source to uncover ``soft'' nature of the shockwaves quantitatively, which is in good agreement with independent theoretical analysis using the computational simulation of fluid dynamics.   

Shock Wave Effects on Polymer Morphology

A gas gun has been used to launch steel ogival-nosed projectiles at a selected impact velocity into steel plates with polyurea, polyurethane and polyethylene coatings. Experiments were performed for two polymer thicknesses, and with the coating cast or loosely held onto the steel plate. Recovered polyurea, polyurethane and polyethylene coatings were characterized by small angle x-ray scattering (SAX) to determine the plastic strain at the constrained metal/polymer interface and the free boundary polymer interface. The difference in plastic strain as found by SAX in polyurea shows both flow orientation and possible fibril morphology compared to only flow orientation in polyurethane and polyethylene. The relative amounts of resulting void damage at the free surface due to reflected rarefaction waves are also described.   

Molecular Dynamics simulations of perpendicular tetracosane films

We present the results of Molecular Dynamics computer simulations of perpendicular tetracosane (C$_{24}$H$_{50})$ films adsorbed onto a bilayer of tetracosane on graphite in the temperature range [100K, 500K]. Various structural and thermodynamic quantities are utilized to characterize the system's temperature evolution. The system goes from the low -- temperature solid phase supporting a perpendicular third layer to the collapse of the perpendicular film near $T$ = 300K to a dense, coalesced patch at high temperature.   

Deterministic simulations of endohedral noble gas release from fullerene clusters

Molecular Dynamics computer simulations are conducted on small ($N$ = 5) endohedral X@C$_{60}$ clusters, where X = He, Ne, Ar, Kr and Xe. The cluster dissociates at a temperature between $T$ = 1150K and 1200K. As temperature is raised further, endohedral atoms begin leaving the cluster near $T$ = 4000K. In the temperature range [4000K, 5000K] escape constants and half lives are calculated for release of the various gases. Helium exits much more quickly than any other noble gas examined, and larger species tend to exit more slowly not only because of their larger collision diameter but also because they stabilize the fullerene cage. Comparisons with and contrasts to experiment are mentioned.   

Systematic Study of magnetic Field Effects in Amorphous Solids at Ultra Low Temperatures.

The dielectric response of some amorphous solids below 100 mK is known to be sensitive to applied magnetic field. While a theoretical explanation for this phenomenon has been proposed, a systematic experimental study of different glasses in a broad parameter range is advised. We investigate and compare the behaviour of Suprasil, BK7, Aluminum-Barium-Silicate, Corning and Durane samples in the temperature range from 2 mK to 200 mK in presence of the magnetic field up to 30 milliTesla. In addition, we hope to find an amorphous solid whose dielectric constant shows no magnetic field dependence, making it suitable for thermometry in applied magnetic fields.   

Atomic Stability as Result of Electrodynamic Stability Condition

According to [1] an electron {\Large{\sf e}}$^{-}$ is formed of an oscillatory massless charge $-e$ in general also traveling at velocity $v$, and the resulting electromagnetic waves of angular frequency $\omega^j$, $j=\dagger $ and $ \ddagger$ for generated in $+v $ and $-v$ directions. The wave energy $\hbar \sqrt{\omega^\dagger\omega^\ddagger}$ equals the charge oscillation energy $\varepsilon_q$ (with the $v=0$ portion) endowed at the charge's creation; $\varepsilon_q/c^2$ gives the electron mass $m_e$, $c$ the wave speed. For an atomic orbiting electron, the charge's $v$ motion is along a circular (or projected-elliptic) orbit $\ell$ of radius $r$; so are its waves. (a) The waves meet in each loop with the charge, are absorbed a portion by it and reemitted, repeatedly, and thereby retained to it; the vacuum, having no lower energy levels for the charge to decay to except in a pair annihilation, is essentially a non-dissipative medium. (b) The two waves, being Doppler-differentiated for the moving source, meet each other over the loops and superpose into a beat, or de Broglie phase wave ${\mit\Psi}$. ${\mit\Psi} =C e^{i(k_d \ell-\omega T)}$ is a maximum if $2\pi r_n = n\lambda_{dn} $, $n$ integer, $\lambda_d=\frac{2\pi}{k_d}=(\frac {c}{v})\lambda$ the de Broglie wavelength and $\lambda=\frac{2 \pi c}{\omega}$, and accordingly yields a stable state. The corresponding overall eigen solutions are exactly equivalent with the QM results. The classical electrodynamic stability conditions (a)-(b) entail the stability of the atomic system. \quad [1] JX Zheng-Johansson \& P-I Johansson, {\it Unification of Classical, Quantum and Relativistic Mechanics and the Four Forces}, Fwd Prof R Lundin, Nova Science, NY, 2005; see also B40.00003, this meeting.   

Domain wall soliton in a trapped spin-1 atomic Bose-Einstein condensate

Solitons are interesting phenomenon in nonlinear systems, including atomic Bose-Einstein condensates. We investigate numerically a soliton state of a ferromagnetically interacting spin-1 condensate confined in a cigar shaped harmonic trap. The dynamics of this soliton, described by the coupled Gross- Pitaevskii equations within the mean field theory frame, is stable and intimately related to the recently observed domain structures in spin-1 condensate and similar to the domain wall soliton in two-component condensates. We present a rotating reference frame in which the local spin dynamics of the soliton becomes time independent.   

Near-wall turbulence structure in Poiseuille-Couette flow

Given recent reports that ultra-hydrophobic surfaces generate slip at the wall in laminar flows, the question arises whether turbulence drag reduction can be achieved over such surfaces. If so, how is the near-wall turbulence structure affected? This work focuses on the effects that a specified wall velocity (or wall slip) can have on the turbulence field. Direct numerical simulations of a Poiseuille-Couette flow channel were used. Computations were completed for Re=5200 (based on centerline mean velocity and channel width). The runs included the cases where one wall moved with 0, 1, 2 and 4 plus velocity units in the flow direction relative to the opposite channel wall. The mean velocity maximum shifted towards the moving wall as the wall velocity increased, as well as the point at which the Reynolds stress crossed zero. The turbulence intensity was lower close to the moving wall side. The velocity correlation coefficients showed that near-wall structures became shorter. The paper will discuss the implications on slip-induced drag reduction.   

A methodology to separate single cells and single mitochondria to determine the location of heteroplasmy in mitochondrial DNA

A mixture of mutated and wild type mitochondrial DNA is referred to as a heteroplasmic population. Mitochondrial DNA heteroplasmies have been studied at the multi-cell level with some being linked to chronic symptoms of mitochondria-based diseases. However, the mechanism producing heteroplasmy is undetermined. One question is whether mitochondrial DNA heteroplasmies are present within single mitochondria. To address this issue we developed a protocol to isolate a single mitochondria from single human leukocyte cells. The cells from an HL-60 cell culture were labeled with Mitotracker Green FM and showed to contain a heteroplasmy at the cellular level (PCR and sequencing showed a 50/50 C/T heteroplasmy at nucleotide position 12071). In order to study heteroplasmy at the single mitochondria level a pulsed UV laser was used to lyse an individual cell. Mitochondria escaped from the cell and optical tweezers were used to transfer single mitochondria into a micropipette tip. Preliminary results suggest that single mitochondria also contain the heteroplasmy.   

Carrier Diffusion Model for Electrical Conductivity Response of Bi3TiNbO9 to Ambient Humidity

The electrical properties of the metastable phase of Bi$_{3}$TiNbO$_{9}$ isomorphic to $\delta $-Bi$_{2}$O$_{3}$, in ceramic pellet form have been established. Pellets are formed by mechanochemical activation (1 to 48 h in an SPEX 8000 automatic mill), and uniaxial pressing, leading to the metastable phase in the 310 to 490 \r{ }C, as the activation time increases. Correspondingly, the stable Aurivillius phase forms in the 500 to 600 \r{ }C range. The current through pellets with $\sim $10 nm thick sputtered Pt electrodes increase by factors between 10 an 2000 in a transient from dry air to dew point of 10 \r{ }C at room temperature. The response time (10{\%} to 90{\%} of the current rise) is in the 15 to 30 s range, and it does not deteriorate under saline ambient, nor does it depend on the dry carrier gas (air, Ar, N$_{2}$, O$_{2}$, or 0.1{\%} H$_{2}$ / He). A bulk carrier diffusion model achieves a close qualitative fit to the current response to bias and humidity cycling.   

Influence of Gas Heating and Vibrational Kinetics on the Ionization Dynamics of Preformed Air Plasma Channels

An extensive self-consistent air-plasma model based upon the Boltzmann equation for the electron energy distribution function, coupled with a heavy particle kinetics was developed to study electric discharges in a preexisting air plasma column [1]. Incorporated in the model are the steady-state balance equations for various nitrogen and oxygen species in ground and excited states, as well as atomic and molecular ions. The influence of the gas temperature is accounted for by reduction of the neutral density, collisional processes such as recombination, dissociation, V-V and V-T reactions [2], and by reactions involving electronically excited states of O$_{2.}$ The model was applied to study the influence of the gas temperature and vibrational kinetics on the breakdown processes in a preformed air plasma channel. Numerical calculations predict that electrical breakdown occurs at relatively low electric field. The calculated self-consistent breakdown electric field is $\sim $10 kV/cm for gas temperature of 300 K, while at temperature of 600 K it drops to $\sim $5.7 kV/cm, in excellent agreement with the experimentally determined breakdown electric field [1]. * NRC-NRL Postdoc [1] Tz.B. Petrova, H.D. Ladouceur, and A.P. Baronavski, 58th Gaseous Electronics Conference, 2005; San Jose, California, FM.00062 [2] J. Loureiro and C.M. Ferreira, J. Phys. D: Appl. Phys \textbf{19} (1986) 17-35   

Defend Science: The Attack on Scientific Thinking and What Must Be Done

``In the United States today science, as science, is under attack as never before (\textit{Defend Science}, defendscience.org).'' Beyond, and underlying, the many particular attacks and outrages in different spheres and policy areas is the question of the scientific method and whether it is going to be upheld and applied, or whether -- even in the realm of science itself -- that method is going to be replaced by something antagonistically opposed to the scientific method. These attacks are increasingly coming from powerful forces, in and out of the Bush administration, with an extreme right-wing political agenda, a Biblical-literalist ideological agenda, and theocratic aspirations for society. Individual scientists may be atheists, agnostics, or may hold various religious beliefs, but if religious and theistic elements are forced into the definition of science, then the scientific process is undermined and science cannot really be practiced. We can and must develop a society wide battle, initiated by scientists, but involving ever growing masses of people to defend science and scientific thinking. Scientists from various fields must be mobilized to issue a public call to millions with this urgent message as the beginning of this effort. I will discuss the necessity, possibility, and some initial efforts toward developing this kind of societal movement in defense of science.   

Tod-Down Physics

Over the past 7 years I have worked to develop two, two-semester course sequences in Theoretical and Computational Physics appropriate for undergraduate education in Physics. Covering material traditionally handled in Classical Mechanics and Electricity and Magnetism, the Theoretical Physics sequence stresses mathematical rigor, physical insight and a project- based paradigm, covering topics such as “Landing on the Moon,” “Realistic Tidal Models,” and “The Solar Sail.” A two- volume text (recently adopted by Mercer University) has been developed for the Computational Physics sequence and introduces students to the essentials of Maple, LaTeX and JAVA as well as web-page (HTML, JavaScript) publishing. While the bulk of the first semester is devoted to software use and algorithm development (i.e., numerical integration) wrote homework is supplemented by ``group’’ quiz and project activities. In the second semester laboratory experiments such as the “Toothpick Toss”, “The Not-So-Simple Harmonic Oscillator” and the “Chaotic Diode” are performed and then computational simulations are developed using various tools (i.e., JAVA, Visual Basic, Matlab).   

Delayed Ionization Studies of Metal-Carbon Clusters

Clusters of molecules, restricted to the nanometer size regime, display reduced size phenomena associated with the effects of quantum confinement. One such occurrence, termed delayed ionization, is a fascinating phenomenon that has been observed in various metal, fullerene and metal-carbon cluster systems. Generally, favorable candidates to display delayed ionization characteristics are clusters containing a significant number of vibrational modes. Furthermore, the magnitude of the cluster's ionization potential must be in competition with its binding energy, whereby dissociation is impeded compared to ionization. The mechanism most commonly used to explain the delayed emission of electrons is termed thermionic emission. In order to interrogate this effect, delayed ionization studies are performed on early transition metal-carbon cluster systems. These clusters are produced in a laser induced plasma reactor cluster source coupled to a nanosecond ultraviolet photoionization laser, and are detected via the microchannel plates of a reflectron time-of-flight mass spectrometer. In this work, interesting delayed ionization characteristics of metal-carbon clusters will be presented in an effort to provide a better understanding of the relaxation processes that occur within clusters.   

Probing minimal scattering events in enhanced backscattering of light using low-coherence induced dephasing

We report the first experimental evidence that the minimum number of scattering events in enhanced backscattering (EBS, also known as coherent backscattering) of light is double scattering in discrete random media, which has been hypothesized since the first observation of EBS of light. We exploit low spatial coherence illumination to dephase time-reversed partial waves outside its finite coherence area, which virtually creates a controllable coherence volume and isolates the minimal number of scattering events from higher order scattering in EBS. In addition, EBS under low spatial coherence illumination possesses unique advantageous features compared to conventional EBS: (i) The spatial coherence length of illumination can be made to be the shortest length scale (except particle sizes) in weakly scattering media such as biological tissue. (ii) A large number of the independent coherence areas provide statistical information about the optical properties of random media in a single measurement, without configuration or ensemble averaging. (iii) LEBS allows varying the spatial coherence length of illumination to control the dephasing rate externally and LEBS does not require complicated sample preparations. Thus, these characteristics of LEBS will facilitate investigations of EBS in weakly scattering random media including biological tissue.   

Femtosecond time-resolved optical studies of collective modes in La$_{1-x}$Ca$_{x}$MnO$_{3}$

We report on time-resolved ultrafast optical measurements on a La$_{1-x}$Ca$_{x}$MnO$_{3}$ (LCMO) single crystal ($x $= 0.3) and thin film ($x$= 0.33). The differential reflectivity shows coherent GHz oscillations due to the excitation of longitudinal acoustic (LA) phonons. The wavelength dependence of the period of such oscillations allows us to determine the sound velocity and elastic constant. Above $T_{C}$, the measured GHz sound velocity is more than 40{\%} larger than the sound velocity in the MHz range as determined by ultrasound measurements. The data is explained by a possible existence of polaron liquid state. Also the differential polarization shows coherent magnons in LCMO. We determine the magnetic anisotropy and spin stiffness constants from the magnetic-field dependence of the frequency of the spin waves.   

Quartz Crystal Microbalance Measurements of Protein Deposition onto Cross-linked polyHEMA Hydrogel

The adsorption of various concentrations of several opthalmologically relevant proteins was measured using Quartz Crystal Microbalance (QCM). Hen egg white lysozyme HEWL, bovine serum albumin BSA, and lactoferrin were measured both individually and in various combinations as they adsorbed onto cross-linked polyHEMA substrate. Results are discussed in terms of the concentration and time dependence of total adhered protein, as well as the amount of desorbable protein. Variations seen during competitive adsorption are also presented.   

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

The S=1/2 Heisenberg antiferromagnet on a triangular lattice is very well represented by Cs$_2$CuBr$_4$ due to its small anisotropy. An unique feature of this system, as predicted by theory and borne out by experiment, is the magnetization plateau at 1/3 of the saturation magnetization. Previous specific heat and magnetocaloric effect measurements in DC fields of up to 20T have been used to determine the magnetic phase diagram in this regime. However, the nature of the phase diagram is unclear in higher fields. Further specific heat and magnetocaloric effect measurements have been made in DC fields up to 33T, the results of which will be presented in order to elucidate the nature of the phase diagram above the magnetization plateau.   

Experiments with Ultra-cold Atoms in an Optical Lattice with Dynamically Variable Lattice Constant

We have implemented a one-dimensional optical lattice whose periodicity may be dynamically varied with ultra-cold atoms in-situ. We have measured atom heating rates in the lattice as a function of lattice periodicity ramp velocities and profiles. We superimpose another one-dimensional lattice with fixed periodicity and measure atom diffraction as a function of the ratio of the two competing periodicities. Finally, we report interesting near field atomic diffraction results for extremely large periodicity lattices.   

Effect of the anisotropy factor of scattering and the finite spatial coherence length of light source on enhanced backscattering

Enhanced backscattering (EBS) of light (also known as coherent backscattering) is a constructive self-interference effect in the backscattered direction due to the photons traveling along time-reversed paths in a disordered medium. EBS can be used for characterization of disordered media and recently been used in cancer detection. Conventional EBS is determined by the scattering transport mean free path of the medium. However, the properties of EBS become more complex and richer in case of anisotropy of scattering (anisotropy factor g$>$0 ) and finite spatial coherence length (Lsc) of a light source. We report our experimental and numerical studies of the effects of g and Lsc on EBS. We demonstrate for the first time that the profile of the EBS peak varies non-monotonically with mean free path length ls, and is a double valued function of ls for a range of values of Lsc and g. Hence, the value of g can be extracted from the EBS profile, which may provide important and previously unattainable information about biological tissue in situ.   

Modeling Low-Coherence Enhanced Backscattering (LEBS) using Photon random walk model of Light Scattering

Enhanced backscattering (EBS) peak from a biological media with transport mean free path length ($l_{s}^{\ast }) \quad >>$ wavelength $\lambda $, is extremely small ($\sim $0.001$^{0})$ making the experimental observation of such narrow peaks to be difficult. Hence, we developed a low coherence enhanced backscattering (LEBS) technique by combining the EBS measurements with low spatial coherence (LSC) illumination and low temporal coherence detection. LSC behaves as a spatial filter preventing longer path lengths and collects photons undergoing low orders of scattering. The experimental angular width of these LEBS peaks ($\sim $0.3$^{0})$ are more than 100 times the width of the peak predicted by conventional diffusion theory. Here we present a photon random walk model of LEBS cones to further our understanding on the unprecedented broadening of the LEBS peaks. In general, the exit angles of the scattered photons are not considered while modeling EBS peaks in diffusion regime. We show that these photon exit angles are sensitive to the low orders of scattering, which plays a significant role in modeling LEBS peaks when the spatial coherence length of the light source is much smaller than l$_{s}^{\ast }$. Our results show that the model is in good agreement with experimental data obtained at different low spatial coherence illumination.   

Highly Ordered Arrays of Silicon Nanostructures Fabricated by Reactive Ion Etching and Atomic Fluorine Etching

High density arrays of conical nanostructures have been fabricated in silicon using a combination of anodic aluminum oxide (AAO) templating and dry etching techniques. Optically, such structures appear black, indicating strong optical absorption. Such structures have applications associated with field emitters and photo-voltaic cells. Silicon directly exposed to an atomic fluorine beam yields an array of sharp silicon spikes. Using the AAO template as a mask with the fluorine beam or reactive ion etcher (RIE) results in highly ordered arrays of nanostructures. In particular, the RIE samples have conical silicon nanostructures, approximately 150 nm in height. These nano-cones are arranged in a honeycomb pattern with a center-to-center spacing of approximately 100 nm and exhibit a high degree of ordering over micron by micron areas. Characterization of these nanostructures was carried out by scanning electron microscopy and various optical techniques.   

Synthesis and Mechanical and Electrical Properties of Carbon Nanotubes Grown at Low Temperatures by Thermal Chemical Vapor Deposition

Carbon nanotubes (CNTs) have shown great potentials in versatile applications such as electron sources, heat interface materials and drug delivery vehicles due to their unique aspect ratio, thermal conductivity, and biocompatibility. CNTs have also attracted wide interests in applications for the next generation microelectronics, including interconnects, mainly due to its high current carrying capacity, $i.e. >$10$^{9}$ A/cm$^{2}$. However, CNTs have been commonly synthesized under high temperatures, e.g. $>$ 1000 $^{o}$C for laser ablation and arc discharge and 600-900 $^{o}$C for chemical vapor deposition (CVD), which is not compatible with the $<$ 450 $^{o}$C requirement for microelectronic technology, and makes it difficult to integrate CNTs into integrated circuit chips. In this study, we present a controlled growth of CNTs at 450 $^{o}$C using a simple thermal CVD method. It has been shown that a combination of catalyst choice and preheating precursors is critical for the formation of CNTs at low temperatures. As-grown CNTs have been characterized using scanning electron microscopy, where vertically aligned dense short nanotubes films with lengths of $\sim $ 400 nm have been observed. For applications in microelectronics, mechanical and electrical properties of short CNT films are tested and the results will be discussed. .   

Magneto-Transport Studies of Molecular Beam Epitaxial Grown Osmium Silicides

Semiconducting transition metal silicides present a possible solution to on-chip integration of optical and electronic Si-based circuitry. Two phases of osmium silicide (OsSi$_{2}$ and Os$_{2}$Si$_{3})$ are predicted to have promising optical characteristics but require additional development to fully determine their feasibility for high-quality devices. This study has been motivated by reports that OsSi$_{2}$ has a bandgap between 1.4--1.8eV [1, 2] and Os$_{2}$Si$_{3}$ may have a direct bandgap of 0.95 eV [3] or 2.3 eV [1]. In this paper we will present temperature dependent (20 $<$ T $<$ 300 K) magneto Hall measurements of molecular beam epitaxial grown osmium silicide thin films. Os and Si were coevaporated onto Si(100) substrates at varying growth rates and temperatures. XRD was performed in order to identify the silicide phases present. We will discuss our results in relation to the known phase diagrams and our growth parameters. [1] L. Schellenberg et al., J. Less-Common Met. \textbf{144}, 341 (1988). [2] K. Mason and G. M\"{u}ller-Vogt, J. Appl. Phys. \textbf{63}, 34 (1983). [3] A. B. Filonov et al., Phys. Rev. B \textbf{60}(24), 16494 (1999).   

Electron transport studies of superconducting Pb single-electron tunneling transistors

We investigate the electronic transport properties of superconducting Pb single-electron tunneling transistors created by electromigration of Pb nanowires. In the superconducting state, the conductance is suppressed by a combination of the Coulomb blockade effect and the absence of density of states within the superconducting gap. The tunneling spectroscopy at 2 K shows a strong even-odd parity effect which is smeared out at 4.2 K. Upon application of a magnetic field, the superconducting state is suppressed and single-electron tunneling behavior for normal metallic nanoparticles is recovered. We will present a theoretical model and compare it with our experimental data.   

Session M's Speakers Promote Evolution and Deny Creation Without Reference to My Widely Published Evidence of Earth's Rapid Creation and Without Reference to My Recent Discoveries Disproving the Big Bang: Congress Should Investigate Why They Did This

I have long and widely described the primordial polonium radiohalos that exist in Earth's foundation rocks all over the world in the world's leading scientific journals. The short half-lves of the relevant polonoium isotopes -- 3 min for Po- 218 -- requires almost instantaneous creation of the host rocks, generally the hard crystalline rocks such as granites, showing that these rocks were the product of creation, not a long slow process of evolution. My publications can be found at www.halos.com. and have been accessible to the worldwide scientific community for over two decades without being refuted. More recently I have discovered fatal flaws in the big bang theory and also a New Cosmic Model that explains eight of the major predictions of the big bang (see CERN Preprint, Ext-2003-021, and CERN Preprint, Ext-2003-022. Additional disproof is available at www.orionfdn.org, which are papers that were submitted to and received arXiv numbers, but were promptly censored from being released, and continue to be suppresed by P Ginsparg and Cornell University. This is a practice borrowed from a totalitarian state and the Congress needs to investigate why supression of evidence of creation continues. There is no excuse for Session M's speakers not to acknowledge this evidence for the Genesis six- day creation of Earth and the visible heavens.   

Mesoscopic light transport properties of a single biological cell : Early detection of cancer

The progression of carcinogenesis involves morphological changes in the internal structure of a biological cell. These changes are reflected in the fluctuations of refractive index within the cell at scales ranging from a few nanometers to microns. We demonstrate that these fluctuations of refractive index can be measured by our newly developed technique -- partial wave spectroscopic microscopy, and can be quantified using the mesoscopic transport theory of light. Our experimental and numerical results show that the statistics of the light reflection coefficient, the statistics of the localization length and the statistics of the refractive index fluctuation agree well with the mesoscopic light transport theory. Furthermore, our results show that we can detect the progress of carcinogenesis in a single biological cell earlier than any existing technique. We conclude that biological cells are nature made interesting disordered mesoscopic systems, and the changes in the statistics of fluctuations of refractive index in a single cell are highly diagnostic for noninvasive early detection of cancer using mesoscopic theory.   

Hydrogenation of Semiconductors Using an Ultra-Violet Light Source

Hydrogenation (also referred to as passivation) of semiconductors using a plasma discharge is routine. However, the process can cause unwanted modification of the surface, and requires contact masking if control of the lateral dosage is required. We will present results of a new technique for the hydrogenation of semiconductors based on the use of an ultra-violet (UV) light. While our studies have been conducted primarily on HgCdTe, we have similar results for the hydrogenation of GaAs and InP. Using the technique of secondary ion mass spectroscopy (SIMS) we have found that D is readily incorporated into semiconductors when the surface of the semiconductor is simultaneously exposed to a hydrogen (deuterium) gas and UV light. No D is observed to be present (to within the SIMS resolution) if the sample is treated under similar conditions without the UV light present. Early studies of the temperature dependence on the D concentration verses depth indicate that the process is diffusion driven. These results strongly suggest that the UV photons are dissociating adsorbed D molecules to atomic D on the semiconductor surface. In addition to our experimental results details of theoretical modeling to account for this phenomenon will be presented.   

Magnetic Force Microscopy of Superconducting Vortices in Ordered Arrays of Artificial Pinning Centers

Ordered arrays of artificial pinning centers were fabricated in Nb thin films using anodic aluminum oxide (AAO) as a template. The nanohole arrays show only a small decrease in the superconducting transition temperature, $T_c\simeq 7.1$ K, from comparable, unprocessed Nb thin films. These artificial pinning arrays have a triangular lattice parameter of 105 nm and antidot diameters of about 50 nm. Anomalous features at the first, second and third matching fields (matching field = 2170 Oe) were observed in the magnetization half-loops of these arrays at 5 K. Magnetic Force Microscopy (MFM) was used to image the nanohole arrays above and below $T_c$. The images clearly show the nanohole lattice. Individual vortices have also been imaged and their movement within the lattice is being investigated. Preliminary measurements of the depinning force of individual vortices at low fields will be presented. This work was supported by NSF grant nos. DMR-0080054 and NSF-0132534.   

Interference patterns in field emission images of Multi-Walled Carbon Nanotubes

Carbon nanotubes are of interest due to their unusual chemical and electrical characteristics and have thus been suggested for use in a wide variety of applications including field emission cathodes. However, there are indications that their emission characteristics, like those of most field emitters, are sensitive to their vacuum environment. Interference patterns in the field emission images of nanotubes have been observed by others and associated with coherent electron emission. Here we report the observation of interference patterns with either a central node or anti-node which appear to be associated with phase shifts introduced by surface adsorbates. This work is supported by the NSF under grant number ECS-0245682.   

Search for a ferromagnetic quantum criticality in YbIn1$_{1-x}$Rh$_{x}$Cu$_{4}$

The geometrically frustrated system YbIn$_{1-x}$Rh$_{x}$Cu$_{4}$ is investigated in order to search for a quantum criticality associated with ferromagnetic correlations. The undoped YbInCu$_{4}$ undergoes a first-order isostructural valance transition at 42K. The valance transition is suppressed with Rh substitution of x$\sim $0.3 and ferromagnetic correlations grow with increasing Rh content as can be seen from the increasing Weiss temperature in magnetic susceptibility. We report low temperature specific heat measurements down to 50mK on YbIn$_{1-x}$Rh$_{x}$Cu$_{4 }$with x around suspected critical concentration x$\sim $0.6 and discuss the relation between the ferromagnetic correlations and non-Fermi liquid behavior.   

Atomic-resolution Spectroscopic Imaging Scanning Tunneling Microscopy of Triple Layer Cuprate: Bi$_{2}$Sr$_{2}$Ca$_{2}$Cu$_{3}$O$_{10+\delta }$

We present preliminary high spatial resolution scanning tunneling spectroscopy (STS) study of triple layer cuprate Bi$_{2}$Sr$_{2}$Ca$_{2}$Cu$_{3}$O$_{10+ \delta }$. The sample is near optimally doped, with T$_{c}$ = 110 K. The local density of states (LDOS) map shows (1) strongly heterogeneous gap distributions, (2) significantly increased average superconducting gap, (3) equivalent energy-dispersive quasiparticle interference patterns and (4) stronger and sharper coherence peaks compared to double layer cuprate Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+ \delta }$ of similar doping. We also have preliminary observations of the disordered bosonic mode energy distribution qualitatively similar to those of Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+ \delta }$.   

Amphiphilic diblock copolymers with adhesive properties: structure and swelling with water

We study asymmetric block copolymers with the simple diblock AB architecture, in the case where the longer block A is both hydrophobic and ``soft'', whereas the shorter block B is hydrophilic and ``hard''. Materials with such a particular combination of physico-chemical and mechanical properties have distinctive advantages, in particular for designing water-compatible adhesive materials. The phase-diagram is established, combining NMR and SAXS characterizations of the materials. The swelling with water is monitored through gravimetry and ``time-resolved'' SAXS. Indications of maintained adhesive properties in a wet environment are given.   

Ionisation Potentials of Metal Carbide Clusters

Photo-Ionisation Efficiency (PIE) experiments have been performed on gas phase niobium and tantalum carbide clusters to determine their ionisation potentials (IPs). For TanCm (n = 3-4, m = 0-4) clusters an oscillatory behaviour is observed such that clusters with an odd number of carbon atoms have higher IPs and clusters with an even number of carbons have lower IPs. Excellent agreement is found with relative IPs calculated using density functional theory for the lowest energy structures, which are consistent with the development of a 2x2x2 face-centred nanocrystal. For the niobium carbide clusters we observe the species Nb4C5 and Nb4C6. Initial calculations suggest that these clusters contain carbon-carbon bonding. Interestingly, the stoichiometry for Nb4C6 is half that of a metcar, M8C12. Preliminary data will also be shown on bimetallic-carbide clusters.