Session U1: High Tc Transport

New measurements of the magnetic penetration depth in YBCO by Gd ESR, Hc, and microwave cavity perturbation.

In superconductors where local electrodynamics are valid, the superfluid density, or more correctly the superfluid stiffness, is directly related to the London penetration depth. For the cuprate superconductors this fundamental property has proven to be difficult to establish reliably, in \textit{any} region of the phase diagram. Materials issues contribute to this difficulty, as do technical problems associated with the various techniques that are used, problems that are exacerbated by the large anisotropies encountered in the cuprates. Here I will describe our efforts to measure the absolute superfluid stiffness in YBCO over the doping range from slightly overdoped (Tc = 88K) to severely underdoped (Tc = 5K), using a variety of techniques, including the novel technique of zero field ESR in Gd-doped samples, measurements of the lower critical field Hc1, and a new cavity perturbation method. These and other measurements give a new picture of the relation between the superfluid stiffness and Tc. Also, the \textit{slope} of the temperature dependence of the superfluid stiffness drops rapidly for the highly underdoped samples. The implication of these results for models of superconductivity will be discussed.   

Evidence of a nodeless superconducting gap in PrCeCuO from magnetic penetration depth measurements

We have measured the inverse-squared magnetic penetration depth, $\lambda ^{-2}$(T), at 50 kHz of films of the electron-doped cuprate superconductor Pr$_{2-x}$Ce$_{x}$CuO$_{4-\delta }$ over a range of Ce dopings, 0.124 $\le $ x $\le $ 0.144, that extends from underdoped to overdoped. The maximum T$_{C}$ was 24 K at x = 0.131. The films were grown by mbe on SrTiO$_{3}$ substrates that had been buffered with a thin layer of the insulating parent compound, Pr$_{2}$CuO$_{4-\delta }$, to obtain the cleanest possible films. Resistivity decreased smoothly and monotonically with doping. We used a two-coil mutual inductance technique to determine the film conductivity $\sigma $ down to about 0.5 K, and we obtained $\lambda ^{-2}$ from $\sigma _{2}$ in the usual fashion. We found that $\lambda ^{-2}$(T) was flat at low temperatures. That is, $\lambda ^{-2}$(T)/$\lambda ^{-2}$(0) changed by less than the experimental noise of 0.15{\%} over a factor of 3 or more change in T. Fits to the low-T data yield minimum a gap value, $\Delta _{min}$(0)/k$_{B}$T$_{C}$, that is unity near optimal doping and decreases with over- and underdoping. This talk will compare our results with other penetration depth measurements that find quadratic behavior at low T, consistent with a d-wave gap and with phase sensitive measurements.   

Universal scaling relation in high-temperature superconductors

Superconductivity at elevated temperatures in the copper-oxide materials has proven to be one of the great challenges in condensed matter physics. Despite 18 years of intensive study, the nature of the superconductivity in these systems is still not agreed upon. Scaling laws express a systematic and universal simplicity among complex systems in nature. We have recently observed a scaling relation in the high-temperature superconductors\footnote{C.C. Homes {\it et al.}, Nature {\bf 430}, 539 (2004)} between the strength of the superconducting condensate $\rho_s$ (a measure of the number of carriers in the superconducting state $n_s$), the critical temperature $T_c$, and the dc conductivity $\sigma_{dc}$ just above the critical temperature: $\rho_s \simeq 35\,\sigma_{dc}\,T_c$. This scaling relation does not depend on the crystal structure, type of dopant, nature of the disorder, or direction. Interestingly, values for the elemental superconductors Nb and Pb also fall close to this line. However, it may be shown from spectral weight arguments that these points correspond to systems in the BCS “dirty” limit (the scattering rate $1/\tau$ is larger than the isotropic energy gap $2\Delta$); in the extreme dirty limit, the scaling relation $\rho_s \simeq 65\,\sigma_{dc}\,T_c$ is recovered. The implications of the clean and dirty-limit approaches within the copper-oxygen planes are discussed. The superconductivity perpendicular to the planes is often described within a BCS framework by the Josephson effect, which interestingly also yields $\rho_s \simeq 65\,\sigma_{dc}\,T_c$, where the superfluid density and the dc conductivity are now taken along the {\it c} axis. Despite the fact that the BCS model considers an isotropic energy gap, and the cuprates are considered to be {\it d}-wave in nature with nodes, the scaling behavior of the dirty-limit and the Josephson effect is in agreement with experimental observations. This suggests that electronic inhomogeneities may play a crucial role in the nature of superconductivity in these materials.   

An Infrared Probe of the Nodal Metal State in High-Tc Superconductors

The focus of this talk will be on the electromagnetic response of the nodal metal state which is initiated with only few holes doped in parent antiferromagnetic systems and extends up the pseudogap boundary in the generic phase diagram of cuprates. The key spectroscopic signature of the nodal metal is the two- component conductivity: the Drude mode at low energies followed by a resonance in mid-IR. The former can be attributed to the response of coherent quasiparticles residing on the Fermi arcs. The microscopic origin of the mid-IR band is yet to be understood. A combination of transport and infrared data uncovers fingerprints of the Fermi liquid behavior in the response of the nodal metal. The comprehensive nature of the infrared conductivity data sets for YBCO and LSCO systems allows us to critically re-evaluate common approaches to the interpretation of the optical data. Specifically, I will re- examine the role magnetic excitations in generating electronic self energy effects through the analysis of the infrared data for underdoped YBCO in magnetic field.   

Thermal and charge transport in low-doped cuprates at very low temperature

Thermal and charge transport measurements were performed in the normal and superconducting states of ultra-pure samples of low- doped YBCO down to very low temperature. The normal ground state, whether accessed by varying doping or applying a magnetic field, is shown to be metallic. Upon cooling towards T=0, the thermal conductivity exhibits a finite residual linear term and the resistivity increases by only a modest amount, in stark contrast to what is observed in LSCO. The continuity of the residual linear term upon leaving the superconducting state points to a normal state with a nodal excitation spectrum. By directly comparing charge and heat conductivities as T $\rightarrow$0 we are able to perform a preliminary test of the Wiedemann-Franz law in underdoped cuprates.   

Session U2: Jamming and Geometric Constraints

Jammed Ellipsoids Beat Jammed Spheres: Experiments with Candies and Colloids

Packing problems, how densely objects can fill a volume, are among the most ancient and persistent problems in mathematics and science. For equal spheres, it has only recently been proved that the face-centered cubic lattice has the highest possible packing fraction $\phi \quad \sim $ 0.74. It is also well-known that the corresponding random (amorphous) jammed packings have $\phi \quad \sim $0.64. The density of packings in lattice and amorphous forms is intimately related to the existence of liquid and crystal phases and is responsible for the melting transition. Geometrical aspects of packing different shapes and the thermodynamic consequences are most readily observed in colloidal systems. Colloids are also useful for building micromachines and there is much more flexibility in colloidal architecture if the building blocks are non-spherical particles. A first step is to understand how such systems densely pack. Here we show experimentally and with a new simulation algorithm that ellipsoids can randomly pack more densely; up to $\phi \quad \sim $0.68 - 0.71 for spheroids with an aspect ratio close to that of M{\&}M's$^{\mbox{{\textregistered}} }$Candies, and even approach $\phi \quad \sim $0.75 for general ellipsoids. The higher density relates directly to the higher number of degrees of freedom per particle, d, and then to the number of contacts per particle Z. We find Z $\sim $10 for our spheroids as compared to Z $\sim $ 6 for spheres, confirming the isostatic conjecture Z=2d. Our results lead to the question as to ellipsoids, or any shaped particle will pack denser randomly than crystalline. In our studies we have found the crystal packings of ellipsoids to a density, $\phi \quad \sim $.7707 which exceeds the highest previous packing.   

Multiscaling at Point J: Jamming is a Critical Phenomenon

We analyze the jamming transition that occurs as a function of increasing packing density in a disordered two-dimensional assembly of disks at zero temperature for "Point J" of the recently proposed jamming phase diagram. Using numerical simulations, we drag a single disk through an increasingly dense assembly of hard disks. We measure the total number of moving disks and the transverse length of the moving region, and find a power law divergence as the packing density increases toward a critical jamming density. This provides evidence that the zero-temperature jamming transition as a function of packing density is a second order phase transition. Additionally, we find evidence for multiscaling, indicating the importance of long tails in the velocity fluctuations of the driven particle. [1] J.A. Drocco, M.B. Hastings, C.J. Olson Reichhardt, and C. Reichhardt, cond-mat/0310291.   

Soft modes and the onset of jamming

Glasses have a large excess of low-frequency vibrational modes in comparison with crystalline solids. We show that such a feature is a necessary consequence of the geometry generic to a {\it marginally} connected solid. In particular, we analyze the density of states of a recently simulated system comprised of weakly compressed spheres at zero temperature. We account for the observed a) constancy of the density of modes with frequency, b) appearance of a low-frequency cutoff $\omega^*$, and c) power-law increase of $\omega^*$ with compression. We predict a length scale $l^*$ below which the boundary conditions strongly affect the system. $l^*$ diverges at the jamming transition when the system becomes isostatic.   

The onset of jamming as the sudden emergence of an infinite k-core cluster

A theory is constructed to describe the zero-temperature jamming transition as the density of repulsive soft spheres is increased. Local mechanical stability imposes a constraint on the minimum number of bonds per particle; we argue that this constraint suggests an analogy to $k$-core percolation. The latter model can be solved exactly on the Bethe lattice, and the resulting transition has a mixed first-order/continuous character. The exponents characterizing the continuous part appear to be the same as for the jamming transition. Finally, numerical simulations suggest that in finite dimensions the $k$-core transition can be discontinuous.   

Spatial structures and dynamics of kinetically constrained models of glasses

In glass-formers and more general jamming systems the microscopic motion is highly constrained because of the interaction with the surrounding particles. An example is the cage effect in glass-forming liquids. Kinetically constrained models encode this in a simple way. They are lattice models in which particles evolve by a stochastic dynamics with kinetic constraints: particles cannot move if surrounded by too many others. We shall show that from these simple dynamical rules highly non trivial physical phenomena emerge as super-Arrhenius behavior, dynamical heterogeneity and finite dimensional glass-jamming transitions.   

Session U3: Unconventional Electronic and Optical Uses of Negative Refractive and Negative Index Materials

Negative Refraction, Left-Handed Materials and Heterostructures in Guided Wave Electronics Using Metamaterials and Nanostructures

A number of remarkable discoveries which are connected to each other have been made in the general subject area of negative refraction in guided wave structures. This triad of discoveries is (1) the unusual electromagnetic field distributions and dispersion diagrams of guided waves [1] in monolithically compatible structures containing left-handed intrinsic materials, which show electric field lines and magnetic circulation patterns never before seen; (2) heterostructure arrangements of uniaxial bicrystals [2] have been discovered to produce electromagnetic fields with asymmetric distributions in guided wave structures; (3) a frequency band exists where propagation using SRR metamaterials [3] is essential lossless. Finding (1) opens up the possibility of creating new electronic devices because of the reconfiguration of the field distributions. Finding (2), based upon the property of broken crystal symmetry of the SO(2) rotation group, offers the possibility of all electronic nonreciprocal devices, something not possible in the last fifty years because of the microwave community's reliance upon the ceramic spin precession physical operation of ferromagnetic materials. Finding (3), using the concepts of effective parameters like rescaled plasma frequencies with direct carrier density dependence removed or severely mitigated, using the associated magnetic and electric linewidths, can have miniscule loss with dispersion in a finite frequency band for the potentially highly dispersive and lossy split ring-rod assemblies employed as unit cells. The theoretical modeling is done analytically and numerically to obtain all of these results, with simulations completed in the microwave and millimeter wavelength regimes, from 5 to 105 GHz using an ab initio anisotropic Green's function solver. [1] C. M. Krowne, ``Physics of Propagation in Left-Handed Guided Wave Structures at Microwave and Millimeter Wave Frequencies,'' Phys. Rev. Letts 92, 053901, Feb. 3, 2004. [2] C. M. Krowne, ``Negative Refractive Bicrystal with Broken Symmetry Produces Asymmetric Electromagnetic Fields in Guided-Wave Heterostructures,'' Phys. Rev. Letts. 93, 053902, 29 July 2004. [3] C. M. Krowne, ``Guided Wave Propagation in Left-Handed Microstrip Structure Using Dispersive SRR Metamaterial,'' submitted PRL Aug. 2004.   

Nonlinear effects in left-handed metamaterials and related structures

We describe a number of nonlinear effects associated with the concept of left-handed metamaterials--composite materials with simultaneously negative dielectric permittivity and magnetic permeability. First, we study transmission of electromagnetic waves through a slab of left-handed metamaterial with a hysteresis-like nonlinear response and describe two types of nonlinear effects: (i) nonlinearity-induced suppression of the wave transmission when an initially transparent left-handed material becomes opaque with the growth of the input wave amplitude, and (ii) nonlinearity-induced transparency of the slab when an initially opaque composite material becomes left-handed (and, therefore, transparent) when the input wave amplitude is increased. We demonstrate, with the help of the finite-difference time-domain numerical simulations, that the nonlinearity-induced wave transmission through an opaque slab is accompanied by the development of modulational instability and the generation of spatiotemporal solitons. Next, we analyze the structure of guided waves supported by a left-handed slab, and the wave transmission through periodic structures made of transparent negative-index (or left-handed) and conventional layers. In addition, we demonstrate novel unique properties of the electromagnetic crystals that include the layers of left-handed metamaterial. In particular, in a sharp contrast with all known results in the theory of wave propagation in periodic media, we demonstrate that a one-dimensional periodic structure with left-handed layers can possess, under certain conditions, a full two-and even three-dimensional spectral gap for the TE- or TM-polarized waves. In this case, the Green function characterizing radiation of a point source becomes exponentially localized in all directions because the electromagnetic radiation cannot propagate through the one-dimensional structure at any angle in the plane.   

Optical Bulk and Surface Waves with Negative Refraction

At optical frequencies the introducing of $\mu (\omega )$ has no physical sense [1]. Using a general approach with a dielectric permittivity $\tilde {\varepsilon }(\omega ,\vec {k})$, we discuss [2] unusual optical nonlinear effects in LHMs and the possibility of seeing negative refraction for optical waves in continuous nonmagnetic media: bulk and surface waves in vicinity of exciton and optical phonon resonances where additional polariton waves [3] have a negative group velocity. The dispersion of surface waves can be engineered by tailoring a surface transition layer [4] to obtain surface waves with negative group velocity. We discuss also a negative refraction in anisotropic transparent media. 1. L.D.Landau, E.L. Lifshits, Electrodynamics of Continuous Media, Pergamon Press,1984. 2. V.M. Agranovich, Y.R. Shen, R.H.Baughman, A.A. Zakhidov, Phys. Rev. B 69 (2004) 165112; Journal of Lumin., December (2004). 3. V.M. Agranovich, V.L. Ginzburg, Crystal Optics with Spatial Dispersion, and Excitons, Springer, 1984. 4. V.M. Agranovich, T.A. Leskova, Progress in Surface Science, 29 (1988) 169.   

Bicrystals Allow Negative and Total Refraction of Electronic and Optical Waves

Recently the so-called left-handed medium (LHM)[1,2] has attracted a great deal of interest primarily for these two reasons: one is that a LHM, when interfaced with a matched right-handed medium (RHM), is able to show an interesting phenomenon -- total and negative refraction of light[2], which is generally believed not possible if only the RHMs are involved; and the other one is the that such total and negative refraction may lead to a very exciting application -- superlensing[3]. Negative refraction or bending has indeed been experimentally demonstrated in a number of ways, but it is typically limited in the spectral region of the microwave and with significant loss[4]. Furthermore, it has now been realized that superlensing can at best be realized under certain extreme conditions. Thus, realistically, what a LHM can offer is a subwavelength resolution[4], which is nevertheless readily achievable using a RHM (e.g., a so-called solid-immersion lens)[5]. One would like to ask: can the phenomenon of total and negative refraction be achieved without using a LHM? Besides the subwavelength resolution, are there any other novel applications for this phenomenon? We will firstly compare different approaches that have been used or proposed for achieving total and negative refraction in terms of their underlying physical mechanisms, then, focus on its realization in a bi-crystal structure[6]. In the bi-crystal approach, none of the components of the permittivity (\textbf{$\varepsilon $}) and permeability\textbf{ ($\mu $}) tensors is required to be negative. The effect relies purely on the dielectric anisotropy in anisotropic RHMs. This approach has offered an experimental demonstration of negative refraction yet with negligible (extrinsic) loss, and it is in principle applicable for any frequency of electromagnetic waves and even for ballistic electrons in semiconductors. A few interesting applications will be discussed for both electrons and light. [1].V. M. Agranovich and V. L. Ginzburg, \textit{Spatial dispersion in crystal optics and the theory of excitons}(1966);V. M. Agranovich, et al., PRB\textbf{69},165112(2004). [2]V. G. Veselago, Sov. Phys. Usp.\textbf{10},509(1968). [3]J. B. Pendry, PRL\textbf{85},3966(2000). [4]J. B. Pendry and D. R. Smith, Physics Today\textbf{57},37(2004). [5]I. Ichimura, et al., Appl. Opt.\textbf{36},4339(1997). [6]Y. Zhang, et al., PRL\textbf{91},157404(2003).   

Session U4: Polymer Microstructures

On the Formation of an Ordered Array of Holes in a Polymer Film:What can Dew Formation Teach Us?

Systems driven far from equilibrium have a remarkable tendency to produce very ordered structures. Such ordered structures have been observed in many a situation where the material subjected to an external perturbing field responds to this perturbation by creating ordered periodic structures. We have used a system driven far from equilibrium to create structures that have subwavelength dimensions and can be made reproducibly. Ordered subwavelength structures are ubiquitous in nature. However, it is only recently that ordered macroporous materials with pore dimensions on the order of the wavelength of visible light have attracted much greater attention. This interest has been in large part due to their anticipated optical properties. We have demonstrated the use of a simple and robust method that uses evaporative cooling for the formation of ordered structures with dimensions that are controllable in a systematic way ranging from about 0.2$\mu $m to 20 $\mu $m. This method uses the formation, and subsequent crystallization of ``breath figures,'' to create the structures. When a cold solid or a liquid surface comes in contact with moist air, moisture condenses on the surface, forming water droplets that grow with time to form patterns on the surface. Such phenomena, referred to as ``breath figures,'' have been studied in detail, starting with the early works of Lord Rayleigh, Baker and Aitken, and more recently by Knobler and co- workers who demonstrated that it was possible to form a hexagonally ordered array of water droplets on a liquid surface as condensation proceeded. We have used ``breath figures'' to form three-dimensional, ordered macroporous arrays with controllable dimensions. We generated breath figures on dilute solutions of polystyrene and other conjugated polymers dissolved in volatile solvents. When solvent evaporation is complete, one is left with a two or a three dimensional array of holes. In this presentation we will discuss the mechanism of structure formation as well as point to some applications for these structured films.   

Block Copolymer Lithography

During the past half decade, extensive resources have been allocated to the development and implementation of new lithographic exposure tools for use by the microelectronics industry to pattern devices with critical dimensions of 50 nm and below. During this same timeframe relatively modest investments were made in the development of imaging materials. As feature dimensions shrink to below 50 nm, however, traditional materials such as chemically amplified photoresists may not be suitable to overcome significant new challenges with respect to problems such as line edge roughness and critical dimension control at the atomic and molecular level respectively. We explore and develop new materials and processes for advanced lithography in which self-assembling block copolymers are integrated into existing manufacturing processes for patterning high resolution structures that are useful for the fabrication of microelectronic devices. A principal concept is to combine the properties of advanced exposure tools (registration, pattern perfection) with the principles of molecular self-assembly (structures of molecular dimensions, thermodynamic control over pattern dimensions and line edge roughness) to meet strict criteria for device manufacturing at the nanoscale. Here we demonstrate that by depositing thin films of ternary blends of block copolymers and homopolymers on chemically nanopatterned substrates with tailored interfacial interactions, we can direct the assembly of perfectly ordered and registered domains with respect to the lithographically defined underlying surface pattern with considerable process latitude. We also demonstrate for the first time that it is possible to direct the assembly of block copolymer domains with non-linear patterns and arbitrary shapes.   

Microstructures of Polymer-Inorganic Hybrids

The study of polymer based self-assembly (bottom-up) approaches to multifunctional polymer-inorganic hybrid materials is an exciting emerging research area interfacing solid state and soft materials and offering enormous scientific and technological promise. By choice of the appropriate synthetic polymers as well as ceramic precursors unprecedented morphology control down to the nanoscale is obtained. Tailoring of the polymer--inorganic interface is of key importance. The structures generated on the nanoscale are a result of a fine balance of competing interactions, a typical feature of complex biological systems. The potential for new multifunctional materials lies in the versatility of the polymer chemistry as well as that of the inorganic chemistry that can be exploited in the materials synthesis. In the present contribution physical insights into the way how to direct microstructures of polymer-inorganic hybrid materials will be presented. In all cases cooperative self-assembly of organic and inorganic species is induced by amphiphilic macromolecules, either block copolymers or extended amphiphilic dendrons, which are blocked species with one block being highly branched. Resulting microstructures are discussed based on the phase behavior of the parent polymer systems that act as structure directing agents. Morphology diagrams of the resulting polymer-inorganic hybrid materials are presented illustrating differences in self-assembly of parent polymer and resulting hybrid systems. Finally, mechanistic structure formation studies are highlighted that elucidate necessary requirements for successful hybrid nanostructure control.   

Use of Polymer Micro-Structures for Drug \& Gene Delivery

The design of polymer microstructures, including polyelectrolyte-surfactant complex formation, plays an important role in the protection and controlled release of drugs {\&} DNA fragments. Two examples are presented: one for drug release and one for gene delivery. Non-viral gene therapy is a challenging problem that has not yet met much success even though numerous attempts have been made. The gene delivery illustration aims to present one specific approach on how DNA fragments can be delivered to a cell by using an electro-spun scaffold as a carrier, i.e., to consider how DNA fragments can be trapped into a scaffold for subsequent release and transfection. Our scheme is to capture the DNA fragments by taking advantage of the DNA coil-to-globule transition and to encapsulate the condensed DNA globule by using block copolymers. The supra-molecular capsule can then be incorporated into a nano-structured biodegradable polymer scaffold by means of electro-spinning. Subsequent DNA release to cells that adhere to the scaffolds was measured by using fluorescence microscopy.\newline\newline \textbf{\textit{Acknowledgements}}\newline \textbf{\textit{Financial Support:}}\newline \textbf{National Science Foundation, Polymers Program (DMR9984102 {\&} Creativity Extension Award), Center for Biotechnology at Stony Brook, ITG Grant, and NIH SBIR Grant to }\textbf{\textit{STAR.}}\newline \textbf{\textit{Main contributors}} include Professors Benjamin S. Hsiao and Michael Hadjiargyrou, Drs. Dufei Fang, Dehai Liang and Kwangsok Kim, Ms. K. Luu and Mr. J. Chiu.   

Functional Microstructures from Iron-Containing Block Copolymers

We have studied the properties of microstructures formed by diblock copolymers composed of an organic block such as polystyrene or polyisoprene, and an iron-containing block such as poly(vinyl ferrocene) or poly(ferrocenyldimethylsilane). We demonstrate that the thermodynamic state of these block copolymers can be controlled by altering the redox state of the ferrocene (Fc) moieties. Oxidizing only 8{\%} of the Fc block results in a 40 K drop in the order-disorder transition temperature. Fc is catalytically active in the oxidized state. Thus one can obtain catalysts from iron-containing block copolymers wherein both the support and the active sites are formed by self-assembly. An interesting property of ferrocene is the fact that its oxidation state can be altered reversibly by the application of small electric fields ($\sim $2V/cm). We are currently exploring the possibility of using electric fields to control the microstructure and function of our iron-containing block copolymers.   

Session U5: Spin Control in Ferromagnetic Semiconductor Structures

Electrical Control of Magnetization in Semiconductors

Ferromagnetic III-V semiconductor (Ga,Mn)As is characterized by $p-d$ exchange stabilized ferromagnetism, small magnetization, and strong spin-orbit interaction [1, 2], thus offering a unique combination of physics related to current-induced magnetization reversal. Here we present our study on (1) current driven magnetic domain wall motion in a lithographically defined (Ga,Mn)As structure [3], and (2) current driven magnetization reversal in fully epitaxial (Ga,Mn)As magnetic tunnel junctions (MTJ's) using GaAs as a barrier [4]. In the former, two regimes are found to be present in the velocity - current density characteristics and the estimated spin-transfer efficiency is as high as 10{\%} or even higher. In the latter, current density required for the reversal in MTJ is found to be lower than that expected from scaling of magnetization. [1] H. Ohno, Science, 281, 951 (1998). [2] T. Dietl \textit{et al.}, Science, 287, 1019 (2000). [3] M. Yamanouchi \textit{et al.}, Nature, 428, 539 (2004). [4] D. Chiba \textit{et al.}, accepted for publication in Phys. Rev. Lett.   

Spin accumulation in forward-biased MnAs/GaAs Schottky diodes

The injection of electrons from ferromagnetic metals into semiconductors has recently received much attention in the field of spintronics since these systems have the potential to serve as room-temperature sources of spin polarization. To date, most research in this vein has focused on electron currents flowing through a tunnel barrier from the ferromagnet to the semiconductor. For example, spin injection has been observed for tunneling through Schottky and aluminum oxide tunneling barriers as well as in more complicated structures such as magnetic tunnel transistors. All of these schemes share the common feature that spin-polarized electrons are injected from ferromagnet to semiconductor. Here we describe experiments demonstrating a new means for the all-electrical generation of spin polarization in ferromagnet/semiconductor epilayers, in which an electron current flows from the semiconductor to the ferromagnet \footnote{J. Stephens et al. Phys. Rev. Lett. 93, 097602 (2004)}. In contrast to the more conventional route of spin injection, we observe spin accumulation at the metal/semiconductor interface of these forward-biased ferromagnetic Schottky diodes. Spatiotemporal Kerr microscopy is used to image the electron spin and the resulting dynamic nuclear polarization that arises from the non-equilibrium carrier polarization. A simple model can be used to describe the spin accumulation effect in terms of spin-dependent interface transmission and reflection coefficients and to estimate its magnitude.   

Interfacial Control of Ferromagnetism in (Ga,Mn)As-based Hetero- and Nano-structures

We discuss recent experiments that demonstrate how heterointerfaces impact the magnetic properties of hetero- and nanostructures derived from the ``canonical'' ferromagnetic semiconductor (Ga,Mn)As. In this material, holes created by the Mn acceptors mediate a ferromagnetic interaction between the Mn ions, and the Curie temperature ($T_{\rm{C}}$) is determined by a complex interplay between substitutional magnetic ions, interstitial defects and holes. Although as-grown epilayers of (Ga,Mn)As typically have $T_{\rm{C}} \leq 110$K, post-growth annealing at low temperatures ($180^{\circ}\rm{C}$ - $250^{\circ}\rm{C}$) significantly enhances the ferromagnetic properties, leading to $T_{\rm{C}} \sim 150$K. We first describe experiments that examine the effects of capping ferromagnetic (Ga,Mn)As epilayers with a thin layer of undoped GaAs [Stone {\it et al}, Appl. Phys. Lett. {\bf 83}, 4568 (2003)]. We find that the overgrowth of even a few monolayers of GaAs significantly suppresses the enhancement of the ferromagnetism associated with low temperature annealing, suggesting that heterointerfaces have a direct impact on the migration of interstitial defects during post-growth annealing. We next demonstrate how nanopatterning allows us to provide alternate defect diffusion pathways, hence remove the constraints on $T_{\rm{C}}$ imposed by the presence of heterointerfaces [Eid {\it et al}, submitted]. Finally, we examine the influence of an overgrown antiferromagnet (MnO) on the magnetic properties of (Ga,Mn)As, demonstrating the first example of exchange biasing of this ferromagnetic semiconductor [Eid {\it et al},Appl. Phys. Lett. {\bf 85}, 1556 (2004)]. Detailed studies show that systematic control over the highly reactive MnO/(Ga,Mn)As interface is essential for the routine achievement of exchange bias in this important spintronic material. This work was carried out in collaboration with K. F. Eid, M. B. Stone, O. Maksimov, K. C. Ku, B. L. Sheu, W. Fadgen, P. Schiffer, T. Shih, and C. Palmstrom. Supported by ONR and DARPA.   

Ferromagnetic Control of Spin-Dependent Electron Currents in a Semiconductor

It is well known that electrons or neutrons scattered against a polarized target become polarized. This talk will show how this principle can be used in variety of ways to generate and to change a spin polarization in a current flowing in a semiconductor interfaced with one or more ferromagnets. In theory it is possible to generate a 100{\%} polarized current or a pure spin current without charge current. The relative merits of the various configurations will be assessed. Experiment tests will be described. Possible device applications provide illustrations of the theory. Work done in collaboration with J.P. McGuire, C. Ciuti, Eric Yang, Yuchang Chen, Thomas Grange, and Ed Yu, and supported by NSF DMR 0099572, DARPA/ONR N0014-99-1-1096 and University of California Campus- Laboratories Cooperation project.   

Unexpected magnetism in thin film dielectric oxides

High temperature ferromagnetism in thin films of dilute magnetic oxides is a widespread phenomenon, of which there appear to be two distinct sources. One is the contribution of the 3$d$ dopant ions themselves, the other is related to crystal defects in the interface region. The latter contributes a magnetic moment of 100 -- 400 $\mu _{B}$ per square nanometer of substrate area, which is largely independent of film thickness or dopant concentration. In very dilute films it seems as if there is a giant ionic moment when the film moment is expressed per 3d cation, but this is because the source of the magnetism is misattributed. It is suggested that the magnetic defects are two-electron or two-hole centres which have a spin triplet as ground state or low-lying excited state. In ZnO or SnO$_{2}$, examples of the latter, the magnetic dopant stabilizes the spin triplet by exchange. However HfO$_{2}$, ZrO$_{2}$ and WO$_{3}$, examples of the former, are ferromagnetic even when undoped. They are 'd-zero' ferromagnets. A characteristic sign of this exotic magnetism is strong anisotropy of the saturation magnetization. Possible links to other systems such as defective graphite or gold/thiol will be discussed.   

Session U6: Physics of Slip Phenomena at Liquid/Solid Interfaces

Slip Behavior in Liquid Nanoscale Films: Influence of Molecular Ordering, Wall Roughness and Patterned Surface Energy

The development of micro- and nanofluidic devices for actuation of liquid films, drops and bubbles requires detailed knowledge of the interfacial forces affecting transport. The small dimension size guarantees that all tranport properties are strongly dominated by boundary effects. The large surface to volume ratios, however, also cause significant frictional losses which can be reduced by generating slippage at the liquid-solid interface. Slippage can be enhanced by surface chemical treatments, textured substrates and nucleation of nanobubbles. High molecular weight polymers also generate large slip lengths, defined as the distance within the solid phase where the extrapolated flow velocity vanishes. While hydrodynamic analyses are useful in providing a continuum description of fluidic response at the microscale, molecular dynamics (MD) simulations offer detailed resolution of the molecular behavior near chemically or topologically modified surfaces, a necessity in constructing nanofluidic devices. In this talk we show how the slip length in nanoscale liquid films is affected by the amplitude and wavelength of surface roughness. We also consider periodic variations in the liquid-solid interaction potential mimicking regions of no-shear and no-slip, as with surfaces covered by nanobubbles. A detailed comparison between hydrodynamic predictions and MD simulations elucidates what geometric and molecular parameters govern the slip length at different length scales. Excellent agreement is obtained when the system size is about an order of magnitude larger than the molecular size. We end this talk with discussion of a simplified model for predicting the dynamic exponent observed in the MD simulations for the power law increase in slip length with shear rate. These studies clearly pinpoint the molecular origin of the dynamic exponent and help explain the different slip laws expected for liquid versus gas flow.   

Low Friction Flow of Liquid at Smooth and Nanopatterned Interfaces

With the recent important development of microfluidics systems, miniaturization of flow devices has become a real challenge. Microchannels, however, are characterized by a large surface to volume ratio, so that surface properties strongly affect flow resistance in submicrometric devices. Although the no-slip boundary condition used for describing simple liquids flows at a macroscopic scale is very robust, it is now admitted that simple liquids may undergo substantial slip on solid surfaces, which cannot be neglected at the scale of tenth of micrometers. However, experimental results on this topic are still controversial~: slip effects reported vary quantitatively (over order of magnitudes) as well as qualitatively (regarding their linear or non-linear variation with the shear rate), without clearcut relation with expected relevant parameters for interfacial hydrodynamics, i.e. liquid-surface interactions and surface roughness. We first report an accurate determination of what we expect to be an \textit{intrinsic} slip length of water and organic solvants on smooth hydrophilic and hydrophobic surfaces. This boundary slip is well defined, does not depend on the scale of investigation (from 1 to several hundreds of nanometers) neither on shear rate (up to 5.10$^{3}$ s$^{-1})$. On smooth highly hydrophobic surfaces, the magnitude of slip is 20 nm, in good agreement with theory and numerical simulations. We then present results showing that the concerted effect of wetting properties and surface roughness may considerably reduce friction of the fluid past the boundaries. The slippage of the fluid is shown to be drastically reduced by using surfaces that are patterned at the nanometer scale. This effect occurs in the regime where the surface pattern is partially dewetted, in the spirit of ``~superhydrophobic' effect that has been discovered at the macroscopic scale. Our results show that in contrast to the common belief, surface friction may be reduced by surface roughness. They also open the possibility of a controlled realization of the ``~nanobubbles~'' that have long been suspected to play a role in interfacial slippage.   

Polymer/Nonpolymer Interactions and Apparent Wall Slip During Flow at High Stresses: An Historical Perspective

The no-slip boundary condition is a valuable empiricism derived from 19$^{th}$ Century experiments on low molar-mass liquids. Data that suggest deviations from the no-slip condition have long been available, but convincing evidence came only through experiments with entangled molten polymers, where the molecular scale over which slip might occur is large enough to result in macroscopic effects. The mechanisms for apparent slip in entangled polymers remain unclear; there is evidence to support both adhesive failure at the melt/metal interface and cohesive failure within the entangled melt. This talk will provide an historical overview and address critical experiments.   

Liquid Slippage over a Hydrophobic Surface: The Effect of Nanobubbles and Nanoroughness.

Micro and nanofluidic devices for manipulating fluids are widespread and are finding uses in many scientific and industrial contexts. Their design and small length scale introduce new research questions and themes to consider, first of all the impact of surface phenomena in controlling the flow. The present talk focuses on the combined effect of roughness and (partial) wettability on the flow, the circumstances that lead to interesting modification of old hydrodynamic problems and new flow responses. New high-precision quantitative methods based on confocal and atomic force microscopy - double focus confocal fluorescence cross correlation and thin film drainage measurements - are detailed. Also covered is the design of the model surfaces with the controlled nanoroughness and wetting properties. A discussion of the theoretical models suggested for a description of experimental configurations is given. Special emphasis is given to the analysis of experimental data. This covers the formation and role of nanobubbles, a contribution from surface forces into accelerating the flow, an interplay between nanoroughness and slippage, and more.   

Slippage, Cavitation and Sharkskin in Polymer Melts

Slippage in polymeric materials has been a subject of intense interest for three primary reasons. First, it is strongly interconnected with extrusion instabilities that commonly occur in polymer manufacturing (sharkskin, gross melt fracture, stick-slip). Second, the effect can be quite strong, the magnitude of slippage can become an appreciable fraction of the largest velocity of the flow. Third, molecular scale theoretical models have been developed that relate the slippage to shear stress, polymer molecular weight and polymer-surface interactions, all of which the experimentalist can control. We have utilized near-field velocimetry to demonstrate slippage within the first 100 nm from a solid wall and found a stress dependent transition from weak to strong slippage as well as a dependence on interfacial interactions. We have also shown that slippage can occur at a polymer-polymer interface when the interaction between them is weak. Allowing slippage at a polymer-polymer interface dramatically reduces the undesirable flow instability known as sharkskin. In the case of mixed flow boundary conditions, we have observed that the polymer can cavitate at the wall.   

Session U7: Nucleic Acid Translocation Through Nanopores

Silicon Nanopore Devices for DNA Translocation and Sequencing Studies

In this talk, I will discuss the recent progress [1-3] in developing solid-state nanopore devices using silicon technology.~ We have demonstrated a novel technique for shaping nanopores in the range of 1-10 nm, using surface-tension-driven mass flow with single nanometer precision.~ This technique overcomes a major technical challenge in silicon technology. I will also discuss the current effort [3] in developing integrated nanopore silicon chips with electrically addressable nanopores. These devices are used for DNA translocation and sequencing studies.~ This work was done in collaboration with the group of Cees Dekker at TU-Delft with partial support from FOM and Guggenheim Foundation. The work at Brown was supported by NSF-NER and NSF-NIRT. \newline \newline [1] A.J. Storm, J.H. Chen, X.S. Ling, H. Zandbergen, and C. Dekker, ``Fabrication of Solid-State Nanopores with Single Nanometer Precision'', Nature Materials, 2, 537 (2003). \newline [2] A.J. Storm, J.H. Chen, X.S. Ling, H. Zandbergen, and C. Dekker, ``Electron-Beam-Induced Deformations of SiO2 Nanostructures'', Journal of Applied Physics (submitted, 2004). \newline [3] X.S. Ling, "Addressable nanopores and micropores" (patent pending).   

Anomalous dynamics of translocation

We consider the passage of long polymers of length $N$ through a pore in a membrane. If the process is slow, it is in principle possible to focus on the dynamics of the number of monomers $s$ on one side of the membrane, assuming that the two segments are in equilibrium. The dynamics of $s(t)$ in such a limit is diffusive, with a mean translocation time scaling as $N^2$ in the absence of a force, and proportional to $N$ when a force is applied. We demonstrate that the assumption of equilibrium must break down for sufficiently long polymers (more easily when forced), and provide lower bounds for the translocation time by comparison to unimpeded motion of the polymer. These lower bounds exceed the time scales calculated on the basis of equilibrium, and point to anomalous (sub--diffusive) character of translocation dynamics. This is explicitly verified by numerical simulations of the unforced translocation of a self-avoiding polymer. Forced translocation times are shown to strongly depend on the method by which the force is applied. In particular, pulling the polymer by the end leads to much longer times than when a chemical potential difference is applied across the membrane. The bounds in these cases grow as $N^2$ and $N^{1+\nu}$, respectively, where $\nu$ is the exponent that relates the scaling of the radius of gyration to $N$. Our simulations demonstrate that the actual translocation times scale in the same manner as the bounds, although influenced by strong finite size effects which persist even for the longest polymers that we considered ($N=512$).   

Translocation and unzipping kinetics of DNA molecules using a nanopore

We have developed a method to dynamically set the voltage applied across a phospholipid bilayer that contain a single $\alpha $-Hemolysin pore[1]. With this method the entry rate of single-stranded DNA or RNA molecules into the nanometer scale pore, and the voltage wave used to induce their unzipping rate, are independently controlled. Thus, hundreds of polynucleotides can be individually analyzed in a short period of time (a few minutes). We have used this method to characterized the unzipping kinetics of DNA hairpin molecules under fixed voltage amplitudes ($V)$, or steady voltage ramps ($\dot {V})$. We found that at high voltages ($V>30$ mV) or at high voltage ramps ($\dot {V}>5$ V/s) the unzipping process can be described by a single step kinetics model with negligible re-zipping probability. But at the low voltage (or voltage ramp) regime re-zipping probability must be included to account for our data[2]. A model that includes re-zipping is introduced and is used to fit our data at low and high voltages. From the fits we estimate the effective DNA charge inside the nanopore and the unzipping rate of the hairpins at the limit of zero force. 1. M. Bates, M. Burns, and A. Meller, Biophys. J. \textbf{84} (4), 2366 (2003). 2. J. Math\'{e}, H. Visram, V. Viasnoff, Y. Rabin and A. Meller, Biophys. J. \textbf{87 }(5), 3205 (2004).   

Simulation of Nucleic Acid Transport Through Carbon Nanotube Membranes

We use molecular dynamics simulations to study the electrophoretic transport of single-stranded ribonucleic acid (RNA) molecules through 1.5-nm wide pores of carbon nanotube membranes. During a total simulation time of $\sim$800 ns, we observe $\sim$170 individual RNA translocation events at full atomic resolution of solvent, membrane, and RNA. By analyzing structure, thermodynamics, and kinetics, we identify key factors for the membrane transport of biopolymers. We find that RNA entry into the nanotube pores is controlled by conformational dynamics. Exit from the pores is strongly affected by hydrophobic attachment of RNA bases to the pore walls. Translocations with and without such hydrophobic binding result in slow and fast exit from the pores. We use a trap-diffusion model to describe the pore-blockage statistics obtained from the simulations and earlier experiments using an alpha-hemolysin pore. The rate of hydrophobic trapping depends only weakly on the applied electric field, whereas the rate of dissociation from the pore walls increases exponentially with the field. In the absence of an external electric field, RNA remains hydrophobically trapped in the membrane despite large entropic and energetic penalties for confining charged polymers inside nonpolar pores. We find that differences in RNA conformational flexibility and hydrophobicity result in sequence-dependent rates of translocation, a prerequisite for nanoscale separation devices.   

Kinetics of RNA translocation through a nanopore

A nanopore is so small that only single-stranded but not double-stranded RNA molecules can pass through it. Thus, if an RNA molecule is driven through a nanopore its secondary structure has to be broken. This couples the dynamics of translocation through the pore to the dynamics of the secondary structure rearrangements of the molecule. Thus, translocation experiments of RNA molecules through nanopores give insight into secondary structure dynamics. In addition there are potential applications to the determination of RNA secondary structures and to the separation of RNA molecules according to their secondary structure features. We will present a theoretical framework in which to study this competition between translocation and structural dynamics. As a first application we study the crossover between a fast translocation regime in which the structure is easily destroyed and the translocation time is linear in the polymer length and a slow translocation regime in which the structure is in thermodynamic equilibrium at all times and the translocation time is dominated by the specific structural features.   

Session U9: Glassy and Random Magnets

Experimental Study of the Phase Diagram of LiHo$_x$Y$_{1-x}$F$_4$

The diluted dipolar-coupled Ising spin system LiHo$_{x}$Y$_{1-x}$F$_4$ has been found to have a very rich phase diagram and some unique magnetic properties. Most notably, as the concentration of magnetic Ho ions is reduced, the material appears to leave the spin glass phase and enter the so-called ``anti-glass'' phase where spins do not freeze down to very low temperatures (D. H. Reich \textit{et al}). This doping regime has also shown some unusual features in the temperature dependence of the specific heat. We are performing measurements on a variety of stoichiometries to refine the phase diagram of LiHo$_{x}$Y$_{1-x}$F$_4$. More specifically we are attempting to better understand the nature of the ``anti-glass'' phase and the range of stoichiometries over which it exists. Recent results from heat capacity and SQUID-based AC magnetic susceptibility measurements at dilution refrigerator temperatures will be presented.   

Aging dynamics across a dynamic Almeida-Thouless line in three dimensional Ising spin glass Cu$_{0.5}$Co$_{0.5}$Cl$_{2}$-FeCl$_{3}$ graphite bi-intercalation compound

Cu$_{0.5}$Co$_{0.5}$Cl$_{2}$-FeCl$_{3}$ graphite intercalation compound is a three-dimensional short-range Ising spin glass with a spin freezing temparature $T_{g}$ ($= 3.92 \pm 0.11$ K). The stability of the spin glass phase in the presence of a magnetic field $H$ has been studied from the time dependence of zero-field cooled (ZFC) susceptibility $\chi_{ZFC}$ after a ZFC aging protocol with a waittime $t_{w}$ ($= 1.0 \times 10^{4}$ and $3.0 \times 10^{4}$ sec). The relaxation rate $S(t)$ (= d$\chi_{ZFC}$/d$\ln t$) exhibits a local maximum at a characteristic time $t_{cr}$, The $t$ dependence of $\chi_{ZFC}$ is well described by a stretched exponential relaxation ($\approx \exp [-(t/\tau)^{1-n}$]) in the vicinity of $\tau \approx t_{w}$, where $t \approx t_{cr}$. The $H$ dependence of $t_{cr}$ and $\tau$ is measured at the fixed temperature $T$ (2.9 K $\leq T < T_{g}$): $\tau$ ($t_{cr}$) decreases with increasing $H$. Contour plots of $H$ and $T$ with constant $\tau$ form lines in the $H$-$T$ plane, depending on the value of $\tau$ chosen. We find that the line with $\tau \approx 2.0 \times 10^{3}$ sec coincides with an Almeida-Thouless (AT) line where the irreversible effect of succeptibility appears. This result indicates that the spin glass phase is unstable in the presence of $H$. There is no AT line for short-range Ising spin glass.   

Correlation length of the two-dimensional Ising spin glass with bimodal interactions

We study the correlation length of the two-dimensional Edwards-Anderson Ising spin glass with bimodal interactions using a combination of parallel tempering Monte Carlo and a rejection-free cluster algorithm in order to speed up equilibration. Our results show that the correlation length grows $\sim \exp(2J/T)$ suggesting through hyperscaling that the degenerate ground state is separated from the first excited state by an energy gap $\sim 4J$, as would naively be expected.   

The Compressible Ising Spin Glass: Simulation Results

We have studied the compressible Ising spin glass using the Edwards-Anderson model with $\pm J$ interactions, primarily in two dimensions. Compressibility is introduced by the addition to the standard spin-glass Hamiltonian of a term which couples the spin-spin interactions to the distance between neighboring particles. A dimensionless parameter $\mu$ relates the strength of the coupling term to the original spin-glass energy. In the simulations, the spin dynamics are modeled via single-spinflip Monte Carlo, while the lattice is relaxed using conjugate-gradient minimization with respect to the particle positions. We find that the total energy of a given spin configuration is shifted from its incompressible value, $E^0$, by an amount which is proportional to $\mu$ and quadratic in $E^0$. Furthermore, the previously discrete energy levels broaden into bands that overlap for sufficiently large $\mu$; in the thermodynamic limit, even an infinitesimal coupling renders the spectrum continuous. Using these results, we have constructed a simple analytic model and investigated the effects of the compressibility on the original spin-glass transition.   

Phase Transition of the Random Field Ising Model at Zero Temperature and Positive Temperature

The random field Ising model (RFIM) is studied numerically at both zero and positive temperature. Thermal states and thermodynamic properties are obtained for all temperatures using the the Wang-Landau algorithm. The specific heat and susceptibility display sharp peaks in the critical region for most realizations of random fields for large systems and strong enough disorder. These sharp peaks result from large domains flipping and are strongly correlated with the domains found in ground states. Although the correlation is higher for stronger disorder, it remains for relatively low disorder. The correlation between ground states and thermal states is a concrete manifestation of the zero temperature fixed point scenario.   

Behaviour of spin glasses in a magnetic field

The low-temperature phase structure of spin glasses is still not understood beyond the mean-field level. In particular, it has not been possible to establish whether the ordered spin-glass phase is stable or unstable against the application of a weak external magnetic field. An answer to this question would allow to discriminate among widely discussed competing theories of spin glass ordering. In this talk, I will present recent numerical investigations of this issue for short-range spin glasses in three, four, and five dimensions, as well as dilute spin glasses, and discuss the results in the light of the various alternative theories.   

Spin-glass behavior probed at the local level using information theoretic measures

This study shows the equivalence of the entropy calculated using an information theoretic method and the thermodynamic entropy in two-dimensional quenched random systems. The information theoretic entropy is calculated via histograms of spin configuration occurrences in shapes planted in the lattice during a Monte Carlo simulation. This method yields a local entropy density, one for the region around each site. These local entropies differ markedly in value across the lattice, but their average coincides with the thermodynamic entropy. Thus, these calculations show how the entropy is spread unevenly across the lattice in glassy systems. We have also calculated the excess entropy in terms of local contributions, which provides a measure of spatial structure and memory in the spin glass. Results yield a way to examine the connections between frustration and the local entropy and excess entropy densities and also provide comparisons between the bimodal and Gaussian cases in two dimensions in terms of these local measures.   

Renormalization study of Ising Spin Glasses in a transverse field

We study an ensemble of Ising spins coupled by random antiferromagnetic/ferromagnetic couplings, and submitted to a (random) transverse field in low dimensions. We use an extension of the Ma-Dasgupta decimation procedure to study the low energy behaviour of this quantum spin glass. Particular attention is paid to the relevance of frustration near infinite disorder fixed points.   

Memory and Quantum Erasure in a Disordered Ising Magnet

Glasses are well known for their slow relaxation and complex history dependence, including their memory of previous aging after a cycle in temperature or field. We study the breakdown of memory via quantum relaxation by measuring the time-dependent AC susceptibility in LiHo$_{0.20}$Y$_{0.80}$F$_{4}$, a diluted, dipolar coupled Ising magnet. Application of a magnetic field transverse to the Ising axis introduces quantum tunneling modes that speed relaxation and provide new pathways through the hierarchical distribution of states. It is possible to compare directly in the same system magnetic aging and memory effects driven by classical (thermal) and quantum (tunneling) channels.   

Magnetic and chemical superstructures in Gd2PdSi3 studied using synchrotron radiation

$R_{2}$PdSi$_{3}$ compounds have stimulate a recent interest due to the coexistence of long-range antiferromagnetic order and spin-glass like behavior. We present x{\-}ray resonant magnetic scattering studies on a Gd$_{2}$PdSi$_{3}$ single crystal at temperatures between 8~K and 30~K performed on the MUCAT beamline, APS, Argonne. Charge satellite reflections were observed with the propagation vectors (1/2~0~1/8) and (1/2~0~1/3). The corresponding chemical superstructure can be described by an ordered distribution of the Pd and Si ions on the B site in the hexagonal AlB$_{2}$ derived structure. Below $T_{N}$~=~23~K antiferromagnetic satellite reflections appear. The propagation vector is incommensurate with temperature dependent values between (0.14~0~0) and (0.13~0~0). This emphasizes the dominance of the RKKY interaction in Gd$_{2}$PdSi$_{3}$ in comparison to the strong influence of crystal electric field effects in the other $R_{2}$PdSi$_{3}$ compounds.   

Frustrated Ternary Insulating Mixed Magnetic Co/Mn/Fe Dichloride Dihydrate

Mixed ternary insulating Co/Mn/Fe dichloride dihydrate has been examined by dc susceptibility and magnetization measurements over a broad range of compositions. The three components are isomorphous antiferromagnets with different ordered spin arrangements. Orthogonal spin anisotropies occur between the Fe and Co, and the Fe and Mn, components. Competing (sign) exchange interactions occur between the Fe and Mn, and the Co and Mn, components. The consequences of these competitions has been determined in previous work on the three possible binary mixtures. Examined here, for the ternary mixture, are high temperature paramagnetic phase properties, and low temperature properties including magnetic ordering and nonequilibrium behavior vs composition. The latter includes field cooled vs zero-field cooled susceptibilities, and thermoremanent magnetization as a function of time and temperature. A T-H irreversibility line is determined for one spin-glass-like composition, and temperature-time scaling of the thermoremanent magnetization confirmed for several compositions. A magnetic phase diagram (T vs composition) is also determined, involving two independent composition variables. *Supported by NSF-Solid State Chemistry-Grant No. DMR-0085662.   

Mixed magnetic behavior in Ce doped perovskites

La$_{(0.67-x)}$Ce$_x$Ca$_{0.33}$MnO$_3$ samples have been prepared for $x$=0.0, 0.10, and 0.20. We report here magnetization measurements leading to hysteresis curves showing several interesting features: low coercive field, magnetization jumps at magnetic fields less than coercive field, loops not quite reproducible, and virgin curve outside of the main loop. These and other characteristic properties are more pronounced for the sample with $x$=0.20. We present a simple model based on an Ising Hamiltonian with mixed but segregated ferromagnetic and antiferromagnetic interactions to qualitatively explain these phenomena. We conclude that groups of spins with in-plane ferromagnetic interactions are overturned at critical values of the field for which pinning due to off-plane antiferromagnetic interactions is compensated. The role of the Ce doping is discussed.   

Absence of an Almeida-Thouless line in Ising spin glasses

We present results of Monte Carlo simulations on Ising spin glasses in the presence of a (random) field. A finite-size scaling analysis of the correlation length shows no indication of a transition in three dimensions, in contrast to the zero-field case. This suggests that there is no Almeida-Thouless line for short-range Ising spin glasses. We also present results on the behavior in a field of the one-dimensional Ising chain with long-range power-law interactions.   

Session U10: Focus Session: Spin Hall Effect

Spin current injection by intersubband transitions in quantum wells.

We show that a pure spin current can be injected in quantum wells by absorption of linearly polarized infrared radiation leading to transitions between subbands. The magnitude and the direction of the spin current depend on the Dresselhaus and Rashba spin-orbit coupling constants and light frequency and, therefore, can be manipulated by changing the light frequency and/or applying an external bias across the quantum well. The injected spin current should be observable either as a voltage generated via extrinsic spin-Hall effect, or by spatially resolved pump-probe optical spectroscopy.   

Electrical generation of spin in crystals with reduced symmetry

We propose a way of generating a spin polarization in crystals with strong spin-orbit interactions. We show that, in the presence of an electric field, there exists an intrinsic torque term which gives rise to a nonzero spin generation rate. This spin generation rate is experimentally observable, as recent experimental progress in the detection of spin accumulation has shown. The wide applicability of this effect is emphasized by explicit consideration of a range of examples: bulk wurtzite and strained zincblende ($n$-GaAs) lattices, as well as quantum well heterojunction systems.   

Spin-Hall effect and related spin-charge transport in one and two- dimensional mesoscopic systems

We study theoretically the spin transport and the spin Hall effect in one and two-dimensional mesoscopic systems with Rashba spin-orbit coupling. The non-equilibrium Green function formalism is used to model the samples with mobilities and Rashba coupling strengths experimentally available. In particular, we propose the realistic H-shape experimental setup where the indirect detection of spin-Hall effect is possible by measurement of voltage through paramagnetic contacts. We confirm the robustness of the intrinsic spin-Hall effect in mesosocopic systems against the disorder in agreement with the exact diagonalization and Born calculations in the bulk. Also, we discuss the influence of the effective Rashba and Zeeman fields on the spin and charge transport in mesoscopic structures of various shapes. Reference: Hankiewicz et al Phys. Rev. B Rapids (2004); cond-mat/0409334.   

Electrical Spin Generation and Transport in Spin-Orbit Coupled Systems

We consider spin generation and transport in bands with built-in spin-orbit coupling. A number of fundamental issues will be discussed: (1) the existence of spin-dipole and torque-dipole of wave packets which model the carriers; (2) source terms in the continuity equation (spin generation and relaxation); (3) the composition of the spin current (Berry phase and more); (4) spin Hall conductivity and its reciprocal; (5) the spin current responsible for spin accumulation.\\[4pt] References:\\[0pt] [1] D. Culcer, J. Sinova, N. A. Sinitsyn, T. Jungwirth, A. H.MacDonald, Q. Niu, `Semiclassical theory of spin transport in spin-orbit coupled systems', Phys. Rev. Lett. 93, 046602 (2004).\\[0pt] [2] P. Zhang and Q. Niu, `Charge-Hall effect driven by spin force: reciprocal of the spin-Hall effect' Cond-mat/0406436.\\[0pt] [3] D. Culcer, Y. G. Yao, A. H. MacDonald, and Q. Niu, `Electric generation of spin in crystals with reduced symmetry', Cond-mat/0408020.   

Nonvanishing spin Hall Effect in a square-lattice system

Recently, it has been debated whether the spin Hall effect vanishes or not in the presence of impurities. To investigate this problem, we study a generic model on a square lattice with broken inversion symmetry, modelling a 2D semiconductor in a heterostructure. We adopt the Kubo formula and Keldysh formalism, assuming the clean limit from the disordered system. In the Keldysh formalism, the less Green function $G^{<}$ has two pieces contributing to the spin Hall effect. One is proportional to $n_{F}$ (contribution from the filled states) while the other is to $n'_{F}$ (contribution from the Fermi surface), where $n_{F}$ is the Fermi distribution. Both of these two pieces are intrinsic, as they are nonzero in the clean limit. In general models they do not cancel with each other. The Rashba model is exceptional as they cancel each other exactly by accident. We discuss a condition when the spin Hall effect becomes larger. In the spin Hall insulators, the spin Hall effect merely consists of the contribution from the filled states.   

Effect of electron correlations on the Spin-Hall conductivity in a 2D Rashba electron system

The spin-Hall effect in 2D electron systems (2DES) is currently an intense field of theoretical research. In this work we study the spin-Hall effect in a two-dimensional electron system (2DES) with Rashba spin-orbit coupling and electron-electron Coulomb interaction. The problem is examined by performing a density-functional calculation of the 2DES and computing the conductivity $\sigma_{SH}$ using the Kubo-Greenwood formula. It is found that the Coulomb interaction renormalizes the spin-Hall conductivity and that the strength of the renormalization depends on the electron density $r_{s}$.   

Cancellation of intrinsic ``spin-Hall'' conductivity in absence of broken time-reversal symmetry

A Streda-type argument is used to obtain the intrinsic dissipationless ``spin-Hall'' conductivity of electronic systems with spin-orbit coupling (SOC). Instead of directly calculating the ``spin-current'' response to an electric field, we calculate the {\it spin-density} response to a magnetic flux density $B$ (from orbital, rather than Zeeman coupling), and transform to a moving frame in which an electric field is present. This is consistent when used to compute the ``anomalous'' and ``quantized'' (electrical) Hall conductivities, and appears to also be so for the ``spin-Hall'' conductivity: the induced ``spin-current'' in the moving frame is interpreted as the induced spin density in the static frame, times the boost velocity. In the 2D model with ``Rashba'' spin-orbit coupling, the spin density induced by linear response to $B$ is cancelled by an ``anomalous'' term from the lowest Landau level, which is ``special'' because it alone is spin-unpaired. Similar unpaired ``special'' Landau levels also occur in the 3D ``Luttinger'' model for SOC of holes in p-type semiconductors. We also argue that a {\it quantized} spin-Hall effect cannot occur in the absence of broken time-reversal symmetry, and conjecture that this is also true for the metallic (non-quantized) spin-Hall effect.   

Resonant manipulation of spin current with a double-barrier structure

In this work, we consider a Rashba-type narrow channel consisting of two AC-biased finger-gates (FG) that orient perpendicularly and lie above the narrow channel. It is shown recently that such a gate configuration can give rise to dc spin current [1]. The dc spin current can be greatly enhanced by an optimal choice of the separation between the FGs. With the introduction of a double-barrier structure in between the FGs, we can explore the interplay between the dc spin current generation and the resonant levels in the double-barrier structure. Our results show that the direction of the dc spin current can be monitored by the chemical potential alone. No charge current, however, is generated in this configuration. [1] L. Y. Wang, C. S. Tang, and C. S. Chu, cond-mat/0409291   

Spin-Hall Effect in Two-Diensional Spin-Orbit Coupled Systems with Disorder

The spin-Hall conductance of a two-dimensional electron system with the Rashba spin-orbit coupling and disorder is calculated numerically by using the Landauer-B\"{u}ttiker formula and Green's function approach. We find that the spin-Hall conductance can be much greater or smaller than the universal value $e/8\pi$, depending on the magnitude of the SO coupling, the electron Fermi energy and the disorder strength. The spin-Hall conductance does not vanish with increasing sample size for a wide range of disorder strength. The position-dependent spin polarization is also calculated. Our result is consistent with recent experimental observation of spin polarization near the edges of a semiconductor channel detected and imaged by using Kerr rotation microscopy.   

Maxwell Equation for the Coupled Spin-Charge Wave Propagation

We show that the dissipationless spin current in the ground state of the Rashba model gives rise to a reactive coupling between the spin and charge propagation, which is formally identical to the coupling between the electric and the magnetic fields in the $2+1$ dimensional Maxwell equation. This analogy leads to a remarkable prediction that a density packet can spontaneously split into two counter propagation packets, each carrying the opposite spins. In a certain parameter regime, the coupled spin and charge wave propagates like a transverse ``photon". We propose both optical and purely electronic experiments to detect this effect.   

Spin current corrleations induced by the Kondo effect

Rapid technological progress over the past two decades has made available electrical conductors on the nanoscale. Due to their small size these conductors often have an effectively reduced dimensionality and one expects electron-electron interactions to play an important role. It is a fascinating but challenging endeavor to observe effects of these interactions in electrical measurements. Generically interactions manifest themselves in correlations. Measurements of current correlations should therefore most naturally be able to probe interactions. At low temperatures the Coulomb blockade in an interacting quantum dot is lifted by the Kondo effect. Although this effect results from interesting many-body correlations, no signatures of them have been found in previous calculations of current correlations. Since the Kondo effect results from spin fluctuations, one may expect it, however, to be observable in spin resolved transport measurements. I will show that this intuition is indeed correct. Kondo fluctuations can induce correlations between spin currents through a quantum dot.   

Topological Spin Current

We show that the $SU(2)$ transformation which diagonalizes the two dimensional spin-orbit hamiltonian has a singularity in the momentum space at $\vec{K} = 0$ which gives rise to non-commuting cartesian coordinates. When an external electric field is applied, the non-commuting cartesian coordinates induce a Hall current. The presence of a random potential in an infinite system causes the single particle occupation at $\vec{K} = 0$ and the Hall current to vanishes. For a finite system, the spin-Hall conductance is quantized in units of $\frac{e g \mu_{B}}{2 h}$, and the charge-Hall conductivity increases with the strength of the Zeeman magnetic field. We propose an experiment to test our theory.   

Is The Intrinsic Spin Hall Effect Measurable?

Despite of the large intrinsic spin Hall conductivity in a spin- orbit coupled material predicted theoretically, we show that the intrinsic spin Hall effect in any diffusive sample is not measurable via conventional transport methods, thus the research on the intrinsic spin Hall effect is limited at the pure theoretical content. After generally defining the intrinsic and extrinsic transport coefficients, we show that the intrinsic magnetization Hall current, which is the sum of the intrinsic spin and intrinsic orbit-angular-momentum Hall currents, is identically zero. More importantly, we demonstrate that the equation of motion for the spin density does not depend on the intrinsic spin Hall current, therefore the transverse spin accumulation is solely determined by the extrinsic spin Hall current. The zero intrinsic magnetization Hall current and the independence of the spin accumulation on the intrinsic spin Hall effect lead us to conclude that the intrinsic spin Hall effect can not be assessed by conventional spin transport experiments based on the measurement of the magnetization current and the spin accumulation at the edge of the sample.   

Session U12: Point Contact Tunnelling and Proximity Effect

Andreev Reflections at the Superconductor-Semiconductor Interface

We present results of experiments involving coupling of a BCS superconductor (niobium) with a heavily doped gallium arsenide based semiconductor system. Silicon doped GaAs is grown by molecular beam epitaxy and capped by InGaAs, with an indium fraction of 30 percent. Silicon delta doping layers increase electron densities into the semi-metallic regime. Intimate contact between superconductor and semiconductor is obtained by \textit{in situ} evaporation of niobium. Evidence of strong Andreev reflections at this interface is observed and analysis of this behavior in accordance with the BTK formalism will be presented.   

Investigation of the Superconductor-Ferromagnet Proximity Region Using the Usadel Equation

Due to its simplicity, the Usadel formulation is widely used to study the superconducting properties of heterostructures composed of superconductors (S), ferromagnets (F), and normal metals (N) in the dirty limit. In particular, with very simple boundary conditions and parameters, it quantitatively explains the non-monotonic relation between Tc and the ferromagnet thickness observed in SF bilayers. However, we find that the same simple analysis fails to explain the density of states measured in the F layer by tunneling spectroscopy. Based on experimental data taken in SF and SN bilayers, we discuss the validity of this approach and which of the ingredients -- boundary resistance, spin-orbit scattering, tunneling barrier height, etc. -- play a role in the real problem. Work supported by DoE BES.   

Quantum conduction through two narrow half-metallic leads with opposite spin coupled to a superconductor

We study quantum conduction through a two-dimensional junction consisting of two narrow half-metallic leads with opposite spin coupled to a wide superconductor at zero temperature. When a half-metallic lead is coupled to a superconductor, conductance vanishes at zero bias. Adding another half-metallic lead with opposite spin, conductance becomes non-zero due to Andreev reflection at the interface when the distance between the centers of the two leads is smaller than the coherence length of the superconductor. With the use of the Bogoliubov equations, we compute the conductance as a function of the distance between the centers of the two half-metallic leads.   

Point-Contact Spectroscopy Study of the Ferromagnetic Superconductor ZrZn$_2$

Superconductivity and ferromagnetism are competing orders, but in ZrZn$_2$ evidence for superconductivity has been observed in the ferromagnetic state [1]. We have performed point-contact spectroscopy measurements on single-crystal samples of ZrZn$_2$ using Au and Pt-Ir tips. We present differential conductance spectra measured down to 100 mK, along with their magnetic-field evolution. Implications of our data on possible pairing symmetry [2] and pairing mechanism [3] will be discussed. *Permanent address: Inst. Surface Chemistry, N.A.S. Ukraine, Kyiv, Ukraine. [1] C. Pfleiderer et al. Nature \textbf{412,} 58 (2001). [2] M.B. Walker and K.V. Samokhin, Phys. Rev. Lett. \textbf{88,} 207001 (2002). [3] D.J. Singh and I.I. Mazin Phys. Rev. Lett. \textbf{88,} 187004 (2002).   

Tunneling Spectroscopy of Superconductor/Ferromagnet Proximity Effect Bilayers

Previous studies of the superconducting proximity effect in ferromagnetic thin films have primarily focused on the non-monotonicity of the transition temperature as a function of film thickness. This behavior is due to the non-zero center-of-mass momentum acquired by a Cooper pair in this presence of an exchange field, which results in position-space oscillations of the pair density in the ferromagnet. In order to provide a complementary view of this phenomenon, we have made a systematic study of the tunneling density of states on the ferromagnetic side as a function of film thickness. We make our samples in a well-calibrated UHV sputtering chamber, allowing us to probe thicknesses both smaller and larger than the coherence length of the ferromagnet, which is on the order of 1 nm. By careful sample preparation, measurement, and data analysis, we are able to observe minute variations in the density of state, characteristic of samples with thicknesses greater than the coherence length. Work supported by DOE BES   

Thouless energy in Josephson junctions with the barrier composed of strongly correlated materials near the Mott transition

The Thouless energy was originally introduced as the inverse of the electron dwell time (mod $\hbar$) in a finite-sized piece of diffusive conductor, where the dimensionless conductivity is determined by the ratio of the Thouless energy to the quantum energy level spacing of the wire. The Thouless energy is also useful in characterizing the proximity-effect coupling in Josephson junctions, consisting of two superconductors separated by a barrier layer of diffusive normal metal; the characteristic voltage across the junction $I_c R_N$ is described by a universal function of the Thouless energy. In this talk, we present a generalized form of the Thouless energy for Josephson junctions made of a strongly correlated metal (insulator) as the barrier layer and use it to illustrate how the quasiclassical picture of transport breaks down as the strongly correlated barrier passes through the Mott metal-insulator transition.   

LOFF superconductivity in the proximity region of d-wave superconductor/ferromagnet tunneling junctions

In the proximity region of superconductor-ferromagnet tunneling junctions, a signature of the Larkin-Ovchinnikov-Fulde-Ferrell (LOFF) state is the decaying oscillation of the LOFF order parameter with the distance from the junction barrier. Using N/I/F/Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8}$ (BSCCO) heterostructures, we investigated the dependence of this oscillatory behavior on the d-wave symmetry of the BSCCO superconductive order parameter. Two junctions were prepared on the same BSCCO crystal, in order to probe two different injection angles $\beta $ in the ab-plane: one close to the nodal line direction ($\beta $=45\r{ }) and the other one close to the maximum gap direction ($\beta $=0). The tunneling spectra obtained on N/BSCCO junctions (i.e., no ferromagnetic layer) with the same orientation presented the usual features: high amplitude zero bias conductance peaks (ZBCP) for $\beta $=45\r{ } and V-shaped gaps for $\beta $=0. However, with a 70 {\AA} thick antiferromagnetic interlayer (Fe), the N/I/F/BSCCO junctions show ZBCP for $\beta $=0 and gaps for $\beta $=45\r{ }. We attribute this opposite behavior to the presence of a spatially modulated LOFF order parameter in the Fe layer leading to flipped spectral characteristics at distances dependent on the ferromagnetic exchange energy. The diffusiveness of the structures is rather low, so that the oscillations for the nodal and antinodal injections are almost in antiphase.   

Point contact tunneling spectroscopy of a Tl2Ba2CaCu2O8 thin film

Point contact tunneling spectroscopy was used to investigate the superconducting gap of Tl$_{2}$Ba$_{2}$CaCu$_{2}$O$_{8}$ (Tl- 2212 cuprate) c-axis oriented thin film of near-optimal doping, with T$_{C} $~=~106~K. The quasiparticle peaks were observed at various bias voltages in the range 30-55 mV, and the 2{\_}/k$_{B}$T$_{c}$ ratio coming from our measurement is $\sim $ 6.6-12. This is the first observation of large energy gap in near-optimally doped Tl-2212 by electron tunneling. The largest values match those found in the~infrared response and Raman scattering on Tl- 2212 (single crystals and oriented thin films) published by other groups. We discuss this in the context of experiments done by relevant techniques on related materials, the Tl-2201, Tl-2223 and Bi-2212 cuprates.   

Point contact spectrscopy of electron-doped cuprates in a magnetic field of 32 tesla

Electron-doped cuprates have low values of the upper critical field ($H_{c2} \sim 10$ T at 1.5 K) and hence it is possible to study their normal state at low temperatures. Such studies have been done before and showed evidence of a ``pseudogap". However, to understand the origin of this pseudogap and which model of high-$T_c$ superconductivity it supports, it is necessary to study the effect of high magnetic fields on this pseudogap. We have performed point contact spectroscopy experiments using junctions between a normal metal (Pt-Rh) and electron-doped Pr$_{2-x}$Ce$_x$CuO$_4$ (PCCO) films for $0.13 < x < 0.17$. To probe the normal state at low temperatures ($\sim$ 0.4 K), we suppressed the superconductivity by applying high magnetic fields (up to 32T). We will show the effects of such high fields on the pseudogap and discuss our results in the context of present theories {\em viz.} preformed pairs and the presence of a quantum critical point.   

Evidence for nodeless gap in superconducting Nd1.85Ce0.15CuO4-y:

The pairing symmetry in a single crystal of Nd$_{1.85}$Ce$_{0.15}$CuO$_{4-y}$ is studied by measuring the point-contact spectroscopy along nodal and anti-nodal directions. For comparison the same measurements on a hole-doped cuprate single crystal of La$_{1.89}$Sr$_{0.11}$CuO$_{4}$ is also presented. A nearly identical spectrum is obtained in Nd$_{1.85}$Ce$_{0.15}$CuO$_{4-y}$ for both directions along Cu-O bond and Cu-Cu bond and no any zero bias conductance peak is observed. This is in contrast to the results of La$_{1.89}$Sr$_{0.11}$CuO$_{4}$, in which an angular dependent spectrum is observed with a remarkable zero bias conductance peak in the nodal direction. Our results support an s-wave like symmetry in optimally electron-doped cuprate Nd$_{1.85}$Ce$_{0.15}$CuO$_{4-y}$ other than the d-wave dominant symmetry as demonstrated in hole-doped cuprates.   

Proximity effect in normal metal high-Tc superconductor contacts

We study the proximity effect in good contacts between normal metals and high-Tc ($d_{x^2-y^2}$-wave) superconductors. We present theoretical results for the spatially dependent order parameter and local density of states, including effects of impurity scattering in the two sides, s-wave pairing interaction in the normal metal side (attractive or repulsive) and subdominant s-wave paring in the superconductor side. For the $[100]$ orientation, a real combination $d+s$ of the order parameters is always found. The spectral signatures of the proximity effect in the normal metal include a suppression of the low-energy density of states and a finite-energy peak structure. These features are mainly due to the impurity self-energies, which dominate over the effects of induced pair potentials. For the $[110]$ orientation, for moderate transparencies, induction of a $d+is$ order parameter on the superconductor side leads to a proximity induced $is$-order parameter also in the normal metal. The spectral signatures of this type of proximity effect are potentially useful for probing time-reversal symmetry breaking at a $[110]$ interface.   

Observation of the current-phase relation of a characterized superconducting atomic point contact

The current-phase relation $I(\varphi )$ in a superconducting atomic point contact (APC) is different from that in a tunnel junction. Beenakker et al. treated this problem based on the idea of Andreev reflection, and obtained a formula for $I(\varphi )$ as a function of transmission coefficient $\tau $ of the contact. Measuring magnetic responses of a superconducting loop with an APC, Koops at al. determined $I(\varphi )$ to find a clear non-sinusoidal behavior. However, no characterization of the point contact was possible in their experiment. In this paper, we propose a new experiment in which we can determine both the transmission coefficients {\{}$\tau _{i}${\}} and $I(\varphi )$ relation: A device we should make is a dc-SQUID which consists of an APC and a tunnel junction. The coefficients {\{}$\tau _{i}${\}} can be determined by analyzing the sub-gap $I-V$ characteristics. On the other hand, $I(\varphi )$ of the point contact is derived from a dependence of the critical current on the magnetic flux, $I_{C}(\Phi )$. Using the EB-lithography and break junction technique, we performed the experiment at 90 mK. The $I-V$ characteristics can be fitted to the theory of multiple Andreev reflection quite well. The critical current is not symmetric to the magnetic field direction nor to the current direction, which means the $I(\varphi )$ is not sinusoidal.   

Calculation of superconductor-normal metal point contact conductance with finite gap decay length

A numerical method is developed for the calculation of differential conductance in superconductor-normal metal contact junctions using the Bogoliubov-De Gennes equations beyond the Blonder-Tinkham-Klapwijk (BTK) approximation. Effects of finite lengthscales of gap onset and contact potential are calculated self-consistently, and exact quasiparticle momenta are retained. For a physical choice of the parameters, the combination of these effects produces a significant departure from the BTK conductance, most notably a suppression of the excess current above the gap.   

Proximity Effect in Nb/Cu/CoFe Trilayers

We have fabricated the Nb/Cu , Nb/CoFe bilayer and Nb/Cu/CoFe trilayer samples by varying the CoFe or Cu layer thickness using DC magnetron sputtering system and measured their superconducting transition temperature Tc. In Nb/Cu(d$_{Cu})$ and Nb/CoFe(d$_{CoFe})$ bilayers , we observed Tc behavior consistent with conventional SN and SF theory. In Nb/Cu/CoFe trilayer , as we increase d$_{Cu}$ with fixed values of d$_{Nb}$ and d$_{CoFe }$, $_{ }$Tc of Nb/Cu(d$_{Cu})$/CoFe trilayer increased rapidly for d$_{Cu} \quad <$ 5 nm and slowly saturated to a limiting value. We analyzed these data using the method based on Usadel formalism and obtained $\gamma _b ^{N}$=R$_{b}$A/$\rho _{n}\xi _{n}$[Nb/Cu]=0.41 , $\gamma _b ^{F}$= R$_{b}$A/$\rho _{f}\xi _{f}$ [Nb/CoFe] = 0.33, and $\gamma _b ^{F}$[Cu/CoFe]=0.35. In our Nb/Cu/CoFe trilayers, as we increased d$_{CoFe}$ with fixed values of d$_{Nb}$ and d$_{Cu }$, we observed the same dip structure as Nb/CoFe(d$_{CoFe})$. We will explain our data with interface resistance and its implications.   

Andreev reflection at the normal-metal / heavy-fermion superconductor (HFS) interface: Point-contact spectroscopy (PCS) studies of CeCoIn5

Andreev reflection between a normal metal and superconductor with highly disparate fermi surface parameters is investigated with PCS using a gold tip and the HFS, single crystal CeCoIn5. Data are taken from 60 K down to 400 mK [W. K. Park et al, cond- mat/0409090] and applied fields up to 9 T. The contact is shown to be in the Sharvin limit with the enhanced sub-gap conductance arising from Andreev refection. The temperature dependence of the zero-bias conductance data are best fit using the extended Blonder-Tinkham-Klapwijk model with a d-wave order parameter [S. Kashiwaya et al. PRB 53, 2667 (1996)]. The highly- suppressed Andreev signal, a signature of normal-metal/HFS junctions, is quantified and theoretical models to account for this are presented. We acknowledge A.J. Leggett, D. Pines, V. Lukic, J. Elenewski, B.F. Wilkin, A.N. Thaler, P.J. Hentges, K. Parkinson, W.L. Feldmann and support by the DoE DEFG02- 91ER45439, through the FSMRL and the Center for Microanalysis of Materials.   

Keldysh study of point-contact tunneling between superconductors

We revisit the problem of point-contact tunnel junctions involving one-dimensional superconductors and present a simple scheme for computing the full current-voltage characteristics within the framework of the non-equilibrium Keldysh Green function formalism. The effects of different spin-pairing symmetries, combined with magnetic fields and finite temperatures, at arbitrary bias voltages are addressed. We propose ways of measuring the effects found when the two sides of the junction have dissimilar superconducting gaps and when the symmetry of the superconducting states does not correspond to spin-singlet pairing.   

Session U13: Devices & Applications II

Measuring equipment for thermophysical properties of droplet electromagnetically-levitated under axial static magnetic field

EML is used for measurement of thermophysical properties melt with high melting point and with high reactivity. Application of a strong static magnetic field is considered to be a promising method to damp convection and motion in electrically conductive fluid, because the Lorentz force is induced by the magnetic field. Therefore, a novel measuring equipment for thermophysical properties of an electrically conductive droplet has been developed to solve the problem mentioned above based on the principle. In order to observe behavior and shape of melt three-dimensionally and in realtime, high-speed camera and CCD camera were mounted at the top and side of a reaction chamber of EML, respectively. Surface temperature of the melt was monitored by pyrometer from the side. Result, vibrations of Si, Ti droplets levitated by the equipment was stabilized and the convection was seemed to be damped under the magnetic field, though the droplets rotated along the magnetic field as rigid bodies. Some obtained results for thermophysical properties of the droplets will be reported.   

Extraordinary Optoconductance in GaAs-In Hybrid Structures

Following the demonstration of extraordinary magnetoresistance (EMR) in semiconductor-metal hybrids\footnote{ S.A. Solin et al., Science {bf289}, 1530 (2000).} it has been realized that EMR is but one example of a general class of EXX phenomena that can be geometrically enhance by the judicious choice of sample geometry, lead placement and the location, size and shape of any inhomogeneities. The second EXX phenomenon to be demonstrated was extraordinary piezoconductance, EPC\footnote{A. C. H. Rowe et al., Appl. Phys. Lett. {bf83}, 1160 (2003).}. Here we report a third EXX phenomenon, extraordinary optoconductivity, EOC. The optoconductivity of a macroscopic 4-contact van der Pauw plate structure consisting of Si-doped GaAs $(n\sim 1 times 10^{18}$ cm$^{-3})$ with an In shunt was compared to that of a shuntless sample. The conductance of each sample was measured as a function of temperature and of the position and wavelength of a focused Ar ion laser beam (spatial resolution of $10 \mu m$). At room temperature the short carrier mean free path $(\lambda)$ resulted in a photovoltaic response that was diminished by the shunt. In contrast, at low temperature the longer $\lambda$ results in EOC that, at 15K, is more than 400$\%$ larger in the shunted sample relative to the the unshunted sample.   

On Power dissipation in information processing

We consider power dissipation during simple switching in the informationally irreversible architecture. First, we investigate two limit cases of sudden and infinitesimal slow switching and then we derive solution for the general problem of the arbitrary speed switching in the two level system. The energy dissipation during errorless switching has a minimum of \textit {kTln(2)} and increases linearly with a switching speed. Both charge-position and spin degrees of freedom behave similarly in this model with the only difference being the relaxation times. We show that for a relaxation time of 1ps, power dissipation due to the finite switching speed at the operational frequencies of around 35GHz will become comparable to the \textit{kTln(2)} and total power dissipation per switch will become \textit{$\sim $2kTln(2).}   

Radiation-Induced Interface Traps in Silicon Bipolar Transistors

Experiments on bipolar transistors have shown that gain degradation increases as the dose rate is reduced for a given total dose of ionizing radiation. We suggest that this effect is caused by competing reactions involving hydrogen released from oxide sites by the ionizing radiation. At low dose rates, most of the hydrogen reacts with hydrogen-passivated Si dangling bonds at the semiconductor-oxide interface to create interface traps (Pb-centers), but at higher dose rates a larger fraction of the hydrogen is consumed in other reactions that depend on the dose-rate. This presentation will discuss continuum calculations of the the interface trap density as a function of radiation dose rate. These calculations will be compared with experimental data for dose-rate dependent irradiation of test structures.   

Electronic Properties of Energetic Particle-Irradiated In-rich InGaN Alloys

InGaN alloys, whose fundamental bandgaps span almost perfectly the solar spectrum, are potential materials for high-efficiency tandem solar cells. We have carried out a systematic study on the effect of irradiation on the electronic and optical properties of InGaN alloys over the entire composition range. Three different types of energetic particles (electrons, protons, and alpha particles) were used to produce displacement damage doses ($D_{d})$ spanning five orders of magnitude. The electron concentrations in InN and In-rich InGaN increase with $D_{d}$ and finally saturate after a sufficiently high dose of irradiation. The saturation of carrier density is attributed to the Fermi level pinning at the Fermi Stabilization Energy ($E_{FS})$, as predicted by the amphoteric native defect model. Electrochemical capacitance-voltage (ECV) measurements reveal a surface electron accumulation whose concentration is determined by pinning at E$_{FS}$. Modeling with a combination of various scattering mechanisms provides an excellent fit with the mobility measurements.   

Energy Loss and Stopping Cross Section Factors for Alphas in Lead Iodide

Lead Iodide is a candidate for use as a room temperature gamma ray sensitive semiconductor similar to mercuric iodide. We report here on values for the energy loss factor and the stopping cross section factor in lead iodide thin films. Vapor diffused purified lead iodide was used to make thick film and thin film samples evaporated on amorphic glass substrates. Thin films were used to take advantage of the surface energy approximation. In addition, separate lead and iodide backscattering peaks from the films are well resolved. Film thickness ranged from 50nm to 1000nm as determined by optical interference methods. The high energy singly ionized helium beam was provided by the CSULA 4 MeV Van de Graff accelerator. Rutherford backscattering was detected at 170$^{\circ}$. Both the input and output energy losses were calculated from the FWHM of the corresponding peaks. The typical energy loss factor was found to be 20.2 eV/angstrom with a 3{\%} uncertainty for a 2.4 MeV input beam. As expected, this value is about one third that of the pure elements.   

Dislocation Pile-up/Grain Boundary Interactions

Dislocation and grain boundary migration contribute significantly to plasticity in metals, but little is understood as to how the interaction between them influence plastic response. A multiscale computational method (CADD) is used to study the effects of dislocation pile-ups on the grain boundary deformation, initiation of failure, and overall mechanical response. Use of CADD preserves accurate atomistic details while allowing a large number of dislocations to pile-up near a tilt boundary. The effects of applied loading, pile-up densities, and geometry on absorption, transmission, and damage initiation at the grain boundary are studied.   

Ideal Shear Strength of Silicon Under Hydrostatic Tension and Compression

The ideal shear strength of silicon is computed using an {\em ab-initio} electronic structure total energy technique applying both hydrostatic tension and compression. Silicon displays a lower ideal strength under hydrostatic compression as compared with hydrostatic tension. This behavior is explained by silicon's desire to retain a more covalent like bonding under hydrostatic tension as compared with a more metallic like bonding under hydrostatic compression. The trend may be correlated with the shrinking of the band gap under the application of hydrostatic compression as compared with the gap predicted under hydrostatic tension. This research was supported by the Department of Energy, Basic Energy Sciences under the Office of Science under contract DE-AC03-76SF00098.   

Simulation of Current Filaments in Photoconductive Semiconductor Switches

Optically-triggered, high-power photoconductive semiconductor switches (PCSS's) using semi-insulating GaAs are under development at Sandia. These switches carry current in high carrier-density filaments. The properties of these filaments can be explained by collective impact ionization theory in which energy redistribution by carrier-carrier scattering within the filament enhances the impact ionization. This allows these filaments to be sustained by fields which are relatively low compared to the bulk breakdown fields. For GaAs, the sustaining field is approximately 4.5 kV/cm. For this talk, a hydrodynamic implementation of the collective impact ionization theory is used to compute the properties of these filaments. These continuum calculations are based on previous calculations in which the steady-state properties of filaments are computed using a Monte Carlo method to solve the Boltzmann equation. The effects of defects will also be considered in the presentation of the results.   

A porous silicon diode as a source of low energy ($<$ 0.1 eV) free electrons and its applications

We have developed a nanoporous silicon (PS) diode that yields free electron currents with energies $<$ 0.1 eV below 77 K. The power dissipated during emission is low so that pulses of electrons can be produced below 100 mK without raising the temperature of the system. Free electrons were generated in liquid $^4$He and $^3$He as well. At 77 K, $>$ 40 nA/cm$^2$ of emission current density was obtained. The results suggest that a Poole-Frenkel type of mechanism accounts for the observed electric field enhanced conduction but the electron emission mechanism is not well understood in the present models of PS. Application of this low energy electron source in a quantum computing system using electrons on the surface of a dielectric film as well as lithography and electron microscopy will be presented.   

Atomic hydrogen cleaning on GaAs photocathodes

The high-gradient-doping technique has been applied to GaAs photocathodes to overcome the surface-charge-limit effect while maintaining high polarization. However, the highly doped layer used in this technique is vulnerable to conventional $600^{\circ}$C heat-cleaning. One technique to reduce the heat-cleaning temperature is to use atomic hydrogen cleaning (AHC). We have systematically studied AHC using GaAs photocathodes, and have successfully reduced the heat-cleaning temperature to $450^{\circ}$C. The effect of AHC on polarization was minimal or zero in our study. In this presentation, we will show latest results from our study. Recent developments and future plans to integrate AHC into the SLAC linac injector polarized electron source will also be discussed.   

Towards SiC surface functionalization: an ab initio study

We present a microscopic model of the interaction and adsorption mechanism of simple organic molecules on SiC surfaces as obtained from ab initio molecular dynamics simulations. Our results for the silicon terminated SiC(001) surface show that at variance with the most exploited semiconductors such as Si and GaAs, the most common functional groups chemisorb to the surface, as a consequence of the substrate polarity with exothermal reactions. The preferential chemisorption of thiolates in particular can lead to the realization of stable self-assembled monolayers, with no requirement of preliminar metallic deposition. Our results open the way to functionalization of silicon carbide, a leading candidate material for bio-compatible devices. Part of this work was performed under the auspices of the U.S. Department of Energy by University of California Lawrence Livermore National Laboratory under contract No. W-7405-Eng-48.   

Integrability constraints for atomic-orbital integrals with applications to semi-empirical modeling of multi-element systems

Semi-empirical modeling of atomic-scale systems is often plagued by several issues related to atomic-orbital integrals. There are typically several sets of parameters which give similar results, suggesting that some combinations of parameters are redundant. The energy eigenvalues are not guaranteed to be real, which can and do result in systems that do not have a calculable energy. The use of $d$- orbitals results in an unreasonable increase in the number of parameters, making it difficult to model the transition metals. And the extraction of parameters for multi-element systems from those of single-element systems is not well-understood. We have improved these issues by accounting for the constraints which arise from the fact that the elements of the Hamiltonian and overlap matrix are specific integrals of specific atomic orbitals. These constraints are implemented using convolution and deconvolution between the two-center integrals and the radial parts of the orbitals. The result is a $50\%$ decrease in the relevant number of parameters for $s$ and $p$ orbitals, and a $83\%$ decrease for the $d$ orbitals. The eigenvalues are now always real. And the parameters for multi- element systems can be obtained using a convolution of the single-element orbitals, eliminating the need for artificial averaging or re-fitting techniques.   

Investigation of the Orbital Ordering Transition in La$_4$Ru$_2$O$_{10}$ using the Mossbauer Effect

There is a structural phase transition in La$_4$Ru$_2$O$_{10}$ from a triclinic phase starting at 140K to a monoclinic phase which is complete by 190K. This is a accompanied by the development of a local moment from $\mu_{eff} \simeq 0.4\mu_B$ to $\mu_{eff} \simeq 2.5\mu_B$ which leads to the identification of this transition with orbital ordering. The Mossbauer Effect(ME) has been measured from 4.2K to 196K in a sample prepared with enriched $^{99}$Ru(97\%). The ME spectrum is fit with two sites consistent with the triclinic structure. The spectra for the two sites are characterized by a quadrupole splitting(QS) and an isomer shift(IS): QS$_1$=0.51mm/s and I$_1$=-0.27mm/s and QS$_2$=0.38mm/s and IS$_2$=-0.32mm/s. At 171K the sample is mostly in the monoclinic phase which has a single-site ME spectrum with QS=0.38mm/s and IS=-0.31mm/s. The less symmetric site with the larger QS has disappeared. Throughout the transition the IS is consistent with a +4 charge state for the Ru site. The Debye temperature is 307K.   

Optimising Magnetoresistance in InSb

The extraordinary magnetoresistance (EMR) geometry produces the highest low field MR to date. Here we address the high field MR of InSb comparing materials from several sources and studying the behaviour when processed into a set of standard and novel geometries. We find that the Corbino geometry still produces the largest high field MR, but the linearity of the high field MR in a micron thick InSb film is enhanced by the fabrication of an array of interconnected circles with high resistance bridges. Nevertheless, unprocessed submicron InSb epilayers also show enhanced linear MR properties. This work was funded by EPSRC and by NSF grant ECS-0329347.   

Session U14: Hydrogen Storage III: Modeling

Physical properties of Ti-doped sodium alanates: First-principles studies and experiments

The recent surge in research on sodium alanates, NaAlH$_4$ and Na$_3$AlH$_6$, has been motivated by Bogdanovic's discovery that, when doped with small amounts of Ti, these compounds can reversibly store more than 4 wt.\% hydrogen. The location of catalytically active Ti and the mechanisms of enhanced H$_2$ sorption kinetics are still poorly understood. We will report on combined first-principles and experimental studies of structural, thermophysical and lattice dynamical properties of bulk alanates. Polarized Raman scattering on single crystals of NaAlH$_4$ has been used to determine the frequencies of the Raman-active vibrational modes between 300 and 425~K, i.e., up to the melting point $T_{\rm m}$. Significant softening (by up to 6\,\%) is observed in the modes involving rigid translations of Na$^+$ cations and translations and librations of AlH$_4^{-}$ tetrahedra. Surprisingly, less than 1.5\,\% softening is seen for the Al-H stretching and Al-H bending modes, indicating that the AlH$_4^{-}$ anion remains a stable structural entity even near $T_{\rm m}$. The phonon mode Gr\"uneisen parameters, calculated using the quasiharmonic approximation, are found to be significantly higher for the translational and librational modes than for the Al-H bending and stretching modes, but cannot account quantitatively for the dramatic softening observed near $T_{\rm m}$, suggesting an essentially anharmonic mechanism. The calculated lattice expansion due to zero-point vibrations is found to be large (1.2 and 1.5\% for the $a$ and $c$ parameters, respectively), as expected for a compound with many light elements. The formation energies of Ti impurities in bulk alanates are found to be high ($>1$~eV), indicating that bulk substitution should not occur under normal conditions. We discuss the implications of these results for the kinetics of hydrogen release and hypothesize that breaking up the AlH$_4^{-}$ anions is the rate limiting step. The enhanced kinetics in Ti-doped NaAlH$_4$ powders is attributed to the effectiveness of Ti in promoting the break-up of the AlH$_4^{-}$ anions at the interface between NaAlH$_4$ and Na$_3$AlH$_6$.   

Atomic and electronic structure of alkali borohydrides: An \textit {ab initio} study

Alkali borohydrides MBH$_4$ (M = Na, K) have attracted great interest recently due to their potential applications as hydrogen storage materials and energy carriers for fuel cells due to the extremely large gravimetric capacity At low temperature the compounds crystallize with a tetragonal structure having P42/nmc symmetry in which the [BH$_4$]- complexes are ordered. We have carried out total-energy \textit{ab initio} electronic structure calculations based on the Projector Augmented Wave (PAW) method to calculate the atomic and electronic structure of this series. The lattice constants and various bond lengths are in good agreement with experiment. Results of the trend of the heat of formation for the hydriding/dehydriding reactions, the band structure, the density of states, and bonding properties of the [BH$_4$]- complexes will be discussed.   

First principles study of absorption of Hydrogen into Pd(111)

~~~ It is well known that Hydrogens interact with Palladium to form a hydride PdH, and numerous studies both experimental and theoretical have been devoted to the system. However, important data such as the energy barriers for hydrogen absorption in the Pd substrate which eventually leads to formation of the hydride are not well known. In order to understand the rationale for hydrogen absorption process, we have carried out first-principles electronic structure calculations for high and low coverages of H on Pd(111). We have found that for subsurface H which is absorbed in the octahedral position below the top layer on Pd(111) the energy barrier to be overcome is 0.6 eV for low coverage and increases for high coverage. For a comparative study we have carried out our additional calculations for H on Pt(111). In this talk, we will compare the coverage dependent barriers for H on Pd(111) with those on Pt(111) and obtain insights from first-principles calculations about how they differ. *Work supported in part by US-DOE under grant DE-FGO2-03ER154645.   

Hydrogens in Metal Clusters

Atomic and electronic structures of hydrogen atoms in metal clusters are presented by the first principles calculation based on the density functional theory using the linear combination of atomic orbital method. Discussions are focused on a formation of the vacancy-hydrogen cluster at the cluster center mainly in bcc metals. We found characteristic double stable positions at the vacancy site. Electronic structures of double minimum states are studied in terms of the hydrogen induced states. We analyze stabilities of clusters with and without the vacancy typically by the cluster models of M$_{50}$H$_{6}$, M$_{50}$H$_{12 }$, and M$_{51}$H$_{6}$, respectively (M= Nb, Mo, V, Cr, Fe, .etc.). Cluster size effects and the maximum capacity of hydrogen at the vacancy site will also be discussed.   

Ab initio investigation of LiNH$_{2}$, Li$_{2}$NH, and Mg(AlH$_{4}$)$_{2}$ complex hydrides

First-principles calculations on the complex hydrides LiNH$_{2}$, Li$_{2}$NH, and Mg(AlH$_{4}$)$_{2}$ were performed to determine their structural stability, electronic structure and formation energy. All these compounds were recently reported the most promising materials for reversible hydrogen storage. We discuss the ionic character and binding implications of other complex hydrides. Possibilities to improve the hydriding/dehydriding reactions are presented.   

Lattice Dynamics and Thermodynamic Properties of Complex Hydride NaAlH$_4$

We present a first-principles investigation of the lattice dynamics and thermodynamical properties of the complex hydride NaAlH$_4$, a promising candidate for hydrogen storage. The calculations are performed within the density functional framework and using a linear response theory. Calculations of the phonon spectrum, Born effective charges Z* of the atoms, dielectric constants in high and low frequency limit are reported. The mode characteristics of zone-center phonons including LO-TO splitting are identified and compared to experiment. The quasiharmonic approach is used to study thermal expansion together with the mean square displacement of each atom and its relation to the melting point. The inclusion of the zero-point motion yields an expanded lattice compared to the static case, while the low-frequency correlated oscillations of Na and AlH$_4$ complexes are found to play an important role in destabilizing the lattice.   

Predicted high storage of hydrogen via H2 complexes on titanium-decorated nanotubes

Developing safe, cost-effective, and practical means of storing hydrogen is crucial for the advancement of hydrogen and fuel- cell technologies. The current state-of-the-art is at an impasse in providing any materials that meet a storage capacity of 6wt\% or more required for practical applications. Accurate quantum mechanical calculations that predict new materials or routes to engineering materials properties are important to overcome this barrier. Here we report a first-principles study, which demonstrates that a single Ti atom coated on a single-walled nanotube (SWNT) strongly binds up to four hydrogen molecules. The first H2 adsorption is dissociative with no energy barrier while other three adsorptions are molecular with significantly elongated H-H bonds. At high Ti coverage we show that a (8,0) SWNT can strongly adsorb up to 8wt\% hydrogen. Simulations at high temperature indicate that the system is quite stable and exhibits associative desorption upon heating, a requirement for reversible storage. These results not only advance our fundamental understanding of dissociative adsorption of hydrogen on transition metals in nano-structures but also suggest new routes to better storage and catalyst materials.   

Confinement effects on chemical reactions in nanostructured carbon materials

Chemical reactions are frequently carried out in nano-structured media, such as micellar or colloidal solutions, nano-porous media, hydrogels or organogels, or in systems involving nano-particles. Nanostructured environments have been shown to enhance reaction rates through a variety of catalytic effects, such as high surface area, interactions with the nano-structure or confinement. In this work, we have used state-of-the-art electronic structure techniques to study the prototypical example of the hydrogen-producing reaction of formaldehyde dissociation (H$_2$CO $\rightarrow$ H$_2$~+~CO) within various configurations of a graphitic pore. Using the Nudged Elastic Band (NEB) method for transition states analysis, we have found that the activation energy of the dissociation can be influenced by the presence of a graphitic pore. In particular, while a graphene surface reduces the activation barrier for the reaction, this catalytic effect is enhanced by the presence of two planar sheets, which mimic the geometry of a nano-pore. These findings will be discussed in terms of the charge transfer and/or polarization mechanism associated with the catalytic process.   

First principles study of ammonia decomposition on Ni and Pd surfaces

Ammonia is considered as an efficient storage for hydrogen. It can be converted to hydrogen on board vehicle by decomposition. The decomposition requires efficient catalyst that still has to be designed based on systematic understanding of the reaction mechanisms. We present results of first principles electronic structure calculations based on density functional theory and the generalized gradient approximation of various stage of the decomposition of NH$_3$ on Ni and Pd surfaces. It is known that the ammonia decomposition rate is much higher on Ni that on Pd. The reaction mostly occurs on surface steps and defects. We calculate, compare and contrast adsorption energies, the paths and energy barriers for NH$_3$ diffusion and dissociation on singular and stepped Ni and Pd surfaces, as well as on those with vacancies. The differences in the characteristics of the energy landscape on the various surfaces are explained through analysis of the local densities of electronic states and valence charge densities calculated for the molecule located at preferred adsorption sites and saddle points on the reaction paths. Contact will be made with available experimental data.   

LiNH$_2$--MgH$_2$ as a Potential Hydrogen Storage Material: a Density Functional Theory Study

LiNH$_2$ possesses high capacity for hydrogen storage [1], but its large hydride formation enthalpy leads to operating temperatures and pressures that lie outside the practicable range for vehicular applications. Partial substitution of Li by Mg can destabilize the system and thus improve the hydrogen de-sorption characteristics, as it has been shown in recent experimental studies [2]. We present and discuss results from our density functional theory investigations of the LiNH$_2$--MgH$_2$ system. The main aim of this study is to understand the bonding characteristics, the Mg-induced destabilization mechanism, and the thermodynamics of hydrogen de-sorption from an electronic structure viewpoint. [1] P. Chen et al., J. Phys. Chem. B 107, 10967 (2003). [2] W. Luo, J. Alloys Compd. 381, 284 (2004).   

Session U15: Focus Session: Synthesis and Doping of Nanostructures

X-ray Analysis of Erbium Doping in Group IV Nanocrystalline Materials

We have produced erbium-doped germanium nanoparticles using a new two cell physical vapor deposition system. Doped nanoparticles are fabricated using two methods: 1) by co-evaporation of Er and Ge and 2) by Er deposition on the surface of undoped Ge nanoparticles. Using elemental specific x-ray techniques [x-ray absorption (XAS) and photoemission (PES) spectroscopy], we are able to monitor band edge shifts as a function of both particle size and Er concentration. In addition, we have used XAS and PES to probe the chemical environment of Er in Ge nanoparticles. We find that large Er/Ge ratios lead to strong spectroscopic signatures in the core level PES spectra. Lower Er/Ge ratios show very little effects in the core level spectra; however, the valence band density of states is altered which allows PES to probe dilute concentrations of Er in Ge nanoparticles. Impact of Er doping on the Ge nanoparticle electronic structure will be discussed. This work was supported by the Division of Materials Sciences, Office of Basic Energy Science, and performed under the auspices of the U. S. DOE by LLNL under contract No. W-7405-ENG-48.   

Doping semiconductor nanocrystals: Theory

The intentional introduction of impurities into semiconductor nanocrystals (NCs) is a poorly understood process whose phenomenology remains largely unexplained. For example, Mn can easily be incorporated into ZnSe NCs using simple precursors, but not into CdSe NCs---despite comparable solubility limits of 50-60 percent in the two bulk crystals. The conventional wisdom is that NCs can ``self-purify'' by expelling impurity atoms to the nearby surface (consistent with the fact that very small NCs cannot generally be doped). Here we propose a very different view: namely, that doping is controlled by the initial adsorption of impurities on the NC surface. For NCs with sufficiently well-defined facets, this view leads to several striking predictions. (i) Dopant incorporation should depend on crystallography. For example, we predict that Mn incorporation will be generally allowed in zinc-blende NCs (such as ZnSe) but suppressed or absent in wurtzite NCs (such as CdSe). (ii) Very small NCs are often known to form cage clusters with strongly reconstructed surfaces. We find that these surfaces do not permit strong dopant adsorption, thus precluding incorporation. (iii) If the equilibrium NC shape can be controlled, doping may be externally tunable or even switchable. Hence, for II-VI NCs grown colloidally, we predict that dopant incorporation will vary with the II:VI concentration ratio in solution.   

Doping semiconductor nanocrystals: Experiment

The intentional introduction of impurities into semiconductor nanocrystals is a poorly understood process whose phenomenology remains largely unexplained. The preceeding talk outlines a theoretical model that addresses this doping problem. Here we experimentally test some of the predictions of this model. In particular, we use photoluminescence (PL), electron paramagnetic resonance (EPR), and inductively coupled plasma atomic emission spectroscopy (ICP-AES) to examine how the doping efficiency in ZnSe:Mn nanocrystals is influenced by various experimental parameters. In agreement with the theoretical model, the doping concentration can be enhanced by increasing the anion (Se) to cation (Zn) ratio in the growth solution. Finally, we exploit the predictions of the model to incorporate individual Mn impurities into previously undopable CdSe nanocrystals. This success indicates that doping is not intrinsically problematic in nanocrystals and a variety of doped particles should be experimentally realizable.   

Shape Manipulation of II-VI Semiconductor Nanocrystals and Heterostructures by Controlled Reactant Injection.

We have developed a general solution-phase method for inducing CdS, CdSe, and CdTe nanorod growth. The shape can be tuned from spheres to rods with aspect ratios up to 20 simply by sequentially injecting precursor solutions to promote epitaxial elongation of the wurtzite crystal structure in the [001] direction (i.e., along the c-axis). Under the appropriate conditions, homogeneous particle nucleation can be avoided upon subsequent precursor injections and the fast growth kinetics in the [001] direction extends the nanorods without increasing the diameter. We have employed this sequential injection approach to form both Type I (Nested) and Type II (Offset) heterostructures of CdS/CdSe/CdS and CdTe/CdSe/CdTe nanorods. Consistent with the Type I band offset, addition of CdS extensions to the ends of the CdSe nanorods significantly enhances the photoluminescence (PL); whereas, the extension of CdTe off the ends of the CdSe rods quenches the PL emission as electrons and holes separate at the CdSe/CdTe interface in the rod.   

EPR Study of Manganese-Doped TiO2 Nanorods

Titanium dioxide nanoparticles and nanorods were prepared by a hydrothermal method using scrolled anatase nanotubes as the starting material. The addition of manganese ions to scrolled nanotube precursors was found to unroll the nanotubes into sheet-like structures indicating a strong adsorption of manganese ions at undercoordinated sites that terminate layers of scrolled titanium dioxide nanotubes. Hydrothermal treatment of Mn$^{2+}$ unscrolled nanotubes was found to result in the formation of doped anatase nanorods (30 x 300 nm). Upon 1 {\%} doping, the optical properties of nanorods change, resulting in the appearance of a broad absorption band at $\sim $650 nm. X-band EPR spectra show unusual eleven line spectrum with 72 G hyperfine splitting and a g factor of 2.007. The total width of the spectrum was 890 G due to the appearance of second order effects. The same hyperfine coupling was found in samples having doping levels in the range 0.1-2 {\%} of Mn$^{2+}$, indicating specific interaction of manganese ions within the TiO$_{2}$ lattice. Adsorption of Mn$^{2+}$ ions onto the surface of nanorods did not change their optical properties and exhibits the typical six line spectrum of Mn$^{2+}$ with 90 G hyperfine splitting in addition to a broad, unresolved solution spectrum of Mn$^{2+}$. The effects of the size and shape of titanium dioxide nanoobjets on the spin multiplicity of manganese dopants are being investigated.   

Synthesis of One-dimensional GaN Nanostructures and Their Implications for Formation Mechanisms

One-dimensional GaN nanostructures were successfully synthesized by gold-catalyzed metal-organic vapor phase approach. It was found that GaN whiskers of various morphologies could be synthesized on Si substrates by just controlling the temperature. Amorphous Ga/nitride nanowires formed at 450C with tadpole-like structures. GaN tubular nanostructures synthesized were observed at 600C. Wavy-like hollow interiors with single crystalline wurtzite phase were seen from high-resolution transmission electron microscope images. With increasing the catalytic temperature, crystalline GaN nano-pyramids, and straight nanowires were formed. It is proposed that the morphologies evolution of GaN whiskers was attributed to the competitions of TMG surface diffusion to the Ga-Au eutectic droplets, Ga bulk diffusion into the catalyst, and GaN seeding. While at low temperature, only Ga atoms at surface react with ammonia and form amorphous Ga@nitride nanowires. With increasing the temperature, GaN seeding and the subsequent growth along the circumferential edges of these seeds leads to the evolution of nanotube morphology. Further increasing the temperature, promoted the nitridation efficiency and axial growth rate and lead to the nanowires and nanopyramids growth.   

Synthesis and Electrical Transport Studies of Zn-doped Ga$_2$O$_3$ Nanowires

Ga$_{2}$O$_{3}$ is a wideband gap material ($E_{g}=4.9eV$). Its one dimensional nanostructures have attracted much research effort. Ga$_{2}$O$_{3}$ nanowire is a promising material in the applications such as blue light emitter, transparent conducting oxide, and chemical sensor. However, the electronic device application of Ga$_{2}$O$_{3}$ nanowire is difficult due to its low electrical conductivity. In this work, $\beta $-Ga$_{2}$O$_ {3}$ nanowires were synthesized via catalytic chemical vapor deposition method. The diameter of the as-grown nanowires ranges from 20 to 80nm. In order to improve the electrical properties, zinc was used as a dopant. A series of material characterizations were performed to study the properties. Electron microscopy shows the morphology and crystal structure, while X-ray diffraction provides the crystal information and composition. In addition, photoluminescence spectra and photoconductivity measurements show trapping states located within the bandgap. The nanowires were also fabricated into field-effect-transistors for transport measurements. And $I-V$ and $I-V_{g}$ curves manifest QTR{it}{p}-type semiconducting behavior, and carrier concentration and mobility are estimated.   

Synthesis and structural analysis of $\gamma $--Fe2O3/CdS nanocrystal heterodimers

Inorganic nanocrystal oligomers with two or more distinct chemical compositions open up interesting avenues of developing building block materials for a variety of research directions. For example, asymmetric dimers, trimers, etc. can provide chemically programmable assembly of nanostructures. Unique properties arising at the nanoscale may also be juxtaposed in a controlled manner (e.g. ferromagnetic behavior with optical properties governed by quantum confinement). $\gamma $--Fe$_{2}$O$_{3}$/CdS heterodimers have been synthesized in solution by annealing Cd and S reagents adsorbed on $\gamma $--Fe$_{2}$O$_{3}$ nanocrystals. While the large lattice mismatch between $\gamma $--Fe$_{2}$O$_{3}$ and CdS leads to dewetting, TEM analysis reveals that certain junction planes can lead to minimized strain allowing dimers to form. Furthermore, both wurtzite and zinc blend structures are observed to grow on the close-packed (1 1 1) plane of $\gamma $--Fe$_{2}$O$_{3}$.   

Fabrication and characterization of SnO2 nanobelt field effect transistors

Single-crystalline SnO$_{2}$ nanobelts have been produced by thermal evaporation of oxide powders in a tube furnace without any chemical catalyst. Individual SnO$_{2}$ nanobelts with thicknesses of 30nm$\sim $300nm and lengths as long as several hundred $\mu $m were dispersed onto a doped Si/SiO$_{2}$ substrate, and multi-terminal metal electrodes were defined on a nanobelt using photolithography. An individual nanobelt was then characterized by measuring current--voltage characteristics as a function of temperature using 4-probe measurement. Temperature dependence of the resistivity is characteristic of a doped semiconductor. A field effect transistor (FET) is formed using a nanobelt as the channel and doped Si as the gate. Electrical measurements revealed characteristic behavior of an n-channel depletion-mode FET, with well-defined linear and saturation regimes, a threshold voltage of $\sim $-15V, and on/off ratio as high as 10$^{3}$. The channel mobility is estimated to be 25 cm$^{2}$/V$\cdot $s, and carrier concentration about 6x10$^{15}$ cm$^{-3}$. The results demonstrate the potential of using SnO$_{2}$ nanobelt to construct high performance nanoFET with possible applications as chemical and biological sensors. This work is supported by NSF NIRT grant ECS-0210332.   

Superparamagnetic Core-shell Silica- Polypeptide Composite Particles

Core-shell composite particles have been prepared, each consisting of a silica-coated cobalt center to which a homopolypeptide shell, either poly ($\varepsilon $-carbobenzyloxy-L-lysine) or poly ($\gamma $-benzyl-L-glutamate), is attached covalently. Core particles were decorated with a mixture of amino groups and passivating groups through silylation reactions. The amino groups initiated the polymerization, with attachment, of $N$-carboxyanhydride monomers, resulting in a homopolypeptide shell. Characterization by dynamic light scattering confirmed the helix-coil transition of the particles through repeated heating and cooling cycles in an organic solvent. The living nature of the polypeptide shell has also been confirmed. The particles have a size and uniformity that leads to formation of colloidal crystals. Magnetometer measurements suggest the particles are superparamagnetic.   

Synthesis of monodisperse nanocrystals via green chemistry

Novel strategy for the synthesis of monodisperse nanocrystals was developed. This new method is cheap, reliable, safe and environmentally benign. The nanocrystals synthesized by this new method, including semiconductor nanocrystals (quantum dots) CdS........$^{1}$, CdSe, CdTe........$^{2}$, PbSe...........$^{3}$, and magnetic nanocrystals, Fe$_{3}$O$_{4}$.()$^{4}$ (magnetite), have wider size range, and narrower size distribution (less than 10{\%}). Through this new method, one can control the size, shape, and crystal structure of the aimed nanocrystals by simply changing the ligands used in the synthesis. With the high quality nanocrystals, some basic physical constants, such as extinction coefficients of semiconductor nanocrystals were accurately measured........$^{5}$. A simple method was also developed to transfer the above-mentioned organic-media synthesized high quality nanocrystals to aqueous media (pure or buffered water). The water-soluble nanocrystals keep their original properties in organic media. For example, water-soluble semiconductor nanocrystals have the same absorption and emission spectra, the same quantum yield, and the same size and size distribution as the ones dispersed in chloroform. The water-soluble nanocrystals are stable in pure water and conventional biological buffers. .$^{1 }$W. W. Yu and X. Peng, \textit{Angew. Chem. Int. Ed.}, \textbf{41}, 2368 (2002). $^{2 }$W. W. Yu, Y. A. Wang and X. Peng, \textit{Chem. Mater.}, \textbf{15}, 4300 (2003). $^{3 }$W. W. Yu, J. C. Falkner, B. S. Shih and V. L. Colvin, \textit{Chem. Mater.}, \textbf{16}, 3318 (2004). $^{4 }$W. W. Yu, J. C. Falkner, C. Yovuz and V. L. Colvin, \textit{Chem. Commun}, 2306 (2004). $^{5 }$W. W. Yu, L. Qu, W. Guo and X. Peng, \textit{Chem. Mater.}, \textbf{15}, 2854 (2003).   

Formation and Properties of Self-Assembled Ni Nanodots and VLS Growth of GaN Nanowires

We report the formation of Ni nanodots and subsequently use them to grow GaN nanowires. For the Ni nanodots both Si(111) and sapphire substrates are used. Layers of Ni are deposited with different thickness (1 to 5 nm) on these substrates using UHV electron-beam evaporation. The layers are annealed ex situ and the nanodot formation is studied for different anneal temperatures and durations. For nanodot formation on Si(111) the process is self limiting at high temperature with distinct facets. Activation energies are consistent with Ni surface diffusion as the primary formation mechanism. Nanostructures on sapphire are droplet shaped, we observe no distinct faceting. The nanodot size can be controlled with initial Ni thickness. Based on these studies, we have grown GaN nanowires on sapphire substrates covered by Ni layers having various thickness. Vapor- liquid-solid growth mechanism is demonstrated using a pulsed metal-organic chemical vapor deposition approach. The GaN nanowires are straight, vertically oriented, and of constant diameter following the same trend as the Ni nanodot diameter with initial thickness. We report control of GaN nanowire diameter and length up to 1 micron.   

Session U16: Photonic Crystals II

Wave Propagation in High Impedance Surfaces

High impedance (hi) surfaces are artificially structured surfaces that form A sub-class of 2-d photonic band gap (pbg) materials. An interesting feature Of metallo-dielectric hi surfaces is the presence of a surface Electromagnetic (em) band gap at frequencies where the wavelength is larger Than the lattice dimension---a feature absent in all-dielectric pbg Materials. Due to this ``sub-wavelength'' behavior, effective medium Theories (emt) can be used to describe their dispersion characteristics. A General emt framework has been developed, and a new mechanism to describe The occurrence of band gaps in hi surfaces is presented. It is shown that The eigenmode of a surface em wave at any frequency can be written as a Linear combination of two ``pure'' modes: a backward mode that propagates Below the surface, and a forward mode that resides on the surface. At the Band gap frequencies these two modes cancel, resulting in no propagation. It Is anticipated that this model will be a powerful tool for understanding and Exploring a large class of periodic systems.   

Design and Modeling of Periodically Loaded Transmission Line Metamaterial Structures

Transmission lines periodically loaded with suitable elements form a class of photonic band gap (PBG) materials that display unusual properties at frequencies where the electromagnetic (EM) wavelength is much smaller than the spacing between periodic loading. So, effective medium theories which use circuit elements as building blocks can be used to describe these systems. A design framework to determine the properties of such meta-material structures has been developed based on circuit models of the unit cells. The models were parameterized using full wave EM field solvers, and the parameterization has been used in subsequent designs of a large class of structures. The predictions of our effective medium model and that of the EM simulations have been validated by measurements. The effective medium model is orders of magnitude faster than full wave EM simulations, reflecting the efficiency of this approach in rapid design. This technique is applied here to 1-D transmission line structures loaded periodically with metal posts. However, it can easily be extended to other types of periodic loading and to 2-D structures as well.   

Control of complete bandgap in two-dimensional photonic crystals with open veins

Two-dimensional photonic crystals of a square lattice with square dielectric rods growing thin (6.5{\%} lattice constant) veins on the middle side of each square dielectric rods in air are proposed. Band structures are calculated by the plane-wave expansion method. The complete photonic band gap (PBG) is found in higher frequency band. Then growing thin veins, this PBG is gradually disappeared. At certain length of thin veins, a complete PBG starts to appear in a lower frequency band. But the greatest complete PBG can be gotten while veins are not connected at the boundary of the unit cell of the lattice. The reason of this finding will be given in this report. Our results may provide a new direction for designing PBG of two-dimensional photonic crystals.   

Electrical switching of light using liquid crystal-infilled 2D photonic crystals.

We have studied two-dimensional photonic crystals of hollow cylinders (made of Si or Ge) that are infilled with the nematic liquid crystal (NLC) 5CB. The dielectric tensor of the NLC cylinders is obtained by minimizing the free energy, which has elastic and electrostatic contributions [1]. Our calculations of the photonic band structure show that an applied electric field can produce switching of the transmitted light; this is realized due to a phase transition from the escaped radial to the axial configuration of the NLC molecules. Specifically, for a square lattice, with propagation in the [110] direction, the light is completely reflected when the field is off; on the other hand, it is partially transmitted when a sufficiently strong electric field is applied parallel to the cylinders. [1] J. A. Reyes-Cervantes, J. A. Reyes-Avenda\~{n}o, P. Halevi. Proc SPIEl \textbf{5511}, 50.   

Electro-Optical Control of Directional Switching Based on Degenerate Defect State Spliting in Photonic Crystal

We study the splitting of the degenerate defect states inside a two-dimensional photonic crystal. Using the group-theory analysis we find the perturbation potentials that allow for the most efficient splitting of the degenerate states. The results of the theoretical analysis are applied to the particular examples of the two-dimensional square and hexagonal photonic lattices of the dielectric rods in vacuum doped by the defect rod, giving the doubly degenerate E state in the first TM band gap. We show how the choice of the perturbation potential can control both the magnitude and symmetry of the splitting. The perturbation potential can be generated either by piezoelectric effect resulting in lattice distortion or by electro-optical effect resulting in change of the dielectric function. We concentrate on the perturbation potential caused by the electro-optical effect. Application of the effect in fast switch of waveguide devices is presented. We also discuss use of the effect in the design of electrically tunable lasers.   

Nonlinear Photonic Crystals as a Source of Entangled Photons

Nonlinear photonic crystals can be used to provide phase matching for frequency conversion in optically isotropic materials. The phase-matching mechanism proposed here is a combination of form birefringence and phase velocity dispersion in a periodic structure. Since the phase matching relies on the geometry of the photonic crystal, it becomes possible to use highly nonlinear materials. This is illustrated considering implementable one-dimensional periodic structures for the generation of light at wavelengths between 700nm and 1500nm. We show that phase-matching conditions used in schemes to create entangled photon pairs can be achieved in photonic crystals.   

Polymer-Based Hypersonic Phononic Crystals

The ability to influence high frequency phonons has great importance for both fundamental science and practical applications. A number of important physical processes, such as thermal energy flow, charge carrier mobility and lifetime, and the superconductivity transition, can be altered by modifying the phononic dispersion relation of a medium. Applications range from thermal management and thermoelectricity, to enhanced microelectronic and opto-electronic devices. In this talk we will discuss the use hypersonic phononic crystals to achieve control over the emission and propagation of high frequency phonons. We fabricate high quality, 2D single crystalline hypersonic crystals using interference lithography and perform direct measurement of their phononic band structure with Brillouin light scattering. Numerical calculations are employed to explain the nature of the observed propagation modes. This work lays the foundation for experimental studies of hypersonic crystals and, more generally, phonon-dependent processes in periodic nanostructures.   

X-ray Microscopy on Thin Metallic Photonic Crystals

We present a high-resolution microscopy experiment that uses hard X-rays supplied by Sector 34-ID C from the Advanced Photon Source. The sample of interest is a two-layered inverted nickel photonic crystal with spherical voids of 1900nm diameter and an expected feature size of c. 200nm. Although existing soft X-ray microscopy techniques can reach a sub-hundred nanometer resolution, nickel does not transmit light with energies in the range of the beams used in these cases (c. 1keV), thus rendering them inappropriate for the imaging of such samples. However, with hard X-rays nickel absorbs very little (c. 2-4 percent) light whose energy lies about its absorption edge, which is at 8.4keV. In this new experiment we were able to obtain a magnified image of the Ni photonic crystal with a resolution of 100nm, a result that is unprecedented in this type of system. The experimental set-up uses advanced hard X-ray optical components, such as Kirkpatrick-Baez mirrors, Fresnel zone plates, and a scintillator screen.   

Enhancement of Magneto-Chiral Effect in Photonic Crystals

We theoretically study a magneto-chiral effect magnified in photonic crystals. A magneto-chiral effect is a directional birefringence even for unpolarized light. This effect occurs in a material such as GaFeO$_3$ in which both time-reversal and inversion symmetries are broken. Unfortunately the wave vector dependence of a dielectric function is typically the order of $10^{-4}$, which is too small to observe. We consider one-dimensional photonic crystals composed of the magneto-chiral medium and air, and calculate the reflectivity. We found that the difference in the reflectivities with respect to different magnetization configurations is thousands of times enhanced compared with that in a bulk material.   

A new picture for gap solitons in nonlinear photonic crystals

We construct a new local-Bloch theory for the interplay between the nonlinearity and the periodicity. Based on this first order theory, we can get gap soliton solutions composed of local Bloch waves over the entire gap. Some important unique properties of the gap solitons are revealed, such as the periodicity-amplified nonlinearity, the periodicity-generated high order nonlinearity, the touching-local-gap-edge property and the intrinsic-ultrashort-pulse property. Besides these properties, the envelope equations of both stationary and time-dependent cases are obtained. The stationary envelope equation is a cubic-quintic nonlinear Schrodinger equation which can be solved exactly. From this simple equation, we can give a clear explanation of earlier theoretical results. The time-dependent envelope equation includes new important high order time-derivative terms, so that it is much more complex than common nonlinear Schrodinger equations. One of those terms is the self-steepening term which leads to a new dynamical instability of the gap solitons.   

Quantum Hall effect analogs in photonic crystals: ``chiral'' (unidirectional) edge modes as ``one-way waveguides''

``Photonic crystals'' constructed from non-reciprocal (Faraday) media with broken time-reversal symmetry can have topologically non-trivial photonic bands with non-zero ``Chern invariants''. In electronic systems, filling such bands produces an (integer) quantum Hall effect (QHE). While photonic (bosonic) bands cannot be ``filled,'' other features of the QHE - ``chiral edge states'' - persist. We present an explicit example of a periodic array of dielectric rods parallel to the Faraday axis of their surrounding medium, with a band gap for photon propagation normal to the rods, and topologically-non-trivial 2D bands. We then examine a ``domain wall'' across which the Faraday axis reverses. As the inevitable consequence of topology, there are modes in the gap which are localized at this interface, and allow flow of electromagnetic energy {\bf in one direction only}, making a ``one-way waveguide'' without counterprogating modes, so it is robust against elastic backscattering at bends (though not against absorbtion, unlike charge flow in the electronic analog). The ``Berry curvature'' introduced by Faraday materials leads to a new class of novel possibilities for ``photonic band-structure engineering,'' with possible technological applications.   

Tuning of the spontaneous emission in a one--dimensional photonic crystal

The optical properties of a photonic crystal (PC) can be tuned if one of its constituents is a semiconductor [1]. Changing the free carrier concentration (electrons or holes) in the semiconductor, it is possible to modify its dielectric constant, and, consequently, the band structure or the density of optical states of the PC. On the other hand, the rate of spontaneous emission of an atom or a molecule depends on the density of states of its environment. Thus, the rate of emission of an atom immersed in a PC is different from the rate of emission in vacuum. In this work, we compute the rate of spontaneous emission of an Er ion in a one-dimensional, tunable PC made of alternating layers of Si and air. The rate of spontaneous emission can be changed about 30 % around its value in the homogeneous medium. [1] P. Halevi and F. Ramos-Mendieta, Phys Rev Lett. 85, 1875 (2000); A. S. Sánchez and P. Halevi, J. Appl. Phys. 94, 792 (2003).   

Polariton-assisted coherent thermal emission by heterogeneous structures

The control of thermal radiation has important applications in thermophotovoltaic devices, solar cells, and space thermal management. Excitation of surface polaritons allows the thermal emission spectra to be modified using nanostructured materials. The coupling of the excited surface polaritons to thermal radiation \textit{via} diffraction by gratings can result in coherent thermal emission. Here, we describe a novel concept of a gratingless coherent thermal source that uses paired single-negative layers: one with a negative permittivity ($\varepsilon )$ and the other with a negative permeability ($\mu )$. We show that coherent thermal emission is feasible for both $s$- and $p$-polarizations, owing to surface polariton excitation at the interface of the negative-$\varepsilon $ and negative-$\mu $ media. Moreover, the emission frequency and emission angle can be controlled by adjusting the film thicknesses. Future development in nanooptical materials with negative-$\mu $ at near-infrared frequencies is critically needed to realize the proposed coherent thermal source.   

The Role of the Absorption in the Stop Band Tuning of Opals and Inverse Opals Through Coating of Semiconductor Materials

In this work we report on the modeling of the optical properties of semiconductor in-filled opals and inverse opals for the visible and near infrared spectral region. The crucial influence of the absorption is theoretically investigated by using the three dimensional Transfer Matrix Method (TMM). Fine-tuning of the stop band positioning is achieved with increasing semiconductor infiltration. The red shift of the stop band can be explained by Bragg's law. However, the optical properties depend strongly on the value of the absorption that is directly related to the imaginary part of the Dielectric Constant (DC). We use a realistic model of the DC for a specific semiconductor (InSb) that takes into account the phononic contributions, intrinsic electron and hole densities. By positioning the stop band in the region of the smaller value of the imaginary part of the DC we optimize the value of the lattice constant in order to optimize tuning of the stop band with fewer losses. We also study the influence of absorption in the Fabry-Perot oscillations and in the higher energy stop bands. This work is motivated by new experimental results that show that absorption in 3D structures can be the limiting factor to obtain a useful structure for tuning.   

Session U17: Optical Response and Spin-Orbit Coupling in Quantum Dots

Properties of polar optical phonons in wurtzite quantum dots

Wurtzite (WZ) ZnO and GaN quantum dots (QDs) have attracted significant attention as promising candidates for optoelectronic applications. To enable the interpretation of the optical response of such QDs, we derive an integral equation that defines both interface and confined polar optical phonon modes in WZ QDs of arbitrary shapes [1]. The analytical solution of the derived equation is found for spheroidal QDs [2]. While the frequency of the confined polar optical phonons in zincblende QDs is equal to that of the bulk crystal, the confined optical phonons in WZ QDs are shown to have a discrete spectrum of frequencies different from those of the bulk crystal. The obtained results have been proven useful for the accurate prediction of interface and confined optical phonon frequencies in the novel class of QDs. This research has been supported in part by the ONR Young Investigator Award to A.A.B. [1] V.A. Fonoberov and A.A. Balandin, Phys. Rev. B 70, in press (Dec. 15, 2004). [2] V.A. Fonoberov and A.A. Balandin, Phys. Stat. Solidi C 1, 2650 (2004).   

Hole-LO phonon interaction in InAs/GaAs quantum dots

Various experimental and theoretical works demonstrate that electrons confined in quantum dots are strongly coupled to the longitudinal optical(LO) vibrations of the underlying semiconductor lattice \cite{hameau1999}. This leads to the formation of the so-called quantum dot polaron, which are the true excitations of a charged dot. The interaction between holes confined in quantum dots and LO phonons has not, until now, been explored. We present a study of holes confined in InAs/GaAs quantum dots doped with Be. The interaction between the holes in the dots and the LO phonons of the lattice is studied experimentally by spectroscopy in the FIR ($50-700\,\textrm{cm}^{-1}$) energy range and under the influence of a magnetic field (0-15T). We observe several resonances in magneto-transmission around $200\,\textrm{cm}^{-1}$. In order to interpret our experimental results, we calculate the coupling between the hole-phonon states, using the Fr\"ohlich Hamiltonian. The resulting polaron states we find are in good agreement with our experimental results. \begin{thebibliography}{99} \bibitem{hameau1999} S. Hameau {\em et al.}, Phys. Rev. Lett. {\bf 83}, 4152 (1999); S. Hameau {\em et al.}, Phys. Rev. B{\bf 65}, 85316 (2002). \end{thebibliography}   

Aharonov-Bohm Beats in Excitonic Luminescence from Quantum Rings and Type-II Quantum Dots

We study the absorption spectrum of neutral magnetoexcitons confined in ring-like structures. Despite their neutral character, excitons exhibit strong modulation effects on the energy and oscillator strength in the presence of magnetic fields [1] that have been recently observed [2]. We calculate the absorption coefficient $\alpha$ for neutral excitons confined in circular ring geometries with radii $R_e$ for electrons and $R_h$ for holes. A particularly interesting situation comes about when $R_e \neq R_h$ and a net radial charge polarization arises. In this case, we consider an attractive Coulomb interaction proportional to $(R_e - R_h)^{-1}$ and the excitonic absorption peak shows oscillatory behavior as function of the applied magnetic field both in position and amplitude. Such oscillations strongly depend on the dipole moment $P=e(R_h-R_e)$ of the exciton and on the dielectric constant of the system. Such intensity changes could in principle be experimentally observed with single dot spectroscopy in quantum rings [3]. Supported by the NSF-IMC and NSF-RUI \newline [1] A.O. Govorov et al. Phys. Rev. B 66 081309 (2002); A.O. Govorov et al. Physica E 13, 297 (2002). \newline [2] E. Ribeiro et al. Phys Rev. Lett. 92 126402 (2004). \newline [3] R.J. Warburton et al. Nature 405 (6789) 926 (2000).   

The Aharonov-Bohm effect in self-assembled InGaAs/GaAs quantum rings

Based on the structural information from X-STM measurements on buried self-assembled InGaAs/GaAs quantum rings, we calculate the electron energy spectra and the magnetization as a function of the applied magnetic field. Since the lateral size of quantum rings substantially exceeds their height, the lateral electron motion is governed by the adiabatic potential related to the fast electron motion along the growth axis. The electron states are calculated by diagonalizing the adiabatic Hamiltonian. The oscillations of the electron orbital magnetic moment versus magnetic field are analysed as a function of angular modulations of the height, width and slopes of the rim as well as the chemical composition of a quantum-ring structure. Although the realistic quantum-ring shape differs strongly from an idealized circular-symmetric open-ring structure, Aharonov-Bohm oscillations survive.   

AC-Stark effect in a semiconductor self-assembled quantum lens

We present a theoretical study of the effects of an ac electric field applied along the direction of axial symmetry of quantum dots with lens shape. This geometry has been found to realistically describe semiconductor quantum dots grown by self-assembly [1]. Using the Floquet formalism, the time-dependent Hamiltonian in the effective mass approximation is solved. A conformal analytical image is designed to map the quantum dot boundary into a dot with semi- spherical shape, allowing one to obtain a complete set of orthonormal functions to characterize the physical problem, while keeping the full lens symmetry. The Hamiltonian for a carrier confined in the quantum lens is correspondingly mapped into an equivalent operator and the corresponding Dirichlet problem is analyzed. We show that the Hilbert space of solutions is separated into orthogonal subspaces with different z- component of angular momentum. We give an explicit analytical representation for the quasi-energy spectrum and electronic states as function of the lens parameters and electric field intensity. [1] M. Muñoz et al., Appl. Phys. Lett. {\bf 83} 4399 (2003).   

SHG from bulk and surface of nanoparticle composites

Three wave mixing processes such as second harmonic generation (SHG) have proven to be a sensitive probe of buried interfaces. Recently SHG has been employed to monitor the surface of Si nanocrystals within a glass matrix. Due to the macroscopic homogeneity and centrosymmetry of the composite, its bulk SHG signal is produced by the inhomogeneities of the fundamental light beam, and can be enhanced several orders of magnitude by employing two crossed beams [1]. On the other hand, the surface contribution to the SHG of the composite is due to the relatively large inhomogeneities of the surface local field which act on the nanoparticles, and is therefore insensitive to the beam profile and is not enhanced in a two beam geometry. We calculate the SHG from a thin nanocomposite material and compare the relative strength of its surface and bulk contributions when illuminated with one and for two fundamental beams. We employ our results to analyze recent experiments in which the large contrast between the signal produced by the composite and the matrix in the one beam geometry was lost in the two beam geometry [1].\\{} [1]Figliozzi et al., submitted to Phys. Rev. Lett.   

Measurement of the separation dependence of the resonant energy transfer between CdSe nanocrystals

We have developed an apparatus to study the separation dependence of the interaction between quantum dots (QD). Our measurement scheme is based on depositing isolated QDs on the flat surface of a solid immersion lens (SIL). The photoluminescence (PL) of these dots (around 615 nm) is collected by the SIL and spectroscopically analyzed. The most novel part of our work resides in exciting these QDs by means of resonant energy transfer from smaller ones (emission at 590 nm). The smaller QDs cover the apex of an aperture probe near-field scanning optical microscope, after dipping it on a colloidal suspension. The combination of spectral and positional filtering allows us to measure the interaction between only one of the smaller dots and one of the larger dots at a time. From the analysis of the PL signal as a function of $z $(separation between two QDs), we expect to obtain what part of the energy transfer is dipole induced (F\"{o}rster interaction), and what part is associated with higher order terms (dipole-quadrupole and quadrupole-quadrupole interactions). Results on the progress of resonant energy transfer will be shown. These results improve our knowledge of the QD's wave function and understanding of decoherence phenomena in QDs   

Relaxation dynamics of excitons in a bimodal distribution of CdSe/ZnSSe quantum dots

We have studied the dynamical behavior of excitons in a bimodal distribution of CdSe/ZnSSe quantum dots by intensity dependent, temperature dependent and time resolved PL. The effect of exciton localization is investigated and described, both theoretically and experimentally by identifying transfer mechanisms due to thermalization and redistribution of excitons. We observe a dominant exciton emission from smaller dots (QDs1) and weaker emission from wider dots (QDs2) at 10 K and at low optical excitation. At high excitation densities a CdSe precursor state becomes visible at the high energy side of the QDs1 emission. Temperature dependent PL studies reveal a thermally activated exciton transfer between the different types of QDs resulting in a dominant QDs2 emission above 60 K. Time resolved PL measurements allow to estimate the characteristic radiative and non-radiative decay rates as well as the transfer rate of excitons between different QDs. The experimentally observed PL is successfully reproduced by a coupled rate equation model.   

Multi-Configuration Time-Dependent Hartree-Fock theory applied to quantum dots

The Multi-Configuration Time-Dependent Hartree-Fock method (MCTDHF) is a tool that can be used to study the electron dynamics of several multielectronic systems under strong laser fields. It is based on a variational principle applied to the time-dependent Schrodinger equation, where the many-particle ansatz wavefunction is a sum over configurations, with both coefficients and single-particle functions being time-dependent and optimized at each time step. In this way, electron correlation is taken into account and a faster convergence within a smaller expansion is obtained when compared to other mean-field theories. This method yields two sets of coupled non-linear differential equations to be solved by a certain integration scheme. These so-called working equations are propagated from a given initial state, yielding the wavefunction under the influence of the external field and allowing the study of the multielectronic dynamics under the laser pulse. As a first application of the MCTDHF theory, we analyze the multielectronic dynamics of isolated and coupled harmonic gated quantum dots driven by an intense laser field.   

The universal Hamiltonian of chaotic quantum dots in the presence of spin-orbit interaction

In a chaotic dot with a large number of electrons only a few interaction terms survive and constitute the interacting part of the so-called universal Hamiltonian. The universal Hamiltonian was derived in the absence of spin-orbit scattering. The presence of spin-orbit scattering in chaotic (or diffusive) quantum dots was shown to introduce new symmetry limits of the single-particle Hamiltonian [1]. We derive the universal Hamiltonian (i.e., including interaction terms) in the presence of spin-orbit scattering in these new symmetry limits, both in the absence and presence of an orbital magnetic field. We also derive closed expressions for the finite temperature conductance through such dots and use them to study the conductance peak height and peak spacing statistics. We identify interesting statistical signatures of the interplay between spin-orbit and exchange interactions. This work has been supported in part by the Department of Energy grant No. DE-FG-0291-ER-40608. [1] I.L. Aleiner and V.I. Fal'ko, Phys. Rev. Lett. {\bf 87}, 256801 (2001).   

g-tensor evaluation in self-assembled quantum dots

In solid state, the first term to be considered in the effective spin Hamiltonian is that representing the electronic Zeeman interaction. In a doublet state with $S=1/2$, the two levels will diverge linearly with the magnetic field ($B$), with slopes $\pm 1/2g\beta B$. In practice, the Zeeman interaction not depends only on the angle between the effective spin vector (\overrightarrow {S}) and $\overrightarrow{B}$ but depends also on the angle that $\overrightarrow{B}$ makes with certain axes defined by the sample symmetry. Taking into account this kind of anisotropy, the effective spin Hamiltonian is $\beta(\overrightarrow{B}\cdot\makebox[0.1cm][l]{\raisebox{1ex} {$\leftrightarrow$}} g\cdot\overrightarrow{S}$), where $\makebox[0.1cm][l]{\raisebox{1ex}{$\leftrightarrow$}}g $ is the g-tensor. Since electrons can be individually trapped into quantum dots (QDs) in a controllable manner, they may represent a good candidate for the successfully implementation of spintronics into semiconductor heterostructures. In this work we realized magneto-capacitance spectroscopy (CV) in order to obtain the localization energies and the evolution of the Zeeman splitting for the s and p electron confined levels in InAs self-assembled quantum dots (SAQDs). The CV experiments were performed at 2K using lock- in amplifiers at a frequency of 7.5KHz. An AC amplitude of 10 mV was superimposed on a varying DC bias ranging from -2 V to 0.5 V with a signal/noise ratio above $10^{4}$. Aligning $\overrightarrow{B}$ with different crystallographic directions, we measured the g-tensor showing the existence of a high anisotropy degree. The g-factor values obtained ranges between 1.9 and 0.7, with $\overrightarrow{B}\parallel001$ and $\overrightarrow{B}\parallel110$ respectively.   

Engineering the g-factor in coupled quantum dots

For applications in spintronics, it is fundamental to control the electron spin within the nanostructures. In this work we propose to use the spin-orbit effect to modulate the g-factor of one electron in coupled quantum dots. Changes in the orbital motion, as obtained by modifying the geometry of the dots, allow to modulate the spin components. The band structure is determined by expanding the wave function in a basis obtained by the Finite Element Method. We will discussed the interplay among the magnetic field and the Rashba and Dresselhaus terms included in the Hamiltonian. We have found that the spin component along the magnetic field can show very strong effects when the coupling between dots is changed or asymmetric dots are used. Results for different semiconductors and the role of Coulomb interaction will also be presented.   

Tuning the g-factor in self assemble quantum dots

The knowledge of electron and hole {\it g}-factors, their control and engineering are key for usage of spin degree of freedom for information processing in solid state systems. The electronic {\it g}-factor will be materials dependent, the effect being larger for materials with large spin-orbit coupling. Since electrons can be individually trapped into quantum dots in a controllable manner, they may represent a good platform for the implementation of quantum information processing devices. In this work we explore the effect of a stress on the {\it g}-factor for the electrons trapped in Self- Assembled Quantum Dots (SAQD) in two different samples. The experiments consist on a magneto-capacitance spectroscopy (CV) performed at low temperature (2K) where the direction of magnetic field, as well as the intensity, can be changed. We demonstrated that {\it g}-factor can be increased by as much as fifteen percent. Finally it is also shown that one can achieve the {\it g}-factor assessment, and engineering in SAQDs in a controllable manner.   

Interband magneto-optical transitions between bound and delocalized states in quantum dots

Semiconductor quantum dots are frequently described as artificial atoms. This description implies that the quantum states and exciton transitions are well isolated from their environment. However, since the dots are embedded in a semiconductor material and furthermore sit on top of an underlying wetting layer quantum well, there exists transitions involving the quantum dot states and the delocalized states in the dot's surrounding environment.\footnote{A. Vasanelli {\em et al.}, Phys. Rev. Lett. {\bf 89},216804 (2002).} We present the results of low temperature (4K) interband photoluminescence excitation measurements on an ensemble of InAs/GaAs dots under strong magnetic field (up to 28T) applied along the growth axis. Along with the expected p-p excited state transitions, we observe several other pics in the PLE spectrums. We attribute these pics to crossed transitions between the wetting layer and the discrete states of the quantum dot. A theoretical model is proposed to describe the above transitions.   

Session U18: Focus Session: Wide Band Gap Semiconductors V

Temperature dependence of the A, B, and C excitons in ZnO over 5-400 K: A modulated reflectivity study.

Optical properties of ZnO, a wide gap semiconductor with wurtzite structure, have generated renewed interest in the material in the context of opto-electronic phenomena and applications. The A, B, and C excitons of ZnO, arising from the combined effects of crystal field and spin-orbit splittings of the valence band, are investigated in the temperature range 5- 400 K, exploiting electro-, photo-, and wavelength-modulated reflectivity. The specimens studied have natural isotopic composition. The temperature dependence of the A, B, and C excitonic band gaps, fitted with a two harmonic oscillator model\footnote{M. Cardona, Phys. Status. Solidi b \textbf{220}, 5 (2000); R. P\"{a}ssler, J. Appl. Phys. \textbf{89}, 6235 (2001)} following Manj\'{o}n \emph{et al.}\footnote{F. J. Manj\'{o}n \emph{et al.}, Solid State Commun. \textbf{128}, 35 (2003)}, yields the magnitudes of the zero-point renormalizations 262 meV (A), 227 meV (B), and 249 meV (C), respectively. Isotopically controlled ZnO is currently being investigated to determine the isotopic mass dependence of the zero-point renormalizations.   

Nature of Room-temperature Photoluminescence in ZnO

The temperature dependence of the photoluminescence (PL) transitions associated with various excitons and their phonon replicas in high-purity bulk ZnO has been studied at temperatures from 12 K to above room temperature (320 K). The evolution of the emission lines with temperature allows us to unambiguously study the PL process in ZnO and to elucidate its nature at room temperature. The room-temperature PL in ZnO is shown to be a free-exciton annihilation process. The process is dominated by the simultaneous emission of photons and longitudinal optical phonons with E$_{FX}$-1LO emission at the maximum due to the strong exciton-phonon coupling in the material. The results explain the discrepancy between the transition energy of free exciton determined by reflection measurement and the peak position obtained by PL measurement.   

Electronic Structure of BGaP Alloys

We present the results of an {\em ab initio} investigation of zinc-blende BGaP alloys in the local density approximation. BP is a wide-band gap semiconductor with promising electronic and structural characteristics for potential optical and electronic applications. With an indirect gap of 2.0 eV, it has been considered as an optical window for silicon photoelectrochemical cells and has also been recently investigated as a thin buffer layer for the epitaxial growth of cubic GaN on silicon substrates. The use of boron, a first row element with deep atomic potentials and no $p$ core electrons in III-V compounds results in signficiant differences in electronic structure. For example, the $\Gamma$ conduction band minimum for BP is $p$- like ($\Gamma_{15c}$) in contrast to the $s$-like ($\Gamma_{1c}$) minimum typical of other III-V compounds such as GaP. We discuss the effect of this difference on the band structure of BGaP alloys and determine the bowing parameters for band gaps.   

Quantum-well depth of cubic “single stacking fault” inclusions in 4H-SiC p-i-n diodes determined by Ballistic Electron Emission Microscopy

Current-induced single stacking-fault (SF) cubic inclusions formed in (1 1 --2 0) oriented 4H-SiC $p-i-n$ diodes were exposed in cross-section by polishing down to the intrinsic layer. Surprisingly non-leaky Schottky barrier (SB) Pt contacts were made on the polished surface, and were investigated by nm-resolution Ballistic Electron Emission Microscopy (BEEM) [1]. Enhanced BEEM current and a $\sim $0.25 eV lower SB height was observed over single SF inclusions, directly confirming they act as $\sim $0.5 nm wide quantum wells (QWs) and support propagating 2D electronic states. This indicates the QW conduction band minimum is $\sim $0.25 eV lower than the 4H-SiC host, consistent with calculations and much shallower than the $\sim $0.53 eV depth of double SF inclusions [1]. We also found that the BEEM amplitude (but not the SB height) is extremely sensitive to polishing scratches, likely due to hot-electron scattering from sub-surface defects. Work supported by ONR and NSF. [1] Yi Ding \textit{et al}., Phys. Rev. B \textbf{69}, 041305 (2004)   

Polarization Properties of III-Nitride Blue and UV Light-Emitting Diodes

Polarization resolved electroluminescence (EL) studies of III-nitride blue and ultraviolet (UV) light emitting diodes (LEDs) were performed. The LEDs were fabricated on nitride materials grown by metalorganic chemical vapor deposition (MOCVD) on sapphire substrates (0001). Transverse electric (TE) polarization dominates in the InGaN/GaN quantum well (QW) blue LEDs ($\lambda $ = 458 nm), whereas transverse magnetic (TM) polarization is dominant in the AlInGaN QW UV LEDs ($\lambda $ = 333 nm). For the case of edge emission in blue LEDs, a ratio ($r=I_\bot /I_{//} )$ of about 1.8:1 was observed between the EL intensities with polarization (TE mode) and (TM mode), which corresponds to a degree of polarization $\sim $ 0.29. The UV LEDs exhibit a ratio $r $of about 1:2.3, corresponding to a degree of polarization $\sim $ -0.4. This is due to the fact that the degree of polarization of the band edge emission of the Al$_{x}$In$_{y}$Ga$_{1-x-y}$N active layer changes with Al concentration. The low emission efficiency of nitride UV LEDs is partly related to this polarization property. Possible consequences and ways to enhance UV emitter performances related to this unique polarization property are discussed. Effects of photonic crystal incorporation on the polarization properties will also be discussed.   

Thermal Studies of Operating AlGaN/GaN/SiC Based High Electron Mobility Transistors

AlGaN/GaN structures grown on SiC substrates can be used to create high power, high electron mobility transistors by exploiting the peizo-electric effect. These materials are highly desirable for their high thermal conductivity, high bandgap, and slow degradation. We have performed Raman and AFM- based thermal measurements on operating and non-operating devices in order to study thermal defects. In addition to measuring temperatures at a sub-micron scale, we find that even structures with excellent electronic and optical properties display sub-micron sized thermal defects, resulting in lowered thermal conductivity for the structures. Finite element analysis was used to better understand the thermal flows and experimental results.   

Growth and Fabrication of III-Nitride Deep Ultraviolet Emitters

In recent years, there has been a great effort to develop AlGaN based compact deep ultraviolet (UV) light-emitting diodes (LEDs) ($\lambda <$ 300 nm) for applications such as bio-chemical agent detection and medical research/health care. To obtain deep UV emission with $\lambda <$ 300 nm, AlGaN quantum well (QW) based LED structures require an active layer with Al composition higher than 40{\%}. As a result, the alloy composition for p- and n-cladding layers should be more than that of the active layer. The high Al composition introduces dislocations and leads to poor p- and n-type conductivity in the cladding layers, which limits current injection. We report here on the epitaxial growth of deep UV LEDs with operating wavelengths ranging from 300 nm to 270 nm by metal-organic chemical vapor deposition (MOCVD). Our UV LED structure was deposited on AlN/sapphire templates. We have achieved deep UV LEDs with an output power of 1.4 mW at 350 mA dc driving at 280 nm. The use of AlN epilayers as templates to reduce the dislocation density and enhance the LED performance will be discussed. Different device architectures for enhanced LED performances will also be presented.   

Nitride Deep Ultraviolet Light-Emitting Diodes with Microlens Array

We report on the fabrication of 285 nm AlGaN-based deep ultraviolet light-emitting diodes (UV LEDs) on sapphire substrates with integrated microlens array. Microlenses with a diameter of 12 microns were fabricated on the sapphire substrate by resist thermal reflow and plasma dry etching. LED devices were flip-chip bonded on high thermal conductive AlN ceramic submounts to improve the thermal dissipation, and the emitted UV light was extracted through the sapphire substrates. With the integrated microlense array, a 55{\%} enhancement in the output power at 20 mA DC driving was achieved compared with the same LED without microlens. The light extraction enhancement is the result of the reduced internal reflections of the light caused by the microlense surface profile. An output power of 0.22 mW at 20 mA was measured for a circular LED with a diameter of 275 microns.   

Direct Measurement of Curvature Dependent Ion Etching of GaN

The formation of ion induced nanoscale patterns, such as ripple or dimples, can be described using a continuum equation including a surface roughening term (curvature-dependent sputtering or asymmetric attachment of mobile adatoms/defects), a surface smoothing term (thermal or/and ion induced diffusion), and noise term. By measuring the roughening coefficient of the continuum equation, we have found the Ehrlich-Schwoebel length is 5.2 nm and the step-edge barrier is 1.2 eV at 733K. A Kaufman ion source was used to supply sub-keV ions from glancing incidence. The surface morphology was examined using RHEED in situ and AFM afterwards. The samples were rotated at 1.2 rpm to preclude ripple generation. Dimple structures with diameters ranging from 30 nm to 800 nm, have been produced using both argon ions and nitrogen ions with energies ranging from 60 - 1200 eV at an ion flux of 3.6 ions s$^{-1}$nm$^{-2}$. Using both RHEED and AFM we measure a minimum in the local roughness near 320 C. From the evolution of the dimple dimensions we obtain the first direct measurement of the curvature driven roughening, and the roughening coefficient is measured to be 43.2 nm$^2$/s. The activation energy for surface relaxation has been measured to be 0.11~eV. \\[1ex] Partially supported by the AFOSR   

Spectroscopy and Modeling of Carrier Dynamics in Al$_x$Ga$_{1-x}$N Alloys with Compositional Inhomogeneity

AlGaN samples grown by plasma-assisted molecular beam-epitaxy on sapphire (0001) substrates, with 20-50{\%} Al content, show intense room-temperature photoluminescence (PL) that is significantly red-shifted from band edge. Low temperature PL shows two distinct peaks: a very bright red-shifted (RS) peak that decreases by a factor of $\sim $7 as the temperature is increased from 10 to 300 K, and a feature associated with band edge (BE) emission that rapidly decreases by greater than 3 orders of magnitude from 10 K to 300 K. Room-temperature monochromatic cathodoluminescence images at the RS peak reveal spatially nonuniform emission. Time resolved (TR) PL data for the BE emission peak show a prompt response at $t$~=~0 and an intensity dependent decay that saturates at higher pump intensity. TRPL data for the RS peak, however, show intensity dependent initial rise times with slow and nearly intensity independent decay times. From fitting to a phenomenological model, the density of the localized states associated with the RS feature, the carrier life times in the BE and the RS states, and the transfer time between the two states are determined.   

Deep UV time resolved optical studies of excitonic transitions in AlN epilayers

Optical properties of excitonic transitions in AlN epilayers have been studied. AlN epilayers were grown by metalorganic chemical-vapor deposition (MOCVD) and the optical properties were probed by deep ultraviolet (UV) time-resolved photoluminescence (PL) spectroscopy. Binding energies and lifetimes of the free-exciton (FX), neutral acceptor-bound exciton (I$_{1})$, and neutral donor-bound exciton (I$_{2})$ transitions have been measured. We found that the undoped AlN epilayer exhibits a strong band-edge emission line at 6.06 eV due to FX transition at 10 K. A PL emission line at 6.02 eV has been observed at 10 K in Mg-doped AlN, which is about 40 meV below the FX transition in undoped AlN epilayer. This transition has been assigned to the recombination of an exciton bound to neutral Mg acceptor (I$_{1})$ with a binding energy of, E$_{bx }$= 40 meV. The recombination lifetime of the I$_{1}$ transition in Mg doped AlN has been measured to be 130 ps. PL studies on Si-doped AlN have found that the I$_{2}$ transition (E$_{bx }$= 16 meV) with a recombination lifetime of 80 ps to be dominant transition at low temperatures. Our experimental study reveals a free-exciton binding energy of 80 meV in AlN, which is the largest exciton binding energy ever reported in semiconductors.   

Resonant shake-up satellites in photoemission at the Ga 3p photothreshold in GaN

Photoemission spectra recorded near the Ga 3$p$ photothreshold from both thin film and single crystal GaN have been found to contain shake-up satellites of the main Ga 3$d$ emission line. The intensity of these satellites resonates at this threshold, and the satellites are associated with a 3$d^{8}$ state. The correlation energies and binding energies for the satellite multiplet have been measured for the satellite and related Auger transitions. The satellite multiplet contains additional constant binding energy features not observed in previous studies of other Ga compounds. The present results are compared to published data for GaP and GaAs, as well as to our preliminary results for thin film In$_{x}$Ga$_{1-x}$N and Al$_{x}$Ga$_{1-x}$N. Our results will be discussed in the context of the degree of correlation and magnitude of the respective band gaps in these materials. \newline   

Resonance Raman Scattering in InGaN

Resonance Raman studies of single phase In$_{1-x}$Ga$_{x}$N epitaxial films with 0 $<$ x $<$ 0.63 and free electron concentrations in the 10$^{18}$ cm$^{-3}$ range are presented. The A$_{1}$(LO) phonon scattering intensity is enhanced for excitation above the direct band gaps of the films. Examination of films with direct band gaps between 0.7 and 1.9 eV and laser energies from 1.96 to 2.71 eV show that the resonance is broad, extending to up to 2 eV above the direct gap. Multiphonon Raman scattering of the LO phonon up to {\em n} = 5 is also observed in alloy samples. Coupling of the electron plasmon to the LO phonon to form a longitudinal plasmon coupled (LOPC) mode, which is observed in the Raman spectra of n-type GaN, is not observed in In$_{1-x}$Ga$_{x}$N for x $<$ 0.15. These experimental results will be discussed in terms of the electron-phonon interaction in InGaN.   

Molten Salt Based Growth of GaN for Native Substrates

The issue of material quality in III-nitrides is nearly ubiquitous in the field owing to the lack of a lattice-matched substrate on which to epitaxially grow thin films for LEDs. Conventional approaches to growth of bulk gallium nitride material require overpressures ranging from 4,000 to 45,000 atmospheres, and the kinetics of such processes is slow and difficult to scale. This work will describe and demonstrate proof of the underlying concepts for a process that may be capable of growing large-area bulk gallium nitride at fast growth rates, reasonable temperatures (450-950ûC), and 1 atm. This method circumvents the typical challenges associated with growth of bulk nitride material by precipitating from a molten salt, which provides the perfect host environment for the reactive nitride anion. Furthermore, molten salts can also function as an electrolyte, permitting electrochemical pre-growth purification processes in addition to several possible methods to electrochemically enhance the growth process, providing further control over the quality of the precipitate. We have recently demonstrated molten salt-based growth of wurtzite GaN crystals as large as 0.9 mm long by 0.6 mm wide in as little as two hours using these techniques. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy under contract DE-AC04-94AL85000.   

Electronic Structure Studies of Silicon Carbide Cationic Nanoclusters

As a continuation of our studies on the high stabilities and associated electronic structure properties of Si$_{8}$C$_{2 }$to Si$_{14}$C$_{2}$ and Si$_{20}$C$_{n }$(n=3-6) clusters,$^{1}$ we report here detailed \textit{ab initio} electronic and geometric structure studies of small $Si_m C_n^+ $(1\underline {$<$ }m, n \underline {$<$} 4) cationic clusters. The theoretical formalism used is the local density approximation (LDA) to density functional theory (DFT) and the \textit{Gaussian03} suite of programs$^{2}$ with an all electron 6-311++G** basis set has been used. Complete geometry optimizations of different possible structures have been carried out. The stability of the clusters varies with the ratio of the number of silicon to carbon atoms in the cluster. In contrast to the neutral clusters,$^{3}$ cationic clusters appear to prefer more open structures. Results will be presented for binding energies, relative energies, fragmentation energies, vibrational frequencies, and adiabatic ionization potentials$^{3}$ for the optimized clusters. Detailed comparisons with published data in the literature will also be presented. * Work supported, in part, by the Welch Foundation, Houston, Texas (Grant No. Y-1525) $^{1}$M. N. Huda and A. K. Ray, Phys. Rev. A (R) \textbf{69}, 011201 (2004); Eur. Phys. J. D \textbf{31}, 63 (2004). $^{2}$ \textit{Gaussian03}, Revision A.1, M. J. Frisch \textit{et al.,} Gaussian Inc., Pittsburgh, PA , 2003. $^{3 }$P. Pradhan and A. K. Ray, J. Mol. Structure (Theochem), in press.   

Electronic Structure Studies of Silicon Carbide Anionic Nanoclusters

As a continuation of our studies on the high stabilities and electronic structure properties of Si$_{8}$C$_{2 }$to Si$_{14}$C$_{2}$ clusters and Si$_{60}$C$_{n }$(n=3-6) clusters,$^{1}$ we report here \textit{ab initio} studies of small $Si_m C_n^- $(1\underline {$<$ }m, n \underline {$<$} 4) anionic clusters. The theoretical formalism used is the local density approximation (LDA) to density functional theory (DFT) and the \textit{Gaussian03} suite of programs$^{2}$ with an all electron 6-311++G** basis set has been used. Complete geometry optimizations of different possible structures have been carried out. Carbon-rich and silicon rich species show distinctly different patterns with respect to the vertical detachment energies. For carbon-rich aggregates, the VDE's show an even odd alternation, similar to that of the carbon anions. We present results on binding energies, relative energies, fragmentation energies, vertical detachment energies, vibrational frequencies, and adiabatic electron affinities$^{3}$ for the optimized clusters. Detailed comparisons with published data in the literature will also be presented. * Work supported, in part, by the Welch Foundation, Houston, Texas (Grant No. Y-1525). $^{1}$M. N. Huda and A. K. Ray, Phys. Rev. A (R) \textbf{69}, 011201 (2004); Eur. Phys. J. D \textbf{31}, 63 (2004). $^{2}$ \textit{Gaussian03}, Revision A.1, M. J. Frisch \textit{et al.,} Gaussian Inc., Pittsburgh, PA , 2003. $^{3 }$P. Pradhan and A. K. Ray, J. Mol. Structure (Theochem), in press.   

Session U19: Fractional QHE/Composite Fermions

Korringa-Like Nuclear Spin-Lattice Relaxation in a 2DES at $\nu = 1/2$

Via a resistively-detected NMR technique, the nuclear spin lattice relaxation time $T_{1}$ of $^{71}$Ga at low temperatures has been measured in a GaAs/AlGaAs heterostructure containing two weakly-coupled 2D electron systems (2DES), each at Landau level filling $\nu = 1/2$. Incomplete electronic spin polarization, which has been reported previously [1,2] for low density 2DESs at $\nu = 1/2$, should facilitate hyperfine- coupled nuclear spin relaxation owing to the presence of both electron spin states at the Fermi level. Within composite fermion theory, a Korringa law temperature dependence: $T_{1}T = constant$, is expected for temperatures $T<1$ K. Our measurements made at temperatures in the range 35 mK $ [Preview Abstract]

Evidence of Composite Fermion interactions in the Fermi sea at $\nu\rightarrow 1/2$

Interactions between composite fermions with two attached flux quanta ($^2$CFs) are explored at filling factors of the fractional quantum Hall effect $\nu\rightarrow 1/2$. Low-lying ($\omega < $1meV) spin flip excitations modes, in which spin orientation and Landau level index of composite fermions change simultaneously, are measured in resonant inelastic light scattering experiments. The measurements uncover a delicate balance between spin reversal and Fermi energies in the Fermi sea of composite fermions that emerges in the limit of $\nu\rightarrow$1/2. A collapse of the spin-flip excitation gap as $\nu\rightarrow$1/2 is linked with vanishing quasiparticle energy level spacings and loss of full spin polarization. This work is supported by the NSF under Award Number DMR-03-52738 and by the DoE award DE-AIO2-04ER46133. It is also supported by a research grant of the W. M. Keck Foundation.   

Fermi Liquid Behavior and the Ground State at Half-Filled Landau Levels

Experiments show an unquantized quantum Hall state at $\nu=1/2$---widely believed to be a Fermi-liquid state of composite fermions. In contrast, at $\nu=5/2$ a Hall liquid is seen, which numerical calculations have shown to be a p-wave paired state of composite fermions.In the third and higher Landau levels transport becomes anisotropic indicating the development of stripe-type order. All three states may be associated with the existence of a Fermi surface. We construct many-particle wavefunctions with periodic boundary conditions and cast them as Slater-determinants of composite fermion plane waves. We obtain the variational energies of the ground state and a number of low lying excited states near the Fermi surface by quantum Monte Carlo simulations for up to 100 electrons. From this and the K-space translational invariance of the Fermi states we extract the effective mass and the first few Fermi liquid parameters. We compare the Fermi liquid parameters, discuss the stability of the isotropic Fermi surfaces, and explore anisotropic Fermi surfaces in these Landau levels.   

Resonant Enhancement of Inelastic Light Scattering in the Fractional Quantum Hall Regime at $\nu=1/3$

Strong resonant enhancement of the inelastic light scattering cross-section is essential in obtaining the sensitivity required to observe inter-Landau level and the intra-Landau level charge and spin density excitations in fractional quantum Hall liquids. We find that at $\nu = 1/3$ the energies of the sharp peaks in the resonant enhancement profiles of inelastic light scattering intensities coincide with the energies of optical excitations measured in photoluminescence, which recently have been assigned to negatively charged excitons. To interpret the observed enhancement profiles, we propose light scattering mechanisms in which the intermediate resonant transitions are to states with charged excitonic excitations. This work is supported by the NSF under Award Number DMR-03- 52738 and by the DoE award DE-AIO2-04ER46133. It is also supported by a research grant of the W. M. Keck Foundation. *Present address: IBM-Almaden Research Center   

Insulating phases in the limit of strong Landau level mixing

We have explored the electronic phases of 2D holes subjected to perpendicular magnetic fields in the new regime of very low densities. The sample density is $1.6 \times 10^{10}$cm$^{-2}$ and it is tunable with a backgate. At the highest densities, beside the $\nu=1/3$, 2/5, and 2/3 fractional quantum Hall states, we observe both of the previously reported high field insulating and reentrant insulating phases. Similarly to the result in higher density samples, the reentrant insulating phase strengthens as the density is lowered. With a further decrease in density, however, the reentrant insulator unexpectedly weakens then it completely disappears. Since both of the insulating phases have been interpreted as electronic solids, the behavior observed can be regarded as a melting of the solid with decreasing density. Such a melting is at odds with expectations and we think that it reflects the influence of the strong Landau level mixing on the quantum fluctuations of the nodes of the solid.   

Observation of fractional statistics

Our present experiment utilizes a novel Laughlin quasiparticle interferometer, where a quasiparticle with charge $e/3$ of the $f=1/3$ FQH fluid executes a closed path around an island of the $f=2/5$ fluid. The interference fringes are observed as peaks in conductance as a function of the magnetic flux $\Phi $ through the $f=2/5$ island, in a kind of the Aharonov-Bohm effect. A similar situation of resonant tunneling in an FQH fluid at filling $f_1 $ surrounding an FHQ island at a different filling $f_2 $ was considered theoretically by Jain et. al.. We observe the interference pattern shift by one fringe upon introduction of five magnetic flux quanta into the $f=2/5$ island, i.e., the Aharonov-Bohm period $\Delta \Phi =5h/e,$ corresponding to excitation of ten $q=e/5$ quasiparticles of the $f=2/5$ fluid. Such ``superperiod'' of $\Delta \Phi >h/e$ has never been reported before. This $\Delta Q=2e$ charge period is directly confirmed in calibrated backgate experiments. These observations imply \textit{relative} statistics of $\Theta _{2/5}^{1/3} =-1/15$, when a charge $e/3$, statistics $\Theta _{1/3}^ =2/3$ Laughlin quasiparticle encircles one $e/5$, $\Theta _{2/5}^ =2/5$ quasiparticle of the $f=2/5$ fluid.   

Spin excitations at $\nu >1/3$: a probe of composite fermions interactions

We present a resonant inelastic light scattering study of spin excitations at filling factors of the fractional quantum Hall effect (FQHE) with $\nu >1/3$, in which composite fermions may condense into higher order quasiparticles. Observations of low lying spin excitations enable us to study the composite fermions (CF) Landau level configuration and the impact of CF residual interactions at filling factors away from the major FQHE sequences. A very low energy spin mode ($\omega\leq0.1~meV$), which displays a marked temperature dependence above T$\sim$100~mK, emerges at filling factors close to $\nu=4/11$ and remains with small changes in energy in the filling factor range $4/11\leq\nu\leq5/13$. The spectral intensity of this spin excitation becomes negligible at temperatures above 300~mK. The marked temperature dependence of the intensity suggest the existence of even lower excitation modes at energies well below 0.1~meV. We discuss the implication of these experiments on the possibility of higher order CF suggested by transport measurements at $\nu=$4/11. \\ This work is supported by the NSF under Award Number DMR-03-52738 and by the DoE award DE-AIO2-04ER46133. It is also supported by a research grant of the W. M. Keck Foundation.   

Generalized Clustered Quantum Hall States

The Read-Rezayi (parafermion) quantum Hall states[1] for bosons can be defined as states where the wavefunction does not vanish when $g$ bosons come together to the same point, but does vanish as $z^2$ as a $g+1$st particle approaches that point. These states can equivalently be defined as the unique ground state of a point contact $g+1$ particle interaction Hamiltonian. Interestingly, the series of Read-Rezayi states appears to describe well the groundstates of rotating Bose condensates with point-contact two body interactions at a series of filling fractions [2]. If one attaches a Jastrow factor to such bose wavefunctions, one obtains fermion wavefunctions that may occur in electronic quantum Hall systems including the ($g=2$) Pfaffian [3] and the ($g=3$) $\nu=13/5$ Read-Rezayi state [1]. In this work, we consider generalized cluster wavefunctions defined by the algebraic manner in which a wavefunction vanishes as $g+1$ particles coalesce. We find Hamiltonians that generate these wavefunctions as their exact ground state. Among this series of states is the previously studied Haffnian wavefunction[4] and a host of states not previously discussed. We catalogue and study the new states and discuss whether any of them might occur in actual physical systems. [1] N. Read and E. Rezayi, PRB{\bf 59}, 8084 (1999). [2] N. R. Cooper, N. K. Wilkin, and J. M. F. Gunn, PRL{\bf 87}, 120405 (2001) [3] G. Moore and N. Read, Nuc. Phys. B{\bf 360}, 362 (1991). [4] D. Green, PhD Thesis.   

Off-diagonal long-range order in the fractional quantum Hall effect

It is generally accepted that the fundamental physics of the fractional quantum Hall effect lies in the topological binding of quantized vortices and electrons. From a microscopic point of view, however, the non-Pauli vortices are not strictly bound to electrons in realistic ground state wave functions. We study the Girvin-MacDonald off-diagonal long-range order at Landau level fillings $\nu=1/m$ ($m$ odd) for bosonic wave functions obtained from fermionic fractional Hall wave functions by a singular gauge transformation. In order to test the robustness of the concept, we work with accurate representations of the Coulomb ground state, constructed using the framework of the composite-fermion theory, and find strong evidence that the exponent describing its long-distance algebraic decay has a universal value $m/2$ independent of the form of the wave function. We interpret this to mean that the topological notion of electron-vortex binding remains generally well defined as a long-distance property.   

Re-emergence of the electron in a fractional quantum Hall fluid

The low energy physics of the fractional quantum Hall fluid is described in terms of fermionic quasiparticles called composite fermions, which are distinct from electrons. We show that a long lived electron- like quasiparticle also exists in the excitation spectrum. Specifically, we find, using wave functions that are demonstrably very accurate for small systems, that there is a non-zero overlap (in the thermodynamic limit) between the state obtained by application of an electron creation operator to a fractional quantum Hall ground state and a high energy bound state complex containing an odd number of composite-fermion quasiparticles. The electron annihilation operator similarly couples to a bound complex of composite-fermion holes. We predict that these bound states can be observed through a conductance resonance in experiments involving a tunneling of an electron into a quantum Hall fluid.   

Collapse of Composite Fermion Spin-Flip Roton at Filling Factor Below $\nu=2/5$

Composite fermions (CF) consisting of an electron with attached even numbers of magnetic flux quanta are formed in the fractional quantum Hall regime. In this work resonant inelastic light scattering methods are used to study low energy excitations at electron Landau level filling factors close to $\nu=2/5$ and at cold finger temperatures below 40mK. Our measurements focus on low lying spin excitations in which quasiparticles change CF Landau level and reverse spin orientation. The evolution of the roton in the dispersion curve of spin-flip modes is measured as a function of changes in filling factors near $\nu<2/5$. These experiments probe CF properties in the range of filling factor close to $\nu=5/13$ in which the population of the excited CF Landau level is close to 2/3 and CF's that are composed of electrons with two attached flux quanta are expected to condense into higher order quasiparticles. This work is supported by the NSF under Award Number DMR-03- 52738 and by the DoE award DE-AIO2-04ER46133. It is also supported by a research grant of the W. M. Keck Foundation.   

Measuring Fractional Charge and Statistics in fractional quantum Hall Jain States

We study quantum noise in a multiple lead setup of a 2DEG in the FQH regime as a means of capturing clear signatures of fractional statistics and fractional charge in Jain states. Quasiparticles in FQH systems are predicted to have fractional charge and statistics. While the fractional charge for Laughlin states was observed recently through shot noise measurements, and found to be consistent with predictions, a clear signature of fractional statistics has yet to be seen. Here we propose an experiment involving the tunneling of quasiparticles in a multiple lead setup, that can unmistakably capture fractional statistics . Of particular interest are non-Laughlin states since in these states charge $Q$ and statistical angle $\theta$ are different fractions (e.g., $\nu=2/5$, $Q=e/5$, $\theta=3\pi/5$) and in principle, one should be able to tell how each contributes to measured correlations of tunneling currents between different leads. By analyzing finite-temperature cross correlations between tunneling currents, we find more boson-like (bunching) or more fermion-like (anti-bunching) statistical behavior of quasi particles depending on their filling fraction $\nu$ and related statistical angle $\theta$. Furthermore we are able to distinguish contributions coming from the fractional charge and fractional statistics.   

Coulomb Correlations in Landau Levels: Novel Squeezed States and Optical Transitions

We report on a novel theoretical formalism developed for dealing with Coulomb interparticle interactions in composite charged complexes in Landau levels. The formalism is based on a unitary transformation of the exact interacting Hamiltonian and describes condensation of an infinite number of degrees of freedom into new squeezed vacuum and excited states. The squeezed states are characterized by built-in correlations between particles and form a complete basis of states, which is compatible with axial rotations and magnetic translations. We introduce and discuss novel and surprisingly simple exact optical selection rules that govern inter- and intra-band transitions of charged composite complexes in magnetic fields. We report on applications of the developed formalism to studying optical transitions and charged collective excitations in a magnetically quantized 2D electron gas.   

Session U20: High-K Dielectrics

Theory of structural and dielectric properties of amorphous high-K dielectrics

Hafnia (HfO$_2$) and zirconia (ZrO$_2$) are leading candidates for replacing SiO$_2$ as the gate insulator in CMOS technology. Amorphous versions of these materials ($a$-HfO$_2$ and $a$-ZrO$_2$) can be grown as metastable phases on top of a silicon buffer; while they tend to recrystallize during subsequent annealing steps, they would otherwise be of considerable interest because of the promise they hold for improved uniformity and electrical passivity. In this work, we report our studies of $a$-HfO$_2$ and $a$-ZrO$_2$ by first-principles density-functional methods. We construct realistic amorphous models by two different techniques: (i) a ``melt-and-quench'' molecular dynamics approach, and (ii) an ``activation-relaxation technique'' (ART). In both cases, the structural, vibrational, and dielectric properties of the resulting models are analyzed in detail. The overall average dielectric constant is computed and found to be comparable to that of the monoclinic phase. Although the ART technique (as implemented by us in the SIESTA package) allows a dramatic saving in computational time, it yields results of similar quality with respect to ab-initio MD simulations done with the VASP code. These techniques show promise for future modeling of high-K dielectric ultrathin films and interfaces   

Dielectric properties of high-k oxides: Theory and experiment for Lu2O3

The quest for a dielectric for the replacement of silica in Si-based devices has focused the attention of the scientific community on the class of high dielectric constant ($\kappa )$ insulators. In this work we unfold the physical origin of $\kappa $ and its direct connection with lattice dynamics and polarizability in these materials, analyzing the specific case of Lu$_{2}$O$_{3}$ in its ground-state bixbyite structure with a combined experimental and theoretical analysis. The vibrational dielectric function of Lu$_{2}$O$_{3}$ thin films grown by atomic-layer deposition was studied by infrared transmission and reflection-absorption spectroscopies, selectively accessing transverse and longitudinal optical frequencies. The static dielectric constant was extracted analyzing the infrared response. We also present first-principles density-functional linear-response calculations, which are in close agreement with experiment, and provide insight into the microscopic nature of vibrational spectra and dielectric properties.   

Novel Chalcogenide Buffer Layer for Oxide Heteroepitaxy on Si(001)

We have developed a novel chalcogenide buffer layer for heteroepitaxial growth of oxides on silicon and applied it to growth of anatase TiO$_2$. Anatase, nearly lattice-matched to Si(001), is of interest for both spintronic and high-K dielectric applications: it can be made ferromagnetic at room temperature by doping with Co, and has a very large dielectric constant. Through use of a sub-nanometer buffer layer, Ga$_2$Se$_3$ grown on As terminated Si(100), we have been able to grow anatase nanocrystals on Si(001) without any detectable substrate oxidation or silicide formation. The As termination prevents silicon-selenide formation, and the gallium selenide prevents substrate oxidation. The cubic structure of Ga$_2$Se$_3$ offers a stable face for TiO$_2$(001) growth. In addition, the Ga$_2$Se$_3$ layer, with a lattice constant between Si and TiO$_2$, has a structure with inherent vacancies that can absorb strain, acting as a strain relief layer for the TiO$_2$ on Si.   

THz Optical Response from Coupled Ferroelectric/LO Phonon Mode in BaTiO3/Si(100)

The large second-order polarizabilities of epitaxially grown ferroelectrics can be used to generate THz-bandwidth electric fields in semiconductors. Epitaxial films of BaTiO$_{3 }$have been grown on Si(100) using oxide-MBE. X-ray diffraction measurements indicate a high degree of structural perfection and an out-of-plane polarization direction. The coupling between BaTiO$_{3}$ and Si is investigated using an optical pump-probe transient reflectivity experiment that uses 10 fs pulses of light centered around $\lambda $=400 nm. The pump pulse excites the LO phonon mode of the Si substrate directly (15.5 THz) as well as the lower frequency soft modes (1-4 THz) of the ferroelectric film. The observed Fano lineshape, obtained by Fourier transforming the time-domain data, suggests a strong coupling between the LO phonon and ferroelectric modes. The photon energy dependence of the observed electro-optic response suggests that the electro-optic response is generated by the LO phonon and is mixed strongly with the ferroelectric response. The high bandwidth coupling of optical and electronic degrees of freedom is promising for silicon-based quantum computing applications. This work was supported by the NSF, MEXT, NIMS, and DARPA QuIST.   

Valence band offsets and interface structure of HfxSi1-xO2 films on Si(111) from photoemission spectroscopy

We have used synchrotron radiation to perform high resolution soft x-ray photoemission spectroscopy measurements on Si(111)/Hf$_{x}$Si$_{1-x}$O$_{2}$ films. Our samples included both thick ($\sim $ 75 {\AA}) and thin ($\sim $ 10 {\AA}) silicate films. All samples were grown by remote plasma enhanced chemical vapor deposition (RPECVD) at a temperature of 300 $^{\circ}$C using hafnium tert-butoxide and silane in a helium carrier gas. Si 2$p$ and Hf 4$f$ core levels were studied along with valence band spectra using photon energies of 64 and 150 eV. Measurements of the valence band edge were also unexpectedly high for the thick silicates, which may be attributable to bound charge in the form of defects in the films. The band offset parameter was estimated for the thin film compositions based on the Si $2p_{3/2}$ substrate peak position and its known binding energy with respect to the Si valence band edge (98.75 eV). Offsets for the thin film silicates ranged from 3.31 -- 3.9 eV, with an average value of 3.42 $\pm $ 0.03 for x $>$ 0.2. Core-level binding energies exhibit a nonlinear dependence with alloy composition in the thick silicate films, while the thin films show a linear dependence. A decrease in binding energy for both the Si 2$p_{3/2}$ [Si$^{4+}$] and Hf 4$f_{7/2}$ was observed as the composition changed from SiO$_{2}$ to HfO$_{2}$.   

Infrared absorption spectra at interfaces from first principles: Origin of LO and TO red shifts in ultrathin oxide films on silicon

The interpretation of the vibrational spectra of thin SiO$_2$ films on Si(100) has recently been challenged by the observation of a pronounced thickness-dependent red shift of the asymmetric oxygen stretching mode in Si--O--Si intertetrahedral bridges. The origin of this red shift has variably been ascribed to compressive strain of the interfacial oxide, to void incorporation, or to the presence of substoichiometric silica. We here clarify the origin of this red shift by using a density functional approach. For this purpose, we first introduce a formalism to calculate both the longitudinal and the transverse infrared absorption spectra of thin films on transparent substrates. Then, we extend our formulation in order to spatially map the absorption intensity across interfaces. When applying this method to a realistic model of the Si(100)-SiO$_2$ interface, we find that that the red shift arises from a softening of the stretching modes in the substoichiometric interfacial layer. The cumulative effect of an interfacial layer with reduced vibrational frequencies and of a stoichiometric oxide with bulk-like modes is shown to consistently explain the thickness-dependent shift observed in experiments.   

Thickness measurement of interfacial layer between HfO$_2$ film and Si substrate by Fourier analysis of x-ray reflectivity

Thickness of interfacial layers between Si-substrates and HfO$_2 $ films have been estimated by Fourier analysis of x-ray reflectivity. It is demonstrated that enhancement of the signals corresponding to the positions of low-density-contrast interfaces can be achieved through careful data processing in Fourier analysis. Details of the data processing procedures and comparison between results of the analysis and TEM measurements are presented.   

Thin single crystal Sc2O3 Films on Si (111) with very sharp interface

We report the MBE growth and single-crystal synchrotron x-ray characterization of very high quality Sc$_{2}$O$_{3}$ films grown on Si (111). The Sc$_{2}$O$_{3 }$films of 3.5 and 18 nm thickness were deposited by e-beam evaporation on Si in a multi-chamber MBE/UHV system. Streaky RHEED patterns with a 4x4 reconstruction along the in-plane axes of Si were observed after an oxide growth $\sim $1 nm in thickness. X-ray diffraction results find that Sc$_{2}$O$_{3}$ films grow epitaxially in Bixbyite structure with the axis orientations aligned with those of Si. The 3.5 nm film yields a mosaic scan width of 0.044$^{\circ}$ (158 arc-sec.) for the Sc$_{2}$O$_{3}$ (444) peak, which is remarkably sharp, and suggests a defect free structure for the epi-layer. Since the bulk lattice constants of Si and Sc$_{2}$O$_{3}$ are mismatched by 9.2{\%}, the observed perfection in the film structure is very unusual. Electrical measurements of the Sc$_{2}$O$_{3}$ film exhibit low-leakage currents and cross-section TEM studies on Sc$_{2}$O$_{3}$/Si show an extremely sharp interface.   

Electronic properties and band alignments of Hf-based gate dielectrics on silicon

Ultra thin Hf-based dielectrics are being considered for possibly replacing SiO$_2$ gate oxide in silicon based metal- oxide-semiconductor (MOS) transistors. In this work we investigated the electronic structure and band alignments of the Hf-based gate dielectrics on silicon, which dictate the device performance of transistors in the sub-90nm devices. The electronic structure of HfO$_2$/Si interface showed dangling bond states at the interface due to the reduced coordination of Hf caused by the intrinsic bond constraints at the HfO$_2$/Si interface. However, our calculations indicate that these dangling bonds could be passivated by hydrogen or oxygen, which can appropriately change the coordination numbers at the interface, thereby removing the dangling bond states. We also considered the interface of HfSiO$_4$/Si and found that there are no dangling bond states at the interface, making HfSiO$_4$ a promising interfacial layer to improve the interface quality. Band offsets at the interfaces with Si were calculated using density functional theory, and it showed that the band offsets vary depending upon the interface stoichiometry. Band alignments were also determined experimentally using XPS and were in excellent agreement with the theoretical results. Incorporation of nitrogen into the HfO$_2$ network resulted in notable changes in the valence band structure of the material and the corresponding band alignments with silicon and is found to depend on the nitrogen concentration in the bulk of HfO$_2$ as well as at the interface.   

High-K MISFET channel mobility from magnetoresistance

Carrier trapping in the gate insulator or at the interface with the silicon can depress the effective channel mobility of high-K MISFETs below the drift mobility. This reduction in effective mobility can be distinguished from true mobility reduction due to carrier scattering by using the Hall effect to measure the channel carrier density [1]. However, channel Hall measurements have traditionally required nonstandard multidrain devices, which must be included in the test chip design. We propose measuring the reduction in drain current of conventional transistors by a magnetic field to determine the Hall coefficient. This method, which requires no multidrain devices, could become a routine diagnostic procedure. It is based on a theorem concerning the magnetoresistance of a rectangular plate with perfectly conducting end contacts [2], which has apparently not been tested experimentally, at least on MOSFET's. The validity of the method can be determined by comparison with channel carrier density determined in other ways, including split capacitance on MOSFETS, conventional Hall effect, and Corbino magnetoresistance on MISFETs. Progress toward these goals is described. [1] N.S Saks and A.K Agarwal, \textit{Appl. Phys. Letters} \textbf{77 }(20), 3281 -- 3283 (2000); R. T. Bate and W. P. Kirk, \textit{Bull. Am. Phys, Soc}. March, 2004, Abstract S6.011 [2] H. H. Jenson and H. Smith, \textit{J. Phys. C: Solid State}, \textbf{5,} 2867-2880, (1972)   

First-principles investigation of oxygen diffusion in compressively strained high-density silicon oxide

Oxidation of Si nanostructures is a key process in the fabrication of future Si-nano devices such as single electron transistors. It is well known that the oxidation is strongly affected by the initial structural size and shape, called pattern dependent oxidation. This characteristic can be intuitively understood by the oxidation retardation by the oxidation-induced strain: The diffusion of oxygen in the oxide is suppressed by the strained high-density oxide region generated in the nanostructures. However, the understanding of this phenomenon on atomic scale still remains unknown. In this work, we investigate microscopic mechanisms of oxygen diffusion in the strained high-density $\alpha$-quartz based on first-principles total-energy calculations. The calculations show that both incorporation of O$_2$ molecules into the oxide and its migration are significant factors of oxygen diffusion in the high-density $\alpha$-quartz. The calculated activation energy increases by 1.2 eV with a 10\% increase of density, indicating that the diffusion of oxygen can be suppressed by the high-density region.   

Size effect in self-trapped exciton photoluminescence from SiO2-based nanoscale materials

Direct evidence for a size effect in self-trapped exciton (STE) photoluminescence (PL) from SiO$_{2}$-based nanoscale materials as compared to bulk type-III fused silica is obtained. The PL was induced by the two-photon absorption of focused 6.4 eV ArF laser light with intensity $\sim $10$^{6}$ Wcm$^{-2}$ and measured in a time-resolved detection mode. Two models are applied to examine the blue shift of the STEPL band with decreasing size of nanometer-sized silica fragments. The first model is based on the quantum confinement effect on Mott-Wannier-type excitons developed for semiconductor nanoscale materials, which is completely unusable in the case of wide-band-gap nanoscale materials. The second model takes into account the laser heating of carriers due to collisions with the boundary of nanometer-sized silica fragments in the presence of an intense laser field. The laser heating of carriers in excess of the activation energy for the exciton self-trapping give rise to the extremely hot STE's. As a result, the blue shift of the STEPL band is originated from the activation of hot (phonon-assisted) electronic transitions.   

Dielectric function of annealed sub-stoichiometric silicon oxide layers

We present an analytical methodology, based on electron energy loss spectroscopy and energy filtered transmission electron microscopy, which allow us to quantify the clustered silicon concentration and the complex dielectric function of annealed sub-stoichiometric silicon oxide layers, deposited by plasma enhanced chemical vapor deposition (PECVD). To put in evidence the Si clusters, formed as a consequence of the high temperature anneal and embedded in a SiO$_{2}$ host, we used energy selected bright field imaging with an energy loss tuned to the value of the Si bulk plasmon. The dielectric function of the sample is determined by fitting the experimental spectrum using the theoretical description proposed by Barrera and Fuchs(1). The model solves the problem of calculating the energy loss rate for electrons passing through a system of spherical particles of equal radii, located at random in a host material. The dielectric function of the host was preliminarily determined by a Kramers-Kronig analysis of reference pure PECVD oxide spectrum. (1) R.G. Barrera and R. Fuchs, Phys. Rev. B52, 3256 (1995)   

Session U21: Biomedical Physics

Optimizing coherence domain imaging system for biomedical imaging

Holographic optical coherence imaging (OCI) system has shown promise in record full-frame depth-resolved images in tumor tissues, allowing real-time video display. Normally a holographic OCI system consists of a low coherence interferometry, a dynamic coherent filter and a CCD camera for image record and display. The key component in the system is photorefractive multiple quantum wells (PRQW) devices that act as the coherence filter passing full-frame image bearing light while rejecting the scattered background. Holographic OCI system is optimized by considering adaptive capabilities, speckle suppression and higher diffraction efficiency of the devices. Both AlGaAs/GaAs and InGaAs/GaAs devices have been examined at different working wavelengths based on degenerate four-wave mixing and non-degenerate four-wave mixing.   

Medical Image Provessing using Transient Fourier Holography in Bacteriorhodopsin Films

A real-time optical Fourier image processing system is demonstrated for early detection of microcalcifications in screen film as well as digital mammograms. The principle is based on recording and reconstructing the transient photoisomerizative grating formed in the bR film. At first Fourier hologram is recorded by spatially overlapping the Fourier transformed object beam with the reference beam in the bR film. Then the object beam is blocked and the reference beam performs the reconstruction of the recorded Fourier hologram. The optimum of diffraction efficiency occurs when object beam intensity is matched to the reference beam intensity. We exploit this technique to process mammograms in real-time for identification of microcalcifications buried in the soft tissue for early detection of breast cancer. A novel feature of the technique is the ability to transient display of selected spatial frequencies in the reconstructing process which enables the radiologists to study the features of interest in time scale.   

Ultra-High Speed Observations of the Mechanism of Sonoporation

Cells that are exposed to varying amounts of ultrasound energy may undergo either permanent cell membrane damage (lethal sonoporation) or a transient enhancement of membrane permeability (reversable or non lethal sonoporation). The merits of each mode are clear: lethal sonoporation constitutes a significant tumour therapy weapon, whilst its less intrusive counterpart, reversible sonoporation, makes for an effective non-invasive and targeted drug delivery approach. Until now, the mechanism of these interactions has remained unknown. We will demonstrate, for the first time, how an innovative hybridization of hologram based optical trapping technology, together with the application of millisecond pulsed ultrasound energy and parallel observation at MHz frame-rates using microscope objectives, has been used to elucidate the fundamental microscopic mechanism behind sonoporation. We will demonstrate the dependence of the permeabilisation mechanism on both the ultrasound field characteristics and the controlled displacement between individual microbubbles and single cells. High speed movies will be used to illustrate each category, whilst parallel fluorescence microscopy allows bioeffect to be quantified. Strategies for sonoporation optimisation are also illustrated.   

GOLD AND IRON-GOLD NANOPARTICLES FOR INTRACELLULAR TRACKING AND IN VIVO MEDICAL APPLICATONS

We have fabricated Au and Fe-Au nanoparticles for potential use in ex vivo experiments such as intracellular tracking, as well as a variety of in vivo medical applications. In order to improve their targeting potential, circulation time and flexibility, gold NPs were surface modified using a hetero-bifunctional poly(ethylene glycol) (PEG, MW 1,500) spacers. A coumarin-PEG-gold NP complex was formed and cell viability studies and optical fluorescence experiments were carried out demonstrating the use of these surface-modified gold NPs for drug delivery, gene therapy and cell trafficking experiments. Fe-Au nanoparticles were also fabricated and show significant contrast enhancement in MRI studies through a substantial reduction of the T2 relaxation time.   

Viscoelastic and ultrasonic measurements of canine tissue

Mechanical properties, of biological tissues, such as the complex modulus, are of interest for assessing the performance of elastographic methods that evaluate the stiffness characteristics of tissue. Determination of the mechanical properties of biological tissues is often limited by proper geometry of the sample, as well as homogeneity of the stress-strain relationship. Viscoelastic measurements were performed on in vitro canine liver tissue specimens, using a dynamic testing system, from 0.1 -- 100 Hz, and ultrasonic attenuation measurements were performed from 6 -- 9 MHz . Both normal tissues as well as thermal lesions prepared by immersion heating at several temperatures were tested. Experiments were conducted by uniaxially compressing tissue samples and measuring the load response. The resulting moduli spectra were then fit to both the Kelvin-Voigt model, as well as the Kelvin-Voigt fractional derivative model. The data agree well with the models and in comparing the results from the normal tissue with that of the thermal lesions, the concept of a complex modulus contrast is introduced and its applications to elastography are discussed.   

Loading of Cervical Spine when Head is Rotated

The neck is more susceptible to injury during an insult in the forward direction if the head is not initially facing straight ahead. (A typical example of this is when a vehicle occupant is checking traffic to the right or left at an intersection before proceeding.) However, the ability to characterize this behavior has not progressed much beyond the qualitative because practical constraints limit testing with conventional physical surrogates. This shortfall is tackled in this study by employing a model validated elsewhere to represent a range of real-world events with the power of great specificity for parameters of importance. Of primary concern is the variation in head angle, which can now be investigated across a wide spectrum of values that was not possible with previous approaches. The quantitative results computed here provide an extraordinarily high level of detail and they show how the potential for injury can change from low to significant within a matter of degrees. This explains why a seemingly harmless impact can cause trauma in some cases when none would otherwise be expected.   

In-Situ Atomic Force Microscopy of Bone Fracture Surfaces Reveals Collagen Fibrils Individually Coated with Mineral Particles of Varying Shape and Size

High resolution AFM images of bovine trabecular bone fracture surfaces reveal individual fibrils coated with extrafibrillar mineral particles. Treating bone with EDTA removes the mineral particles on these fibrils, and reveals the underlying collagen structure. The mineral particles show distinctly different size and morphology in different regions. Significantly, we rarely observed bare collagen fibrils in fracture surfaces before EDTA treatment. This implies that fractures propagate between the mineral particles on one fibril, and the mineral particles on another fibril. Thus, to understand the mechanics of fracture on the molecular scale, it will be crucial to understand the molecular nature of the adhesion between the mineral particles that coat adjacent collagen fibrils, because this is the weak interface that fails during fracture.   

High-Speed Photography during Compression Testing Human Trabecular Bone

The mechanical properties of healthy and diseased bone are extensively studied. Most of this research is motivated by the immense costs in health care due to osteoporosis. To address the problem of assessing bone microarchitecture and concomitant microcracking behavior, we recently combined mechanical compression testing of trabecular bone with high-speed photography. In an exemplary study, we investigated healthy, osteoarthritic, and osteoporotic human vertebral trabecular bone. Bone samples were loaded along their principal load-bearing axis at high strain rates simulating boundary conditions as experienced in individuals during falls. Even at small global strains huge local deformations could be seen in the recorded high-speed photography frames. Moreover, strained trabeculae were seen to whiten with increasing strain, which could be associated with areas of high deformation using a motion energy filter. Presumably the effect seen is due to microcrack formation in these areas, similar to stress whitening in synthetic polymers. This hypothesis is currently tested applying en bloc microcrack staining and histology.   

Investigations on the Feedback Loop that controls Bone Remodeling

Bone is a living tissue that can adapt its shape and inner architecture to withstand mechanical loading experienced in daily life. The bone tissue is continuously renewed in a process called bone remodeling involving specialized cells: osteoblasts deposit bone, osteoclasts remove bone. The process is regulated by a feedback loop, where the probability for bone deposition (resorption) is related to a local mechanical stimulus. Our interest is the effect of different relations (i.e., different remodel laws) on the resulting bone mass and geometry. We developed a computer model [1], where the spongy architecture of trabecular bone is mapped onto a lattice and the local mechanical loading is calculated using a simple algorithm. Depending on the remodel law, the simulations show remarkable differences in the heterogeneity of the bone architecture and the reaction to perturbations (e.g., changes in the osteoclastic activity). Comparison of these results to data obtained from real bone gives further insight in the underlying feedback loop. \\ $[1]$ Weinkamer et al., PRL {\bf 93}, 228102 (2004)   

Effect of Polymer Matrix on the Electrophoretic Mobility of Linear and Branched DNA in Polymer Solutions

The electrophoretic mobility of linear T2 and 3-arm star branched DNA is studied in low polydispersity index linear polyacrylamide (LPA) solution of varying molecular weights in tris-acetate buffer. In semidilute solution (above the overlap concentration) we have found that the linear and the star-branched DNA of similar size have similar mobility below a certain threshold concentration. This threshold concentration is observed to increase with LPA molecular weight and correspond to about 10 blobs (hydrodynamic screening length) per polymer chain N/g. At concentrations below this threshold, the biased repation with fluctuations model and constraint release could not explain the observed electrophoretic mobility dependence on LPA concentration and molecular weight, and the DNA mobility is found to be independent of the DNA conformation, sensitive to the ionic concentration and the electric field, and determined by the local non-Newtonian viscosity due to shear thinning of LPA in the electric double layer (EDL).Whereas above this threshold concentration, star branched DNA is found to have mobility lower than that of linear DNA and both the star branched and the linear DNA mobility are found to depend on the entanglements among polymer chains characterized by N/g.   

Interactions between the HIV TAT domain and cell membranes

Biologically active molecules such as proteins and oligonucleotides can be transduced into cells with high efficiency when covalently linked to a Protein Transduction Domain (PTD), such as the TAT domain in the HIV virus. All PTDs have a high content of basic amino acids resulting in a net positive charge. Electrostatic interactions between cationic PTDs and the negatively charged phospholipids that constitute the plasma membrane seem to be responsible for peptide uptake, but no detailed structural studies exist. We present recent results on the structures of self-assembled complexes of the cationic TAT domain and anionic lipid bilayers using synchrotron x-ray scattering and electron microscopy, and examine possible mechanisms of the anomalous transduction.   

Transfection efficiency and structural studies on nonviral gene carriers containing cholesterol and other sterols

Lipid based nonviral gene delivery currently focuses on cationic liposomes, which typically consist of a mixture of cationic and neutral (helper) lipids. Motivated by the plasma membrane composition of mammalian cells, which contain large amounts of cholesterol, this molecule is often used as a helper lipid. The presented work investigates the effect of cholesterol and structurally related molecules on the transfection efficiency (TE) of cationic lipid-DNA (CL-DNA) complexes in mammalian cells. Previous studies have identified the membrane charge density as a universal parameter, predicting TE for CL-DNA complexes in the lamellar L$\alpha ^{C} $ phase [1,2]. Addition of cholesterol to low transfecting CL-DNA complexes results in dramatic improvements in TE that significantly deviate from the TE model for lamellar complexes. A model system using negatively charged giant vesicles has been developed to mimic the cell membrane and understand the behavior pattern of CL-DNA complexes containing cholesterol. Funding provided by NIH GM-59288. [1] Lin AJ, Slack NL, Ahmad A, George CX, Samuel CE, Safinya CR, \textit{Biophys. J.}, 2003, V84:3307 [2] Ahmad A, Evans HM, Ewert K, and Safinya CR, \textit{J. Gene Med., }accepted   

Energy Partitioning in FEL Tissue Ablation

The wavelength-dependence of FEL tissue ablation has been attributed to partitioning of absorbed energy between protein and saline. We have taken two approaches to test the hypothesis that such energy-partitioning allows wavelengths targeting the protein component to diminish the structural integrity of tissue before water vaporization commences. First, models of this process predict that energy-partitioning should play little role in near-threshold ablation, but become much more important as the fluence is increased. Thus, we have measured the ablation efficiency on cornea across large swaths of the fluence versus wavelength parameter space. We find that the effects of energy-partitioning do grow as the fluence is increased. Second, we have analyzed the protein components of the ablation plume for signatures of protein structural change. FTIR and NMR spectra of plume components reveal that the secondary and tertiary structure of the protein (collagen) fibers has been lost. These spectra also reveal new functional groups in the ablated material, most likely nitriles or alkynes that could have arisen via oxidative degradation of the protein. However, there is no evidence for widespread scission of protein backbones.   

Charged binary fluid confined to cylindrical monolayer: Pattern formation

Stoichiometric mixtures of acidic and basic peptide--amphiphile molecules (PA) composed of a hydrophobic block and a peptide block that favors $\beta$-sheet formation with a charged end--group co-assemble in the water solution at physiological pH--conditions into long cylindrical nanofibers. These fibers form a network that resembles extracellular matrix found in living tissue. PA--molecules self--aggregate because of competition between hydrophobic--hydrophilic interactions and stabilize aggregate structures by forming hydrogen bonds between peptide blocks and surface charges. On the other hand, small chemical distinction between cationic and anionic components may cause local segregation and formation of patterns. We investigate the phase behavior of stoichiometric charged two-component fluid confined to cylindrical monolayer to describe pattern formation on the surface of self--assembled cylindrical aggregates, such as peptide amphiphiles. The net incompatibility among different components results in appearance of segregated domains which growth is inhibited by electrostatics. We find that the transition from isotropic to striped phase begins from the formation of small domains and proceeds through an intermediate state governed by defects. Detailed results of study of heat capacity, static structure factor, susceptibility and cluster asphericity parameter independence on the radius of the cylinder and degree of incompatibility we will present during the talk.   

Today's ``safe" radiofrequency (RF) exposure limits DON'T protect human health near transmitters!

Maxwell's theory implies that electromagnetic (EM) radiation carries both energy and momentum. ``The momentum may have both linear and angular contributions; angular momentum [AM] has a spin part associated with polarization and an orbital part associated with spatial distribution. Any interaction between radiation and matter is inevitably accompanied by an exchange of momentum. This often has mechanical consequences ..."$^2$ Voluntary consensus standards [ANSI C95; NCRP; INCIRP] protect human health from most {\it thermal} [energy transfer] effects, but no standards yet exist to protect health against {\it athermal} [momentum transfer] effects, though laboratory transfer of spin AM was reported by 1935$^3$ and of orbital AM by 1992$^2$ for an optical vortex [tip of Poynting vector (PV) traces a helix about the beam axis]. In the {\it far field} of a dipole RF transmitter, radiation is linearly polarized ({\it minimal} spin AM) and locally approximated by a plane wave ({\it zero} orbital AM), but in the {\it near field} the orbital AM is {\it non-zero} [tip of PV traces an ellipse$^4$ in air] implying an {\it athermal hazard} [{\it e.g.}, brain tumors in cellular phone users] against which {\it no standard now in use anywhere in the world} protects! \hfil \break $^2$ L. Allen {\it et al}. Phys. Rev. {\bf A 45}:8185-9(1992). $^3$ R.A. Beth, Phys. Rev. {\bf 48}:471(1935); {\bf 50}:115-25 (1936). $^4$ F. Landstorfer, Archiv f\"ur Elektronik und \"Ubertragungstechnik {\bf 26}:189-96(1972) [in German].   

Mechanical Asphyxiation in Proximity with Ceramic Surface

A lack of air reaching the lungs may involve several components, including obstruction of some part of the breathing path. When such an obstruction is effected by external constriction of the trachea, some level of pressure must be applied over some period of time to cause death by asphyxia. The nature of these physical quantities depends on the anatomy of the person concerned and on the geometry of the circumstances. Special emphasis is placed in this research on the interaction between anatomy and geometry in order to calculate the forces associated with the reported rest position of a deceased female against a bathroom commode. The first step in the analysis is to derive the distribution of body weight for this particular individual by employing extensive anthropometric measurements. This methodology produces an optimum representation with a large number of body segments, whose finite number of arrangements are examined to find the forces at certain points of interest. The values obtained are reviewed to see if the apparent rest position is consistent with elementary laws of physics.   

Monitoring stiffness contrast in elastography

Elastography is an imaging modality used to image tissue strains resulting from external quasi-static compression of tissue. Therefore, elastograms can be used to study variations in the stiffness of thermally coagulated regions of tissue. In this study, the variations in stiffness contrast of lesions formed by radio frequency (RF) ablation of canine liver tissue have been investigated. RF ablation was performed on in vitro canine liver tissue over a range of temperatures from 70 - 100 degrees C, and over a range of ablation times from 1 -- 8 minutes. Elastography was then performed on these samples and on normal tissue. It was expected that stiffness contrast would increase with increasing lesion temperature and ablation duration, on the basis that higher temperature and greater ablation durations lead to increased protein denaturation. This increase was seen with ablation duration, but is not obvious with ablation temperature. These and other results will be discussed.   

Session U22: Transport and Kinetics in Biological Systems

Asymmetric exclusion process models for translation and biological transport

In the totally asymmetric simple exclusion process (TASEP), particles travel unidirectionally along a one-dimensional lattice and interact with each other by hard-core exclusion. Variants of the TASEP have been used to model the movement of molecular motors along biopolymers, including ribosomes on mRNA and various motors on cytoskeletal filaments. Ribosomes synthesize proteins as they traverse an mRNA molecule, so their motion is important in understanding the kinetics of protein production. We model protein synthesis as a TASEP with quenched disorder in the particle hopping rates. The hopping rates are determined by gene sequences and the availability of biomolecules. We use a statistical ensemble method in fitting the model to experimental data. The model is able to explain much of the nonlinear relationship between mRNA and protein levels.   

Diffusion and binding of finite-size particles through tubes

The classical treatment of the diffusion of particles through their environment ignores all of the interactions between these particles. The diffusion of biological particles, however, often occurs in crowded environments (e.g. the cellular matrix), where steric interactions between particles are important. In this talk, we investigate the effects of steric interactions on diffusion by considering the diffusion of finite-size particles within a tube whose diameter is comparable to the size of the particles. When the particles diffuse freely through the tube, steric interactions have only a trivial effect on their diffusion; however, steric interactions dramatically alter the diffusion when the particles can reversibly bind to the walls of the tube. Using a simple lattice model of this process, we calculate the steady-state current and density of particles in the tube. We also perform simulations of this system, and find excellent agreement with our analytical results.   

Nucleation and Transport in a Low-Dimensional Driven System

Based on high precision Monte Carlo simulations, we discuss the formation of domains in a quasi one-dimensional model of two species driven in opposite directions. We argue that the presence of a single macroscopic cluster is an intermediate stage of a complex nucleation process. A careful finite size analysis reveals many interesting properties: for small systems the single cluster destabilizes and on large systems we observe the formation of many clusters. We also show results for the dependence of the cluster growth exponent on the stochastic parameters and on different lattice geometries.   

Exact Solution for a class of Mass Transport Models, Condensation Transitions, and the Nature of the Condensate

We study the phenomenon of real space condensation in the steady state of one dimensional mass transport models. These models, including the Zero-Range Process and the Asymmetric Random Average Process, have been used to describe a variety of physical systems, e.g., bio-molecular motors, vehicular or pedestrian traffic, force propagation through granular media, etc. The dynamics consists of stochastically transferring a portion of the mass, from site to neighboring site, according to some prescribed distribution. For a class of these models, we find an easy test to check if the steady state (full multi-site) distribution is `factorizable,' and if so, a simple method to construct the solution explicitly. Based on this approach, we not only verify the criterion for the existence of a condensation transition (where, a la Bose-Einstein, a finite fraction of the total mass condenses into a single site) but also elucidate the nature of the condensate. Specifically, we find two regimes: one where the mass of the condensate is Gaussian distributed with normal fluctuations, and a second regime with non-Gaussian distributions and anomalously large fluctuations.   

Will jams get worse when slow cars move over?

Motivated by an analogy with traffic, we simulate two species of particles (`vehicles'), moving stochastically in opposite directions on a two-lane road. In this simple modification of the asymmetric exclusion process, each species prefers one lane over the other, controlled by a parameter $0 \leq b \leq 1$ such that $b=0$ corresponds to random lane choice and $b=1$ to perfect `laning'. We find that the system displays one large cluster (`jam') whose size increases with $b$, contrary to intuition. Even more remarkably, the lane `charge' (a measure for the number of particles in their preferred lane) exhibits a region of negative response: even though vehicles experience a stronger preference for the `right' lane, more of them find themselves in the `wrong' one! For $b$ very close to $1$, a sharp transition restores a homogeneous state. Various characteristics of the system are computed analytically, in good agreement with simulation data.   

Subcellular protein localization in {\it E. coli}: diffusion and membrane attachment of MinD molecules

In {\it E. coli}, accurate cell division depends upon the oscillation of Min proteins. We provide a model for polar localization of MinD based only on diffusion, a delay for nucleotide exchange, and different rates of attchment to the bare membrane and occupied membrane. We derive analytically the probability density, and correspondingly the length scale, for MinD attachment zones. Our simple analytical model illustrates the processes giving rise to the observed localization of cellular MinD zones.   

Molecular trafficking in tissue engineered cartilage constructs

Tissue processing in vitro requires an effective trafficking of biologically active agents within three-dimensional constructs for induction of appropriate and enhanced cellular growth, biosynthesis and tissue remodeling. Moreover, nutrients and waste products need to move freely through the cellular constructs to minimize the presence of regions with necrotic and/or apoptotic cells. In tissue-engineered cartilage, for example, during the time of culture, cells seeded within the three-dimensional constructs lay-down their own extracellular matrix and this may lead to a heterogeneous distribution of transport properties both in time and space. In this work the diffusion coefficient of BSA and 500kDa dextran has been measured with FRAP thecnique in agarose gel chondrocytes constructs at different position and time during the culture. The diffusion coefficient of both molecular probes within the developing tissue well correlated with the ECM production and assembly. Moreover the comparision between BSA and dextran transport parameters revealed a selective hindrance effect of the neo tissue on high interacting molecules.   

Minimal Model for Noise-Driven Locomotion

We consider a pair of dissimilar particles, bound to each other by a non-centrosymmetric pair potential and restricted to move on a line, in the presence of white noise and nonlinear damping. We show analytically and numerically that the absence of an equilibrium fluctuation-dissipation relation, together with the asymmetry of the pair potential, causes the relative coordinate of the pair to drive a systematic mean motion of the centre of mass, without the aid of an external ratchet potential. This remarkably simple model of noise-driven self-propulsion illustrates a principle that should apply to a variety of driven systems, from biology to granular matter.   

Queueing and Cooperativity in Ligand-Receptor Binding

We compare the kinetics of two types of receptors: receptors that only allow ligands to attach to the binding sites in a specific order, and those that allow binding in any order. For equivalent rate constants and cooperativities, we find that receptors with sequential ligand binding are more likely to have all of its binding sites fully occupied than receptors that bind ligands in any order. The mean occupation of sequentially loaded receptors is also higher. However, starting from a totally empty receptor, we find that the mean first passage time to full occupancy is smaller for low-mean-occupation, random adsorption receptors. Our results are contrasted with Hill-like descriptions of receptor occupancy. Scenarios in which distinction of receptor types may be important are also discussed.   

A model of the kinetic cycle of single cytoplasmic

We use Monte Carlo simulations to model molecular motor function at the single molecule level. For kinesin, we show that the simulations of the kinetic cycle reproduces accurately the dependence of velocity on ATP concentration and applied load, described by Michaelis-Menten kinetics. More importantly, the Monte Carlo approach allows us to implement nonlinear models of more complicated branching enzymatic pathways, like those found in enzymes such as cytoplasmic dynein. Our dynein simulations reproduce the main features of recent single molecule experiments that found a discrete distribution of dynein step sizes depending on load and ATP concentration. The theory relates dynein's chemical/enzymatic properties to its mechanical force production. It proposes the existence of negative cooperativity of ATP binding at secondary binding sites, which is required to reproduce the experimentally observed step distribution and improves dynein's ATP economy by suppressing small steps under high-ATP/ no-load conditions.   

Probing convection and diffusion in macromolecular gels

Transport of molecules within three-dimensional biological tissue occurs by both diffusion and convection. While diffusion is relatively well studied in the literature, there is a paucity of data on convection parameters, even if is the most effective transport mechanism for large molecules. Pressure-driven flow through complex macromolecular gels can provide different probe velocity depending on the diffusant molecule and matrix interaction and so far no specific measurements have been performed. Furthermore the complexity or heterogeneity of the system may cause differences with the position in the convection properties of the sample. In this study both diffusion coefficient and velocity of several fluorescent probes in macromolecular gels have been measured with a high spatial resolution (100$\mu $m). The macromolecular velocity has been evaluated by adopting the video-FRAP technique, through an algorithm to separate the fluorescence recovery due to the brownian motion and that due to a bulk convection. Combination of the two transport process is very relevant in tissue engineering and drug delivery application.   

Dynamics of rigid and flexible extended bodies in viscous films and membranes

The mobility of inclusions (e.g. proteins or lipid ``rafts'') in membranes is a fundamental physical parameter controlling a number of cellular processes. In this talk, we examine the motion of rod-like inclusions in continuum viscous films and membranes as a representative example of the general problem of determining the mobility of arbitrarily shaped, extended bodies moving in membranes or at liquid/liquid interfaces. We demonstrate an important difference between rod mobilities in films/membranes and in bulk fluids, which is present even when the dissipation is dominated by the fluid stress: For large inclusions we find that rotation and motion perpendicular to the rod axis exhibit purely local drag, in which the drag coefficient is algebraic in the rod dimensions. We also study the dynamics of the undulation modes of a semiflexible filament embedded in the membrane and find two dynamical regimes in the relaxation spectrum.   

Session U23: Statistical Physics and Disordered Nonlinear Systems

Bethe Ansatz for the 1D Hubbard model: from the finite lattice to the thermodynamic limit

We prove that the norm of the Bethe Ansatz wavefunction for the one-dimensional Hubbard model does not vanish for all but finitely many values of the interaction U, concluding the proof that it does indeed give the correct ground state of the model. For the finitely many values where the wavefunction could in principle vanish, we propose a method to determine the genuine ground state. We also show the existence of a thermodynamic limit to the distribution function of the parameters of the Bethe Ansatz states, establishing the connection between our previous work on the existence of solution to the Bethe Ansatz equations and the exact solution of the model at half filling in the thermodynamic limit, by E. H. Lieb and F. Y. Wu.   

The Chemical Potential

The definition of the fundamental quantity, the chemical potential (c.p.), is confused in the literature, there being at least three distinct definitions in various books and papers. Major differences among them can occur for finite systems. We resolve the situation by arguing that the chemical potential defined by the symbol $\mu$ conventionally appearing in the grand canonical density operator is the uniquely correct definition, the grand canonical ensemble being the only one of the various ensembles usually discussed (microcanonical, canonical, Gibbs, grand canonical) that is appropriate for statistical thermodynamics, whenever the c.p. is physically relevant. The derivation of the zero-temperature limit of this $\mu$ for rather general interacting-electron systems by Perdew et. al.,[1] is discussed and extended. The enormous finite-size corrections (in systems $>>$ a cm$^3$) for one rather common definition of the c.p., found by Shegelski [2] within the standard effective mass model of an ideal intrinsic semiconductor, are discussed. The quantum dot is mentioned as a small-system application. \newline 1. J. F. Perdew et. al., Phys. Rev. Lett. \textbf{23}, 1691 (1982); J. F. Perdew, in \emph{Density-functional methods in Physics}, edited by R. M. Dreizler and J. da Providencia, Plenum Press, 1985. \newline 2. M. R. A. Shegelski, Solid State Commun. \textbf{38}, 351 (1986); Am. J. Phys. \textbf{72}, 676 (2004).   

The thermodynamics of reversible thermoelectric nanomaterials

Irreversible effects in thermoelectric materials limit their efficiency and economy for applications in power generation and refrigeration. While electron transport is unavoidably irreversible in bulk materials, here we derive conditions under which reversible diffusive electron transport can be achieved in nanostructured thermoelectric materials via the same physical mechanism utilized in the three-level amplifier (thermally pumped laser) and idealized thermophotovoltaic and thermionic devices. From a broader physical perspective, the most interesting aspect of this work is that it suggests that all of the above-mentioned solid-state devices may be unified as a single `type' of heat engine which achieves reversibility when heat transfer via particle exchange between reservoirs is isentropic (but non-isothermal), in contrast to heat engines such as Carnot, Otto or Brayton cycles, which achieve reversibility when heat transfer between the working gas and heat reservoirs is isothermal.   

Single Particle Jumps in a Glass: Statistics and History Dependence

We study a binary Lennard-Jones mixture below the glass transition via molecular dynamics simulations. To investigate the dynamics of the system we define single particle jumps via their single particle trajectories. We find two kinds of jumps: ``reversible jumps'' where a particle jumps back and forth between two or more average positions and ``irreversible jumps'' where a particle does not return to any of its former average positions. For both the irreversible and reversible jumps we present as a function of temperature the number of jumps, jump size and waiting time between jumps. With increasing temperature $T$ particles undergo both more jumps, and the percentage of irreversible jumps versus reversible jumps increases. Similarly the jump size increases with increasing $T$. The distributions of jump lengths and waiting times are in accordance with subdiffusive behavior. Whereas the number of jumping particles is dependent on the history of the system, the jump size and waiting times are independent of the history of the glass.   

Step Strains in a Disordered Foam

Foams consisting of gas bubbles separated by liquid walls are a unique material system that can exhibit solid like properties under small strains and interesting fluid properties under larger strains including stress fluctuations and intermittent flow. Their nonlinear flow behavior is characterized by a viscosity dependence on shear rate and the emergence of a ``yield stress.'' Foams are also interesting as part of a more general class of materials that can be referred to as ``complex fluids'' (granular systems, emulsions) that have been considered in the theoretical framework of jamming. One question raised in studying foams in particular and complex fluids more generally is what is the fundamental feature (including a length scale and time scale) that most prominently describes the flow behavior of these systems near the jamming transition. What properties are universal? To help answer some of these questions, we will discuss an experiment to probe the mechanical properties of a bubble raft (two dimensional foam) by considering step strains applied to this system and focusing on the system's response (stress drops).   

Diffusion in a Rough Energy Landscape

We re-examine Zwanzig's model of diffusion in a rough energy landscape [PNAS (USA) 85, 2029 (1988)]. We interpret the one- dimensional coordinate as a mesoscopic degree of freedom of the system. It is shown that the fluctuating potential corresponds to a broken symmetry. The corresponding order parameter is associated with long- range elastic stress in the system. We derive a Landau-type expression for the free energy of the system from which the activation energy for barrier crossings can be obtained.     

Dielectric Susceptibility Studies of the Glass Transition of Glycerol at High Pressure

We have measured the dielectric susceptibility of glycerol as a function of frequency (0.01Hz-10kHz), temperature (190K-250K) and pressure (0-9kB). The glass transition temperature $T_g$ increases with increasing pressure, however, the thermal fragility, which measures the rate of approach to $T_g$, is independent of pressure. This result is in contrast to studies based on viscosity measurements which probe a higher frequency range, where it was found that fragility increases with pressure. We have also found that the width of relaxation when plotted as a function of the relaxation frequency is only weakly dependent on pressure within this range.   

Glassy Behavior of Interface States in Al-AlOx-Al Tunnel Junctions

The complex impedance of a tunnel junction can be modeled as the parallel combination of a resistance, which is exponentially sensitive to barrier parameters, and a frequency-dependent complex capacitance, which is dominated by the presence of charge traps at the electrode interfaces. We present a study of the time evolution of these interface states by measuring in vacuum the \textit{in-situ }complex impedance of Al-AlO$_{x}$-Al trilayer structures as a function of age $t$. After a sample-dependent settling time, both the resistance and capacitance simultaneously exhibit irreversible logarithmic aging. Application of a dc bias voltage at $t$~=~$t_{0}$ gives rise to a resistance transition (with power-law dependence on $t$~--~$t_{0})$ to a new aging trajectory with a smaller logarithmic slope. This behavior is correlated with barrier parameters determined from the dependence of the resistance on ``witness'' bias sweeps. By examining the dependence of barrier asymmetry on the sign of the voltage bias, we demonstrate that this glassy system retains memory of the state associated with previously applied bias voltages. * This work is supported by NSF under contract DMR 0404962   

Nanoscopic studies of ferroelectric domain walls in epitaxial perovskite thin films

Understanding the behavior of ferroelectric domain walls (DW) is important for applications using multi-domain structures. Microscopic studies of these systems are needed to discriminate between periodic lattice and disorder pinning. Epitaxial perovskite films are an excellent model systems for such studies. Using atomic force microscopy on PbZr$_{0.2}$Ti$_{0.8}$O$_3$ thin films we have investigated the static configuration and subcritical dynamics of ferroelectric DW. Measurements of individual nanoscopic domains showed initial nucleation at the AFM tip, followed by radial DW motion. We have demonstrated this motion to be a creep process with a non-linear velocity response to electric fields: $v\sim \exp [-C/E^\mu]$. The dynamical exponent $\mu$ ranged between 0.5-1 [1,2]. Independent measurements of DW roughness in these films revealed a power law growth of the relative displacements correlation function $B(L)\sim L^{2\zeta}$ at short length scales L, with a wandering exponent $\zeta \sim 0.26$. Together, these results give an effective DW dimensionality of 2.5. These results cannot be explained by lattice pinning, but agree with calculations for two-dimensional elastic interfaces in the presence of random-bond disorder and dipolar interactions.\\ 1. Tybell, PRL {\bf 89}, 097601 \\ 2. Paruch, cond-mat/0411178   

The stochastic dynamics of micron and submicron scale mechanical oscillators

The stochastic response of micron and submicron scale oscillators of arbitrary geometry immersed in a viscous fluid will be discussed. It will be shown that by using the fluctuation-dissipation theorem it is possible to calculate, in a straight forward manner, the stochastic dynamics that would be measured in experiment through deterministic calculations of the fluid and solid equations. This approach is used to investigate the motion of single and multiple oscillators for a variety of experimentally realistic geometries.   

Heterogeneous slow dynamics in a two dimensional doped classical antiferromagnet

We introduce a lattice model for a classical doped two dimensional antiferromagnet which has no quenched disorder, yet displays slow dynamics similar to those observed in supercooled liquids. We calculate two-time spatial and spin correlations via Monte Carlo simulations and find that for sufficiently low temperatures, there is anomalous diffusion and stretched-exponential relaxation of spin correlations. The relaxation times associated with spin correlations and diffusion both diverge at low temperatures in a sub-Arrhenius fashion if the fit is done over a large temperature-window or an Arrhenius fashion if only low temperatures are considered. We find evidence of spatially heterogeneous dynamics, in which vacancies created by changes in occupation facilitate spin flips on neighbouring sites. We find violations of the Stokes-Einstein relation and Debye-Stokes-Einstein relation and show that the probability distributions of local spatial correlations indicate fast and slow populations of sites, and local spin correlations indicate a wide distribution of relaxation times, similar to observations in other glassy systems with and without quenched disorder.   

Session U25: Nanowires II: Metals and Oxides

{\it Ab initio} molecular dynamics study of pure and contaminated gold nanowires

Gold nanowires have the ability to form linear chains that are one atom wide and that have just a few atoms in length. One of the unexpected features of these wires is that before rupture quite large interatomic distances of $\simeq$ 3.6 {\AA} have been observed, which are most likely due to the presence of impurities. In view of these facts we recently studied [1] the effect of H, B, C, N, and S impurities on the breaking of Au nanowires, in particular how they affect the maximum Au-Au bond length. Out of all these impurities, under quasi-static pulling conditions the only one that produced an Au-X-Au close to 3.6 {\AA} was hydrogen. All the others produced distances of the order of 3.9 {\AA} or larger. As the calculations were all performed at zero temperature, it is not obvious if, or how, the vibrational motion of the atoms could change these conclusions. In order to investigate these issues we will present results of {\it ab initio} molecular dynamics for pure and contaminated (H atoms) nanowires. In particular, temperature effects cannot rule out the presence of H atoms in Au nanowires, as recently claimed by Legoas {\it et al.} [2]. [1] F. D. Novaes {\it et al.}, Phys. Rev. Lett. {\bf 90}, 036101 (2003). [2] S. B. Legoas {\it et al.} Phys. Rev. Lett. {\bf 93}, 216103 (2004). We acknowledge support from FAPESP, Capes and CNPq.   

Martensitic phase transitions in metallic nanowires

We use the NRL tight binding method (TB) and modified embedded atom method (MEAM) to investigate the spontaneous phase transition from fcc to bct for Au nanowires oriented in the (001) direction. Employing density functional theory (DFT) calculations of the energy of bulk Au under uniaxial strain along the Bain path, we find that bulk Au has a minimum along this path for a bct structure, which, under uniaxial stress of greater than $\sim $2 GPa, would become energetically favorable to the fcc structure. This state, however, is unstable with respect to shear deformation. The TB method predicts the same behavior. TB simulations of Au nanowires smaller than 2.0 nm diameter, however, indicate that that these nanowires will spontaneously relax from an original fcc structure to a bct structure even at 0 K and zero external pressure. The driving force for the transition is the surface stress along the outer boundary of the nanowire, which provides the necessary 2 GPa of total stress to effect the phase transition. Furthermore, the surface stress stabilizes the bct structure with respect to shear. Its stability is verified by TB simulated annealing and large-scale MEAM simulated annealing simulations. We will also discuss a TB model for the shape-memory NiTi alloy and its use in nanowire simulations.   

The comparison of metal coating growth on nanofibers with metal film growth on flat surfaces

Recent experiments showed that physical vapor deposition is a powerful technique to form novel one-dimensional nanostructures such as metal coated nanofibers and metallic hollow nanowires. In order to have a better understanding of metal coating growth on nanofibers and to determine it's differences with metal film growth on flat surfaces, molecular dynamics simulations are performed. Adsorption, reflection and etching events are analyzed and corresponding reaction probabilities are calculated for both flat and cylindrical coating surfaces with different radii. Our investigations showed that reaction probabilities for metal coating growth on nanofibers are very different from the reaction probabilities for metal film growth for higher kinetic energies or for large off-normal angles of incidence of Al atoms. If one considers only the reaction rates, diffusive transport of Al ions in the plasma of physical vapor deposition is found to be more favorable than ballistic transport of Al ions for the growth of Al coatings on nanofibers. These investigations provide us important insights for the growth of metal coatings on nanofibers and for the formation of hollow nanowires with different surface morphologies.   

Structural and Electronic Properties of Mo$_{6}$S$_x$I$_{9-x}$ Nanowires.

We investigate the equilibrium geometry and electronic structure of recently synthesized Mo$_{6}$S$_x$I$_{9-x}$ nanowires using {\em ab initio} Density Functional calculations. Our structure optimization calculations suggest a well-defined atomic structure within these nanowires, which are energetically unusually stable in view of their sub-nanometer diameter. For particular stoichiometries, we find the Mo$_{6}$S$_x$I$_{9-x}$ nanowires to be rather soft with respect to axial compression, and also to be metallic. We characterize the quantum conductance in these nanowires using a self-consistent nonequilibrium Green's function approach within the Landauer-Buttiker formalism. We find the charge density near the Fermi level to be delocalized along the wires, suggesting a high polarizability. For particular metastable geometries, the nanowires also exhibit a magnetic instability. Combination of atomic-scale perfection with a high structural stability and unusual electronic and transport properties lends itself to potential applications of these nanowires as unique building blocks in hierarchically assembled electronic nanocircuits.   

Non-linear current-voltages character of Au quantum point contact

In this study, we simultaneously observed the configuration and the non-linear current-voltages character (I-V) of gold quantum point contacts (Au-QPC). UHV Transmission Electron Microscope (UHV-TEM) which combined with Scanning Tunneling Microscope (STM) enabled us to observe the configuration of QPC. TEM images were synchronized with the measured I-V. The bias voltage to Au-QPC swept from 0V to 0.3V at room temperature in UHV($\sim $1$\times $10$^{?|7}$[Pa]). The Au-QPC with short length($<\sim $1nm) showed the non-linear I-V which were fitted to a cubic function ( I=aV+cV$^{3 })$. The value of c/a in our results ($\sim $20[1/V$^{2}$]) was lager than that of previous reports (0.3$\sim $2[1/V$^{2}$]). Simultaneous TEM images revealed a changed of the width of Au-QPC. The width was found to increase from 1.1nm (0.02V) to 1.9 nm (0.27V). On the other hand, the Au-QPC with long length (nanowire $>\sim $1nm) showed the linear I-V, and the width was kept constant. We suggested that the changing of the width caused the non-linear I-V. The mechanism of increasing the width should be solved by further investigation.   

First-principles simulation of the field emission from noble metal nanowires

We carry out a theoretical study on the field emission from the nanowire which is composed of noble metal elements such as silver or gold. Our calculations are based on the first-principles density functional theory within a localized basis scheme using the SIESTA package. Electronic states and the potential of the system under finite applied voltages are determined self-consistently. Through explicit time evolution of the wavefunction, we obtain the emission current and the shape of the charge density distribution of the emitted electrons from noble metal nanowire tip.   

Free-Standing Vertical Gold Nanowires from Template Synthesis

Gold nanowires are electrochemically grown in a track-etched polycarbonate membrane inside a Teflon cell containing gold plating solution. Using this method we have grown gold nanowires with diameters in the range of 20 - 200 nm and lengths on the order of 1 - 10 um. By controlling the membrane-dissolving process, we can deposit randomly oriented nanowires with the length in the plane of a substrate, or we can leave the nanowires vertically free-standing with one end still attached to a conducting base. We are currently exploring the vertical configuration in order to study the physics of individual nanowires or groups of nanowires. Quantities of interest include the cantilever mechanical resonance frequencies and the mechanical quality factor.   

Time-resolved x-ray excited optical luminescence studies of II-VI semiconductor nanowires

Due to quantum confinement effects nanostructures often exhibit unique and intriguing fluorescence behavior. X-ray excited optical luminescence (XEOL) provides the capability to chemically map the sites responsible for producing low energy (1-6 eV) fluorescence. By taking advantage of the time structure of the x-ray pulses at the Advanced Photon Source, it also possible to determine the dynamic behavior of the states involved in the luminescence. In this presentation we show how this technique can be utilized to understand the XEOL from ZnS, ZnTe, and ZnO nanowires. Time-gated optical spectra show that the high-energy, band-edge states have a short lifetime while the lower-energy, deep-levels have a relatively long lifetime. X-ray excitation curves are obtained using the relevant optical photons as signals and compared to the corresponding x-ray absorption spectra. We will show how these results enable us to determine the local structure of the luminescent site(s).   

Synthesis, transport studies and applications of In2O3 nanowires

Single-crystalline indium oxide nanowires were synthesized using a laser ablation method and characterized using various techniques. Precise control over the nanowire diameter down to 10 nm was achieved by using monodisperse gold clusters as the catalytic nanoparticles. In addition, field effect transistors with on/off ratios as high as 10$^{4 }$were fabricated based on these nanowires. Detailed electronic measurements confirmed that our nanowires were n-type semiconductors with thermal emission as the dominating transport mechanism, as revealed by temperature-dependent measurements. Furthermore, we studied the chemical sensing properties of our In$_{2}$O$_{3 }$nanowire transistors at room temperature. Upon exposure to a small amount of NO$_{2}$, the nanowire transistors showed a decrease in conductance of up to six orders of magnitude, in addition to substantial shifts in the threshold gate voltage. Our devices exhibit significantly improved chemical sensing performance compared to existing solid-state sensors in many aspects, such as the sensitivity, the selectivity, the response time and the lowest detectable concentrations. We have also demonstrated the use of UV light as a ?gas cleanser? for In$_{2}$O$_{3 }$nanowire chemical sensors, leading to a recovery time as short as 80seconds.   

Structure of Nanocrystals by the Atomic Pair Distribution Function Technique

Knowledge of the atomic-scale structure is an important prerequisite to understand and predict the properties of materials. In the case of crystals it is obtained from the positions and the intensities of the Bragg peaks in the diffraction data. However, many materials of technological importance, in particular nanophase materials, are not perfect crystals. The diffraction patterns of such materials show only a few Bragg peaks and a pronounced diffuse component. This poses a real challenge to the usual techniques for structure determination. The challenge can be met by employing the so-called atomic pair distribution function technique. The basic features of the technique will be introduced and its potential demonstrated with results from recent structure studies of V$_{2}$O$_{5}$ nanotubes. \textbf{Acknowledgements: }This work was supported by NSF through grant DMR-0304391.   

Gallium oxide nanostructures

Crystalline $\beta $-Ga$_{2}$O$_{3}$ nanowires with two distinct morphologies have been synthesized through simple physical evaporation of Te doped GaAs powder in argon atmosphere. Nanowires as long as hundreds of micrometers with diameters in the range of 10-100 nm have been produced with a high yield. Absence of Tellurium in the nanowires indicates that the growth mechanism is not VLS based. Substitution of sulfur in place of tellurium resulted in similar nanostructures. Some of the nanowires exhibit herringbone structure morphology and the TEM images showed hexagonal crystallites ordered in regular spacing along the nanowires axis and the crystal planes of the nanowires were parallel to one of the facets of the crystallite. The other nanowires morphology is essentially single crystalline nanoribbons. The structures of the nanowires were characterized by SEM, TEM, XRD, EDX, and Raman spectroscopy.   

A Generic Synthesis of Transition Metal Oxide Core-Shell Nanowires

A generic nonequilibrium synthesis technique has been developed to produce novel transition metal oxide nanowires, including YBa$_{2}$Cu$_{3}$O$_{6.66}$, La$_{0.67}$Ca$_{0.33}$MnO$_{3}$, PbZr$_{0.58}$Ti$_{0.42}$O$_{3}$ and Fe$_{3}$O$_{4}$. Key to our success is the growth of vertically aligned single-crystalline MgO nanowires, which worked as excellent templates for epitaxial deposition of the desired transition metal oxides and led to high-quality core-shell nanowires. Transport studies on La$_{0.67}$Ca$_{0.33}$MnO$_{3}$ nanowires have revealed the remarkable persistence of metal-insulator transition and magnetoresistance down to nanometer scale. Our technique will enable various in-depth studies such as phase transition in nanoscale oxides and may pave the way for novel applications of these fascinating materials.   

Formation of super arrays of periodic nanoparticles and aligned ZnO nanorods - simulation and experiments

It had been demonstrated that large-scale honeycomb-like nanoparticle arrays could be fabricated inexpensively by the process of monolayer nanosphere self-assembly. Here we report that a double-layer masking procedure can be effectively used to overcome the restriction of honeycomb order in an array resulted from a monolayer mask. By varying the relative angle between the two layers, different arrangement of nanoparticles could be obtained. The relative angle can be directly controlled with the aid of diffraction patterns from illuminating the layers by a laser beam. Experimental results were fully confirmed by computer simulations. Using these nanoparticles as catalysts, we have grown arrays of aligned ZnO nanorods with various orders.   

Session U26: Amorphous and Nanocrystalline Materials: Theory and Experiment

Theory of hydrogen related meta-stability in disordered silicon

Density functional electronic structure calculations are employed to examine hydrogen for a variety of configurations in silicon. A novel complex is found for hydrogen in amorphous silicon. The complex involves the breaking of weak silicon bond to form two Si-H bonds with both hydrogens in between the original silicon atoms. This complex provides a microscopic model for new metastable complexes observed in amorphous silicon. Mechanisms for hydrogen-related metastability will be discussed for amorphous and ppoly-crystalline silicon.   

Nucleation and growth simulation of Si nanocrystals in Si-rich oxide

Ion implantation profile in SiO2 layers on Si substrate calculated by TRIM was used as initial conditions for a diffusion-driven nucleation and growth model. Nucleation was initiated at randomly chosen seeds satisfying the Poisson distribution, which were used as centers in a Voronoi tessellation of space. The volume fraction of implanted Si ions in each Voronoi polyhedron was calculated based on the polygon volume and the fluence. The nanocrystal growth was considered to occur at the nucleation centers by diffusing of Si ions, which become localized at the seeds. Zero net flux across each of the surfaces of the Voronoi polyhedra was assumed for the diffusing Si species. The calculations were by with periodic boundary conditions in the directions normal to the implantation. We report size and depth distribution of the Si nanocrystals formed under the above conditions.   

Studies of Thermally Annealed Graphitic Amorphous Carbon Resulting in a Decrease of Quasi-Stone-Wales Defects and Increase in Bandgap

We used a novel vibrational dynamics model for planar disordered materials (the embedded ring approach) to determine the structural evolution of thermally annealed graphitic amorphous carbon (g-C). The vibrational model assumes that constituent atoms of a material are arranged in n-membered planar rings embedded in the effective medium, a continuous random network of atoms. Standard structural models of g-C-a ubiquitous form of disordered carbon present in the production of diamond films, fullerenes, graphenes, nanotubes, and graphite-suppose that g-C is composed primarily of a structural distribution of such carbon rings with 4 to 8 atoms. We have calculated the in-plane normal modes and frequencies for embedded carbon rings and used these frequencies to fit Raman spectra of g-C annealed to temperatures ranging from 22 \r{ }C to 1050 \r{ }C. From the relative intensities of the different frequency peaks, our procedures provide quantitative ring statistics for the structure of g-C. In particular, we have found that unannealed g-C can have many 5- and 7-membered rings, but that the fraction of 6-membered rings increases with annealing temperature consistent with the known result that g-C evolves to nanocrystalline graphite under high T annealing.   

Magnetic Rare Earth (Gd) Doped Amorphous Carbon

Previous studies on rare earth (RE) doped amorphous silicon ($a-$RE$_{x}$Si$_{1-x}$, RE=Gd, Tb) have shown remarkable physics for compositions near the three-dimensional metal-insulator transition: many orders of magnitude negative magnetoresistance (MR) at low temperatures, and a high onset temperature (T*) where the effects of magnetic dopants ``turns on.'' Both MR and T* are significantly reduced by substituting Ge for Si, an effect we suggest is due to the reduced band gap and consequently larger dielectric constant and larger electron screening of Ge. This suggestion is supported by a systematic decrease in T* and MR with increasing x and by data on ternary alloys (with non-magnetic Y additions). To test this theory, we have prepared samples of $a-$RE$_{x}$C. Amorphous C has the unique feature of a band gap which can be tuned by varying the sp$^{2}$/sp$^{3 }$bonding ratio. As anticipated, $a-$RE$_{x}$C$_{1-x}$ shows even larger negative MR at low temperatures and higher characteristic temperature T*. Chemical and structural properties were studied by RBS, TEM, Raman spectroscopy. The temperature and magnetic field dependence of conductivity and magnetic properties and comparisons to previous work will be discussed. Thanks to the NSF for support.   

Probing the Intermediate Range Order in Novel Rare Earth Phosphate Glasses Using Neutron Diffraction

Neutron diffraction has been used to study the atomic structure and especially the coordination environment of rare earth ions for (x)R$_{2}$O$_{3}$ (1-x)P$_{2}$O$_{5 }$,where R is praseodymium or neodymium and x ranges between 0.05 and 0.28. Such information can help further developing these exciting materials for potential optical and magnetic applications. In the case of neodymium containing samples, the method of isotopic substitution was used to measure the first order difference function involving only neodymium correlations. Merits of this technique as applied to rare earth phosphate glasses as well as the dependence of the atomic structure on the R/P ratio will be discussed.   

Velocity of sound and elastic properties of lanthanum gallo-germanate glasses

The velocity of sound of a group of lanthanum gallo-germanate glasses is obtained by the ultrasonic pulse-echo measurements, at room temperature. Both longitudinal and transverse velocities of these glasses are composition dependence. The experimental results are used to obtain the elastic constants. The correlation of elastic stiffness, the cross-link density, and the fractal bond connectivity of these glasses are discussed. The derived experimental values of Young's modulus, bulk modulus, shear modulus and Poisson's ratio for our glasses are compared with those theoretically calculated values in terms of the Makishima-Mackenzie model. A possible existence of both tetrahedral (four-fold coordination) and octahedral (six-fold coordination) of Ga and Ge in the structure of these glasses is discussed.   

Crystallization Behavior of Chemically Prepared Nanoparticles of Amorphous Fe-B*

The crystallization behavior of amorphous Fe-B nanoparticles prepared by reducing an aqueous solution of Fe$^{+2}$ ions with NaBH$_{4}$ has been studied by differential scanning calorimetry (DSC), vibrating sample magnetometry (VSM), and x-ray diffraction (XRD) measurements. At a heating rate of 10 \r{ }C/min the DSC measurements show a sharp and well defined exothermic peak at a temperature of about 475 \r{ }C and a Kissinger analysis of the shift in the position of this peak as a function of the heating rate yields an activation energy of about 3.6 eV/at. The VSM measurements also exhibit a sharp increase in the magnetization at a temperature 475 \r{ }C (at a heating rate of 10 \r{ }C/min). X-ray diffraction measurements on samples heated to temperatures slightly above 475 \r{ }C verify that the observed DSC and VSM signals correspond to the transformation from the as-prepared amorphous structure to a crystalline structure. * This work has been supported by AFRL DARPA METAMATERIALS contract no. F33615-01-2-2166, ARO DEPSCOR grant no. W911NF-04-1-0264, and the Undergraduate Research Program at the University of Delaware.   

{\textit In situ} Raman Scattering Studies of High-Pressure Stability and Transformations in the Matrix of a Nanostructured Glass-Ceramic Composite

High-pressure Raman scattering studies were performed on a glass-based composite with nanometer-sized gallium oxide aggregates embedded in a potassium-silicate host glass. The aim of our studies was to advance the understanding of pressure-driven structural transformations in the glass matrix of the composite. Throughout the studied pressure range the Raman spectra confirmed that the glass matrix undergoes a range of structural transformations comparable to that reported previously for a pure SiO$_{2}$ glass. Compression from ambient up to 10.8 GPa was completely reversible on decompression to ambient pressure. At higher pressures the Raman spectra demonstrated a breakdown of the intermediate-range order in the glass matrix and a permanent reduction in SiO$_{4}$ ring statistics toward smaller than six-ring configurations and a coordination change of the silicon atom. The overall spectral profile at the end of the decompression cycle indicated the occurrence of permanent reconstructive structural changes in the glass matrix.   

Absence of Dipole Glass Transition for Randomly Dilute Classical Ising Dipoles

Randomly dilute dipoles with long range dipolar interactions appear in a variety of solid insulating materials. Based on theoretical studies of spin glasses with long range interactions, one would expect such dilute dipolar systems to undergo a spin glass-like transition as the temperature decreases. However, there has been no experimental evidence for such a transition in very dilute systems. One example where such a transition has not been definitively observed is two level systems that dominate the physics of glasses at low temperatures. Another is LiHo$_x$Y$_{1-x}$F$_4$ with $x=4.5\%$. We have investigated the absence of a phase transition in dilute dipolar glasses. Using Wang-Landau Monte Carlo simulations, we show that at low concentrations $x$, dipoles randomly placed on a cubic lattice with dipolar interactions do not undergo a phase transition as the temperature decreases. We define a characteristic ``glass'' temperature $T_g$ as the temperature where the distribution $P(q,T)$ is flattest. $q$ is the overlap order parameter. We find that in the thermodynamic limit $T_g$ goes to zero as $1/\sqrt{N}$ where $N$ is the number of dipoles. The entropy per particle at low temperatures is larger for lower concentrations ($x=4.5\%$) than for higher concentrations ($x=20\%$).   

Discontinuous molecular dynamics study of the diffusion of fluids in dynamic random media

The static and dynamic properties of dimeric hard sphere fluids in random media are studied using discontinuous molecular dynamics. The media is composed of a random collection of hard spheres that are dynamic in the sense that they are connected by a string to their respective initial positions, and can move in the spherical volume defined by the length of the string, $l$. The fluid diffusion coefficient is calculated as a function of $l$ for different volume fractions of the fluid and media. In the $l\rightarrow 0$ and $l\rightarrow \infty$ limits, the system reproduces the limits of a fluid in a static media and in a hard sphere liquid, respectively. Much of the phenomenology of glass forming materials is reproduced by this model. For example, this model mimics experimental studies for impurity diffusion in glass forming materials. The diffusion behavior changes from a power law behavior (in $l$) above a critical $l$ to an Arrhenius behavior below this critical $l$.   

Glass Relaxation and the Dielectric Constant Of Moist, Porous Rock

The low-frequency, dielectric constants of moist, porous rocks are important properties related to hydrocarbon saturation and mineral exploration. Numerous theoretical and experimental studies have attempted to explain the unusually large values of the dielectric constant at frequencies below 1000 Hz. This study tests the hypothesis that a disordered arrangement of dynamically-coupled, electrochemical dipoles in the pore fluids explains the dielectric constant. The dynamic coupling leads to relaxation times on the order of tens of minutes and divergent dielectric constants as frequency approaches zero. Temperature-dependent relaxation measurements test and support the hypothesis.   

Terrace Selection at an Icosahedral Quasicrystal Surface

Quasicrystals are aperiodic, but well-ordered, intermetallics. Using scanning tunneling microscopy, we investigate the effects of annealing temperature on the structure of a fivefold surface of icosahedral Al--Pd--Mn. After annealing at 900-915 K shallow void-rich terminations are created although the density of the voids are nearly zero after annealing at 925-950 K. The terminations that are consumed by voids have a distinctive atomic local configuration, very similar to ``rings'' identified in the model of Papadopolos and Kasner [1]. During the coalescence and the growth of the voids, a different termination becomes exposed. We suggest that the shallow steps associated with the voids, and the rings, signal a surface that is at an intermediate stage of structural equilibration. These exposed terraces give us a new insight into the structure of quasicrystal. [1] Papadopolos, Z., et al., Phys. Rev. B 66, 184207 (2002)   

Structure and Phase Separation in Ultrathin Ag/Cu Amorphous Alloy System

The structure of disordered metallic alloys is an important but unsolved problem. Previous studies on Ag-Cu system showed that relatively homogeneous solid solutions formed at liquid nitrogen temperature decompose into separate phases or evolve into crystalline structure at a higher temperature. In this research project, we prepared ultra-thin Ag-Cu films on amorphous carbon support by HV magnetron sputtering with both targets. With high energy Ag and Cu atoms bombarding on the carbon substrate, they are forced to form amorphous alloy or nano-crystalline thin film at room temperature. We have investigated the structure of ultra-thin Ag-Cu films by examining their pair distribution function (PDF) using electron diffraction and observed phase separation process directly in STEM images. In the STEM Z-contrast images, since the contrast is directly related to the atomic number (Z) of the components, we can see clearly the phase separation process. Experimental results show that the sample morphology evolutions are different in samples with different thickness, and the phase separation depends on various Ag/Cu atomic ratios. In Ag$_{50}$Cu$_{50}$ sample, early stage phase separation is associated with increasing Cu crystallite size, indicates that Cu diffuse out of Ag-Cu solid solution phase.   

Amorphization of Aluminum Nanoparticles

The melting behavior of aluminum nanoparticles with an oxide passivation layer is examined using a differential scanning calorimetry (DSC). Both broad and narrow size-distributed particles are studied, and the weight-average particle radius ranges from 8 nm to 50 nm. With decreasing particle size, the melting response moves towards lower temperatures, as predicted by Gibbs-Thomson equation. The latent heat of fusion also decreases and is significantly smaller than that predicted by the surface tension; the heat of fusion is only 20 percent of the bulk value at our smallest particle size. An analysis suggests that a passivated aluminum nanoparticle of 6 nm radius will become amorphous and have no heat of fusion due to the presence of defects induced by the particle's small size. At the onset of amorphization, we calculate that in this system, approximately one defect will exist for every seven atoms.   

Session U27: Focus Session: Carbon Nanotubes: Growth

Critical silicon dioxide thickness for CVD growth of single-walled carbon nanotubes

Chemical vapor deposition (CVD) has shown remarkable control over the efficient and directed assembly of single-walled carbon nanotubes, making CVD a primary growth method for device applications. Due to the high temperatures involved in CVD, the chemical compatibility between the substrate, feedstock, and catalyst must be understood. Using x-ray photoelectron spectroscopy (XPS), we have studied the evolution of the chemical state of an iron nitrate catalyst during the initial temperature ramp of a standard CVD process. Heating the catalyst on clean silicon or on silicon with a native oxide leads to the formation of a silicide at 800~$^{o}$C, inhibiting single-walled nanotube growth. By 900~$^{o}$C, a typical growth temperature, all of the iron catalyst has been incorporated into the silicide. Thicker silicon oxide layers, on the order of 10~nm, effectively prevent silicide formation, enabling high yield growth.   

Evolution of the Catalyst Nanoparticles during CVD Growth of Carbon Single-Walled Nanotubes

Despite intense studies, the growth mechanism of carbon single- walled nanotubes (SWNTs) is still debated and current synthesis methods do not allow for full control over the growth. There has been much discussion of whether the active catalytic species are in the liquid or solid phase during SWNTs formation, which is a key to understand and to control the growth of these materials. However, the actual phase of the catalyst and its evolution during carbon SWNTs growth still has to be experimentally verified. We report the observation of carbon induced solid-liquid and solid--liquid-solid phase transitions of the iron nanocatalyst during the synthesis, using differential scanning calorimetry and Raman scattering measurements. We found that as long as the nanocatalyst is in a liquid state, SWNTs growth occurs and continues until its solidification. Moreover, no growth was observed below the eutectic point, when the catalyst is always in solid phase.   

Growth of vertically aligned multiwall and uniform carbon nanotubes on self-assembled ferromagnetic Fe and Co nanowires

A novel approach to grow vertically aligned and uniformly separated carbon nanotubes on self-assembled \textit{$\alpha $}-Fe is reported. We have previously demonstrated that the growth of LaSrFeO$_{3}$ perovskite oxide by Pulsed Laser Deposition under reducing environments leads to spontaneous formation of an array of single crystalline \textit{$\alpha $}-Fe nanowires embedded in an antiferromagnetic LaSrFeO$_{4}$ matrix. The diameter and spacing of these ferromagnetic nanowires can be controlled by changing the temperature of growth. We now show that these thin films containing self-assembled $\alpha $-Fe nanowires can be used as a template to grow vertically aligned carbon nanotubes using Plasma Enhanced Chemical Vapor Deposition. Acetylene (C$_{2}$H$_{2})$ and ammonia (NH$_{3})$ were used as a carbon source and diluting gas, respectively. Self-assembled \textit{$\alpha $}-Fe nanowires serve as a nucleation sites for the growth of vertically aligned multiwall carbon nanotubes (MWCNTs). The size of carbon nanotubes can be controlled by changing the diameter of \textit{$\alpha $}-Fe nanowires. The results of Transmission Electron Microscopy, Raman spectroscopy and field emission data will be presented. By means chemical mechanical polishing we can achieve atomically smooth surfaces and improve the quality of the carbon nanotubes grown on these nanowires.   

Comparison of Efficiencies of Binary and Ternary Composite Catalysts in Arc-Discharge Synthesis of Single-Walled Carbon Nanotubes

The catalyst composition is a major factor determining the efficiency of single-walled carbon nanotubes (SWNTs) synthesis. The binary catalyst composed of transition metal (TM) and rear earth metal (REM) proved to be the most efficient combination for the arc-discharge technique. Recently we proposed a quantitative procedure to assess the relative carbonaceous purity of bulk quantities of SWNT soot on the basis of solution phase NIR spectroscopy.[1] We applied this technique to obtain direct comparison of the efficiency of variety of binary and ternary TM-TM, TM-REM, TM-TM-REM composite catalysts for the arc-discharge synthesis. We found that substituting of either component of the most popular Ni-Y composition by different transition metal (Fe, Co) and rare earth metal (Se, La) affects significantly the efficiency of the SWNT synthesis and modifies the SWNT diameter distribution. This work is supported by DOD/DARPA/DMEA under Award No.DMEA90-02-2-0216. . [1] M.E. Itkis et al., \textit{Nano Lett.} \textbf{2003}, $3$, 309-314.   

Growth of Single Wall Carbon Nanotubes from Iron Oxide Catalyst

Carbon nanotubes are cylindrical nanostructures with hexagonal networks of carbon atoms with interesting electronic and mechanical properties. CNT synthesis route involving the catalytic decomposition of hydrocarbons on metal particles have been widely popular. We have shown that single wall carbon nanotubes of diameters less than 2 nanometers can be grown directly from catalyst particles comprising of maghemite ($\gamma $-Fe$_{2}$O$_{3})$ on a Silicon substrate using the conventional CVD process. The sizes of SWNT were measured using Atomic Force Microscopy. The average tube diameter was measured to be 1.0$\pm $0.2 nm. FTIR and X-ray diffraction measurements were performed to characterize the catalyst iron oxide. Scanning Electron Microscopy measurements revealed that the catalyst oxide particles formed in clusters of 100 nm diameters. Transmission M\"{o}ssbauer measurements at room temperature showed the presence of only a superparamagnetic doublet, characteristic of nanophase iron oxides. The crystallographic, morphological and magnetic properties of the catalyst metal powders and the properties of the resulting SWNTs will be presented. .   

Current-Controlled Nanotube Growth and Zone-Refinement

We present methods by which the growth of a single carbon nanotube (CNT) can be precisely controlled by an electrical current. In one method a CNT is grown to a predetermined geometry inside another nanotube, which serves as a reaction chamber. Another method allows a preexisting marginal quality multiwall CNT to be zone-refined into a higher quality multiwall CNT by driving a catalytic bead down the length of the nanotube, which can be many microns long. In both methods the speed of nanotube formation is adjustable, and the growth can be stopped and restarted at will.   

Highly nitrogen and boron doped nanotubes: a route to synthesis and study of their properties by spatially resolved EELS

Doping C-nanotubes with B and/or N is expected to be a particular interesting way for tuning electronic and mechanical properties. BN nanotubes are predicted to behave as insulators whereas B(N) doped C-nanotubes are expected to be metallic, independent of their structure. In this framework, we have developped, both at Onera and GDPC, original routes to the synthesis of BN singlewall nanotubes (BN-SWNTs) and to highly doped nitrogen multi wall nanotubes (CN$_{x}$-MWNTs). CN$_{x}$-MWNTs were produced by a CVD method, using an aerosol injector which sprays in the reactor, heated at 950\r{ }C, a liquid mixture of organic compounds with a controlled N/C ratio and suitable metal complexes as the catalyst precursors$^{1}$. This procedure leads to dense amounts of MWNTs with controlled N/C ratios which can exceed 15-20{\%} in average. Upon doping, tubes get a characteristic compartimentalized structure with a reduced number of layers identified in transmission electron microscopy. Using spatially resolved electron energy loss spectroscopy (EELS), N is found to be preferentially localized in inner layers and in the compartiments where the concentration can exceed 40 at.{\%}. Structure of core losses in EELS reveals a high dependance of the N environment to the local concentration :chemical bonding of N can be graphitic, pyridinic or pyrrolic, this latter case being found for highest N concentrations. Relationships between these structural properties and formation mechanism will be discussed$^{2}$. BN-SWNTs are issued from the vaporization of a BN target by a continuous CO$_{2}$ laser under a N$_{2}$ atmosphere$^{3}$. We present here the first investigation on their electronic properties by two ways: first, analysis of the dielectric response of low loss EELS recorded on individual tubes provides the first identification of plasmons and of interband transitions in these tubes and the first measure of their gap found to be close to 5.8eV$^{4}$. Second, optical absorption spectra measured on macroscopic samples strongly suggest the existence of a Frenkel exciton with a binding energy in the 1eV range$^{5}$. 1-M. Glerup et al Chem. Commun 2542 (2003). 2-M. Castignolles et al submitted to Phys. Rev B (2005) 3- R. Lee et al, Phys. Rev. B Rapid Comm 64, 121405-1 (2001) 4-R. Arenal et al, submitted to Phys. Rev. Lett. (2005) 5-J.S. Lauret et al, Phys. Rev. Lett. (2005) in press. Coauthors: M. Castignolles$^{1,2}$, R. Arenal$^{1}$, O. St\'{e}phan$^{3}$, M. Glerup$^{2,4}$,$^{ 1 }$LEM, CNRS-ONERA, Ch\^{a}tillon, France, $^{2 }$GDPC, Universit\'{e} Montpellier II, France, $^{3 }$LPS, Universit\'{e} Paris-Sud, Orsay, France, $^{4}$GHMFL, MPI-CNRS, Grenoble, France.   

Growth Kinetics of Water-Assisted Single-Walled Carbon Nanotube Synthesis-``Super-Growth''

Recently, we have reported the highly efficient synthesis of vertically aligned, highly dense and pure single-walled carbon nanotubes (SWNT) by chemical vapor deposition through the introduction of a small, controlled level of water in the growth ambient [1]. The dramatic increase in catalyst efficiency resulted in the growth of SWNT forests as tall as 2.5 millimeters in a 10-minute growth time. The need to fully utilize the catalytic enhancement by water requires the optimization of the growth conditions and the understanding of the growth mechanism. Here we report the growth kinetics describing water-assisted SWNT synthesis, which we attained through a systematic investigation of the yield [2]. Our extensive analysis of water-assisted growth revealed an unexpected simplicity, in that the growth evolution could be completely described by two characteristic quantities: the initial growth rate and the catalytic lifetime. Furthermore, our studies revealed how these quantities reflected changes in the relative water and ethylene levels. [1] K. Hata \textit{et al}, Science, \textbf{306,} 1362 (2004). [2] D.N. Futaba\textit{ et al}, Nature Materials (\textit{submitted}).   

Water-Assisted Highly Efficient Synthesis of Impurity-Free Single-Walled Carbon Nanotubes-``Super-Growth''

We demonstrate an extremely efficient chemical vapour deposition synthesis of single-walled carbon nanotubes where the activity and lifetime of the catalysts are enhanced by water [1]. Water-stimulated enhanced catalytic activity results in massive growth of super-dense and vertically-aligned nanotube forests with heights up to 2.5 millimeters that can be easily separated from the catalysts, providing nanotube material with carbon purity above 99.98{\%}. Moreover, patterned highly organized intrinsic nanotube structures were successfully fabricated. The water-assisted synthesis method addresses many critical problems that currently plague carbon nanotube synthesis. [1] K. Hata, et al., Science, 306, 1362 (2004).   

Atomic Step-Templated Formation of Single-Wall Carbon Nanotube Patterns

\textbf{Single-wall carbon nanotubes catalytically produced on miscut C-plane sapphire wafers grow along the 0.2nm-high atomic steps of the vicinal $\alpha ${\-}Al}$_{2}$\textbf{O}$_{3}$\textbf{ (0001)~surfaces, yielding highly aligned, dense arrays of discrete nanotubes on a dielectric material [1]. The nanotubes reproduce the atomic features of the surface, including steps, facets and kinks. Microscopy, X-ray diffraction and single-nanotube Raman spectroscopy [2] shed light into the possible structure and mechanism of the step-templated carbon nanotube growth. The orientation, density and morphology of the atomic steps can be macroscopically controlled by the crystal cutting process. Hence, these findings open up the possibility of assembling nanotube architectures by atomic-scale surface engineering.} [1] A. Ismach, et al.,\textit{ Angew. Chem. Int. Ed.} \textbf{2004}, 43, 6140. [2] M. Souza, et al., \textit{Phys. Rev. B} \textbf{2004}, 241403R.   

Ultra-thin films of Single-walled Carbon Nanotubes

Recently the fabrication and optical properties of homogeneous, uniform-thickness films of pure, single-walled carbon nanotubes sufficiently thin to be optically transparent were described. We will discuss progress in the fabrication of transparent films in which the nanotubes are aligned along an axis within the plane of the film. The uniformity of these films also provide additional opportunities, both for study of the nanotubes and for their application. We will describe results of electronic transport measurements in the pure and modified films that further distinguish their properties from those of common 3-D and 2-D systems.   

Thermal Conversion of Bundled Carbon Nanotubes into Graphitic Ribbons

The morphological evolution of purified bundled single-walled carbon nanotubes (SWNTs) heat-treated in a dynamic vacuum from T = 200-2200 \r{ }C is investigated by transmission electron microscopy, UV-Vis and resonant Raman spectroscopies. The coalescence of neighboring tubes was observed to begin at $\sim $1400 \r{ }C in both materials. HiPCO (ARC) tubes exhibited $\sim $ 100{\%} (70{\%}) coalescence of the tubes that survive 1600 \r{ }C. At $\sim $1800 \r{ }C, the ARC material exhibits a much stronger conversion to multiwall nanotubes (MWNTs) with $\sim $3-5 shells, and, in contrast to the HiPCO material, these MWNTs are often bundled and collapse into graphitic ribbons. To our knowledge, this is the first report of these multilayer nanofilaments. With increasing temperature, Raman scattering and TEM indicate a preferential early loss of small diameter tubes. The small d tubes in HiPCO material appears to produce fragments that coat the walls of the MWNTs and lead to a more structurally disorganized material at 2200 \r{ }C. Raman scattering spectra indicate that some coalesced SWNTs of d $\sim $ 2.5nm survive vacuum annealing for $\sim $6 h at 2000 \r{ }C.   

Rings of Functionalized Carbon Nanotubes: Characterization and Properties of a Novel Magnetic Material

Carbon nanotubes are a promising material for electronic, optical and biological applications. However, applications that require large nanotube quantities are hindered by difficulties in processability, due to the perennial presence of amorphous carbon impurities, and to the tubes' insolubility. Side wall covalent functionalization is a possible solution to generate soluble nanotubes but large scale purification remains a challenge. Here we show that single walled carbon nanotubes, appropriately functionalized, spontaneously assemble into rings of varying size. A thorough scanning probe microscopy analysis shows that each ring is formed by bundles of nanotubes and is composed by the assembly of shorter building blocks. Models explaining the ring formation will be presented. Also, magnetic measurements show that eddy currents flow through the rings when an alternating magnetic field is applied. Thus these rings exhibit a strong magnetic response both in solution and when assembled onto a substrate. We show that this property can be used to separate nanotubes from non-magnetic impurities; opening up new possibilities for large scale carbon nanotube purification.   

Growth of Vertically Aligned Carbon Nanotube Films: Single- versus Multi-walled

Vertically aligned high density small diameter carbon nanotube films were deposited by microwave CVD technique. The iron catalyst was prepared by E-beam evaporation on thermally grown silicon dioxide n-type Si(100) substrates. Experiments show that by continuous reduction in the thickness of Fe ($\sim $ 3-5), smaller diameter carbon nanotube can be achieved. Scanning electron and high-resolution transmission electron microscopy show that the diameter of carbon nanotubes ranged $\sim $ 1 - 5 nm and the films are comprised of both the single- and double-wall carbon nanotubes. Visible Raman spectroscopy was used to further verify the diameter of nanotubes. A thick iron film (80 nm) was also used to grow nanotubes for comparison. The results show that the catalyst islands become greater than hundred nanometers with increasing thickness and induce multi-wall and bamboo-like microstructures. While for thinner layer of iron films smaller sizes of catalyst particles/droplets produce hollow concentric tubes without bamboo structure and with less number of walls (single-wall and double-wall carbon nanotubes). The base growth was the most appropriate model to describe the growth mechanism for our films. The electron field emission properties such as field electron emission microscopy (FEEM) in conjunction with the temperature dependence (T-FEEM) were measured to investigate the emission site density and their intensity variation. These findings in terms of the role of adsorption will be briefly discussed.   

Session U29: Organic Light Emitting Diodes

High Efficiency Organic Light-Emitting Devices Having Charge Generation Layers

A new type of organic LEDs having charge generation layers (CGLs) were developed. By applying voltage, holes and electrons are generated at CGL and injected to adjacent organic layers to recombine with the carriers with opposite polarity. Thus, current efficiency can be greatly improved. An extremely high current efficiency of 130 cd/A, which is equivalent to more than 30 % external quantum efficiency, was observed from a device. One of the devices exhibited the lifetime of over 1,000,000 hours at the initial luminance of 100 cd/m2.   

White polymer LED and its integration with polymer transistor

Bright white emission with peak luminance near 10,000 cd/m$^{2}$ is achieved in multi-layer homojunction polymer light-emitting diode (PLED) fabricated by multiple spin coating. The homojunction has the advantages of exciton confinement, carrier balance, and reduced cathode quenching. In order to be applied in an all-polymer active matrix display, multi-layer PLED is integrated with polymer transistor to form a polymer active pixel without the patterning of any polymer layer. The key idea is to replace the conventional conductive hole-transport layer (HTL) for the PLED by a semiconductor, which can then be shared with the transistor in the integrated structure. In this integration both the semiconductor layer and the emissive layer can be spin-coated in large area covering the whole active matrix. We use high mobility polymer polythiophene for the HTL and the transistor. Peak luminance of 3000 cd/m$^{2}$ for white emission on P3HT is reached. A 200 $\mu $m$\times \mu $m polymer active pixel free of patterning of any organic layer is demonstrated.   

High Performance White Organic Light-Emitting Diodes

White organic light-emitting diodes (OLEDs) are being considered as potential solid-state lighting sources. Some of the challenges toward that goal include low-cost fabrication of white OLEDs with high brightness and efficiencies using simple device architectures. White OLEDs were fabricated from multilayers or blends based on poly(9,9-dioctylfluorene) (PFO) and poly(2-methoxy-5(2'-ethyl-hexyloxy)-1,4-phenylenevinylene) (MEH-PPV). Insertion of a non-emissive polymer buffer layer between MEH-PPV and PFO allowed regulation of energy transfer between the two emissive polymers, affording efficient white OLEDs. Bright white light with a brightness of 1446 cd/m$^{2}$, an external quantum efficiency (EQE) of 0.94{\%}, and a device efficiency of 1.1 cd/A was observed. The polymer blend OLEDs gave white light with CIE coordinates of (0.33, 0.34), a luminance of 4000 cd/m$^{2}$, an EQE of 3.1{\%} and a luminous efficiency of 3.7 cd/A. The emission color and the performance of the blend devices were highly dependent on the composition and the morphology of the blends.   

Highly Efficient Blue Electroluminescence from n-Type Conjugated Oligoquinolines

Achievement of blue electroluminescence (EL) with high efficiency, color purity and stability remains a challenge for full-color organic light-emitting diode (OLED) based displays. A series of n-type, thermally robust (glass transition temperature T$_{g} \quad >$ 130 \r{ }C) oligoquinolines based on the 6,6'-bis(4-phenylquinoline) core has been synthesized and used as emissive and electron transport materials for blue OLEDs. Simple bilayer diodes gave stable blue EL with CIE coordinates at (0.15, 0.16), maximum luminance of 4000 cd/m$^{2}$ and luminous efficiency of 7.9 cd/A (at 945 cd/m$^{2})$. These results represent one of the best blue OLED performances reported to date from non-doped, fluorescent organic emitters. The high T$_{g}$s render the amorphous oligoquinoline films very stable with excellent EL spectral stability. These results demonstrate that oligoquinolines are promising blue emitters and electron transport materials for developing high-efficiency OLEDs with a simple architecture.   

Electroluminescent devices from ionic transition metal complexes

During the last fifteen years dramatic advances have been achieved in the performance of organic light emitting diodes (OLEDs), and these devices can now be found in several consumer electronic products. A recent trend in OLEDs involves the use of ionic transition metal complexes as the electroluminescent layer. The mechanism of operation of OLEDs based on these materials is determined by a complex interplay between ionic and electronic charge. As a result of this interplay, efficient devices can be fabricated using air-stable electrodes. Moreover, large-area lighting panels that operate straight from the outlet without any additional circuitry can be fabricated. Materials issues that need to be addressed for these devices to succeed in applications will be discussed.   

A Time-Dependent Density Functional Theory Study of One-and Two-Photon Absorption: Stilbene- and Fluorene-Based Donor-Acceptor Chromophores

In our ongoing theoretical studies to predict the photophysical properties of optical materials, we report the results for one-photon, and two-photon absorption (OPA and TPA) spectra, for a series of compounds, in which electron donating and accepting groups are attached to a core having a delocalized electron structure, such as stilbene or fluorene. Linear response time-dependent density functional theory, with hybrid exchange-correlation functionals, was applied in all calculations. We find that the calculated excitation energies are generally in good agreement with experiment, particularly when compared to measurements carried out in a nonpolar solvent. Predicted TPA cross-sections, applying the two-state approximation, are also in relatively good agreement with experiment; however, a lack of systematic experimental data on solvent effects limits a detailed comparison as yet.   

Optically Detected Magnetic Resonance (ODMR) Studies Critical to the Determination of the Yield of Singlet Excitons in Fluorescence-Based OLEDs

Recent ODMR studies, including (1) photoluminescence (PL)-detected magnetic resonance (PLDMR) of small $\pi $-conjugated molecules, (2) electroluminescence (EL)- and electrically-detected magnetic resonance (ELDMR and EDMR, respectively) studies of small molecular OLEDs, (3) double modulation-PLDMR studies of $\pi $-conjugated polymers, and (4) joint PLDMR and thermally stimulated luminescence (TSL) studies of $\pi $-conjugated polymers are reviewed. The results of each of these studies are inconsistent with the model in which the positive spin 1/2 (polaron) resonance is due to enhanced delayed PL from nongeminate polaron recombination (``the delayed PL model''). Since the delayed PL model is the basis for the previous ODMR studies which predicted the yield of singlet excitons (SEs) in OLEDs, the recent ODMR studies reopen this issue. It is shown that all of the ODMR results obtained to date are consistent with ``the quenching model,'' in which the population of polarons and triplet excitons (TEs) is reduced by magnetic resonance conditions, and leads to reduced quenching of SEs by polarons and TEs. A detailed quantitative model confirms that the mechanism which causes the reduction in the polaron and TE population is the enhanced annihilation of TEs by polarons, whose populations are much larger than that of SEs under normal excitation conditions. *Operated by Iowa State University for the US Department of Energy under Contract No. W-7405-Eng-82.   

Quenching of Photoluminescence and Electroluminescence in OLEDs by Exciton-Charge and Exciton-Dopant Interactions

Electronic dopants such as the strong acceptor F4-TCNQ are used for p-type doping of hole transport layers (HTL) in organic light-emitting diodes (OLEDs). These molecules are found to quench the electroluminescence (EL) if they diffuse into the emissive layer. We have observed EL quenching in OLEDs with a HTL doped with F4-TCNQ. To separate the effects of exciton-dopant quenching from exciton-polaron quenching we have intentionally doped the emissive layer of Alq3 with three acceptors (A) of different electron affinity: F4-TCNQ, TCNQ and C60. We have also taken photoluminescence spectra of Alq3 films doped with identical concentrations of the three acceptors in order to separate the effects of these dopants on electroluminescence and photoluminescence. These results are presented, and channels for energy and charge transfer between excitons and both neutral and charged dopant molecules are described.   

Chain Conformations and Photoluminescence in Poly(di-$n$-octylfluorene)

The diverse steady-state spectroscopic properties of poly(di-$n$-octylfluorene) are addressed from a molecular-level perspective. Modeling of representative oligomers support the experimental observation of at least three distinguishable classes of conformational isomers with differing chain torsion angles. One class appears to be populated by a single compact structural isomer and this appears to conform to the so called $\beta$ phase. A rigorous Franck-Condon analysis of the photoluminescence in conjunction with Frenkel-type exciton band structure calculations are performed. These results accurately reproduce all major spectral features of the photoabsorption and those of singlet exciton emission.   

Enhanced triplet formation in polyfluorene blends

Formation of triplet excitons may be an important loss mechanism in organic light-emitting diodes (LEDs) and photovoltaics. Here we use photoinduced absorption spectroscopy to study the generation of triplet excitons after photoexcitation of a blend of the fluorene-based conjugated polymers poly(9,9'-dioctylfluorene-co-benzothiadiazole) (F8BT) and poly(9,9'-dioctylfluorene-co-bis-N,N'-(4-butylphenyl)-bis-N,N'-phenyl-1,4-phenylene-diamine) (PFB). The triplet generation rate is found to be $\sim $10 times higher in F8BT:PFB than in F8BT alone. We attribute this effect to increased intersystem crossing in the charge-separated states formed at the polymer/polymer heterojunctions in the blend. Applying an electric field dissociates these states and thus reduces the rate of triplet state formation. We will discuss the implications of this result for the operation of polymer blend LEDs and photovoltaics.   

Aggregation can enhance the O/PLED efficiency

In general, the aggregation effects are expected to quench the luminescence. Here, we show in two instances that the intermolecular interaction can enhance the O/PLEDs internal quantum efficiency. At first instance, for the organic LEDs, the siloles molecules exhibit exotic emission behavior, namely, non-luminescent in solution form, but highly luminescent in aggregation. After a detailed theoretical calculation on the non-adiabatic decay rate, we find that it is the aggregation inhibit the radiationless route, thus allowing the radiative decay in solid state. In terms of PLEDs, we take into account both the electronic correlation and electron-phonon coupling, and we find that the interchain coupling effects can actually allow PLEDs to have much higher internal quantum efficiency than the 25{\%} spin statistical limit.   

Field-induced switch from heterojunction to bulk charge recombination in bilayer light-emitting diodes

Optoelectronic devices made from semiconductor polymers often employ partially phase-separated binary polymer blends with distributed heterojunctions in the polymer film. We investigate the photo- and electroluminescence from bilayers of electron- and hole-transporting polyfluorene derivatives at different device temperatures. For low driving voltages (below 2.4$\times$10$^{5}$\,V/cm$^{2}$ at room temperature), we give direct evidence for barrier-free charge capture at the heterojunction. In this mechanism, charge capture produces an interfacial excited state (exciplex) directly and bulk exciton electroluminescence is only achieved through endothermic transfer (activation energy 200\,meV) from the exciplex. For high driving voltages (above 8.3$\times$10$^{5}$\,V/cm$^{2}$ at 43\,K), however, we find that charges are injected over the heterojunction barriers and subsequent charge capture occurs in the polymer bulk. Furthermore, if bulk excitons migrate to another heterojunction site within their lifetime they are re-trapped at the interface and again form exciplex states or dissociate completely. We demonstrate that in polymer blend light-emitting diodes this can reduce the exciton population by more than 70\% and strongly influences the emission spectrum. We then analyze exciton re-trapping in detail using time-resolved photoluminescence spectroscopy on blends with different morphologies and find that for nm-scale phases exciton emission is completely suppressed.   

Physics of Electroluminescent Devices Based on Ionic Transition Metal Complexes

A recent trend in OLEDs involves the use of ionic transition metal complexes as the electroluminescent layer. The mechanism of operation of OLEDs based on these materials is determined by a complex interplay between ionic and electronic charge. We carry out forward time integration of rate equations to solve the bipolar current problem in the presence of ionic charge. We discuss the transient behavior of the current and radiance, as well as steady-state parameters (electric field and charge distribution, recombination profile) as a function of the mobilities of electronic and ionic carriers.   

Session U30: Polymer Thin Films: General

Rheological Response of Ultrathin Polymer Films

A novel microbubble inflation experiment has been developed for the purpose of measuring the biaxial creep compliance response of ultrathin polymer films. Here we have performed experiments on two polymeric films, poly(vinyl acetate) (PVAC) and polystyrene (PS) having thicknesses ranging from 25 to 100 nm. Three findings come from these studies. 1) PVAc films show a segmental dynamics that is quantitatively the same as in the bulk with no reduction in the glass temperature T$_{g}$ for even the thinnest films; 2) early results suggest that for the thinnest PS films the T$_{g}$ decreases by up to 30 $^{o}$C which demonstrates dramatically and quantitatively that the segmental dynamics of nanometer thick polymer films do not follow a universal behavior, i.e., the results seem to depend on chemical structure; 3) for both polymers we find that the creep compliance increases from the glassy value to a rubbery plateau that is greatly reduced from the bulk behavior, i.e., the stiffness is increased. This result suggests that entanglements in ultra thin films are much more effective than they are in the bulk---perhaps because of the two-dimensional aspect of the film.   

Direct measurement of the counterion distribution within swollen polyelectrolyte films

A depth profile of the counterion concentration within thin polyelectrolyte films was measured \textit{in-situ} using contrast variant specular neutron reflectivity to characterize the initial swelling stage of the film dissolution. We find a substantial counterion depletion near the substrate and an enrichment near the periphery of the film extending into the solution. These observations challenge our understanding of the charge distribution in polyelectrolyte films and are important for understanding film dissolution in medical and technological applications.   

Crumpling and uncrumpling of thin polymer films

With a view to study the forced crumpling of thin elastic sheets, we have developed a procedure for creating thin copolymer films of thickness, $h \quad \sim $ 50nm and lateral dimensions $X \quad \sim $1cm x 1cm. This geometry yields an aspect ratio, $X/h \quad \sim $ 2x10$^{5}$, much closer to the theoretical idealization of infinitely thin sheets than can be obtained with macroscopic sheets. Even though the films are exceedingly thin, they are robust enough that they can be mechanically manipulated without tearing. The films are crumpled by forcing them into small volumes. The copolymer is tagged with a fluorescent dye that allows us to study the three-dimensional crumpled structure of the film by confocal microscopy. The film can then be uncrumpled at an oil-water interface to study the statistics of the plastic creases created in the course of the crumpling process. We will also show data on the kinetics of uncrumpling - a little-studied process with many promising applications. We acknowledge support from NSF-DMR 0305936 and the REU program at the polymer science MRSEC at UMass-Amherst   

Grain Structure in Block Copolymer Thin Films Studied by Guided Wave Depolarized Light Scattering

A new optical technique for characterizing the grain structure of ordered block copolymer thin films has been developed. The technique is an adaptation of previous work wherein polarized light was used to characterize the grain structure in bulk block copolymer samples. Thin films of a cylindrically ordered poly(alpha methyl styrene-\textit{block}-isoprene) copolymer were prepared on flat fused silica substrates. A plane-polarized laser beam was coupled into and out of a transverse magnetic (TM) mode of each film, which acts as planar waveguide. The polarization of some of the incident light changes due to encounters with randomly oriented optically anisotropic grains. This results in the coupling of light into transverse electric (TE) modes in the sample. We show that the TE intensity of samples with well-developed grain structure is significantly larger than that obtained from samples with poorly developed grain structure.   

Swelling and surface modification of ultrathin chitosan films

Chitosan is a biodegradable polysaccharide derived from seashell waste products. The high water absorbency and biocompatibility of chitosan have enabled its use as a hydrogel in specialty biomedical applications. We present the results of several experiments focused on characterizing properties of ultrathin films of chitosan critical to their use in techniques such as wound dressings, medical implants and drug delivery systems. Uniform thin films with thicknesses of 15 to 600 nm and rms roughness of the order of 1 nm were prepared using techniques previously developed in our research group. The swelling of these films in the presence of high humidity has been characterized using reflection ellipsometry, atomic force microscopy and quartz crystal microbalance techniques. The effects of exposure to elevated temperature and UV/ozone (a common surface modification technique) on the surface properties such as hydrophobicity are described.   

Surface Tension Driven Laser Lithography of Thin Polymer Films

We have developed a technique for the non-destructive laser lithography of supported polymer films. Using a focused laser beam we induce a sharp thermal gradient within the film, which leads to a variation in surface tension across the free surface. As a result of the gradient in the surface tension, material flows away from the center of the beam via the thermo- capillary effect. This non-destructive process can be used to rapidly write trenches and patterns in polymer films, with fine control over the spatial dimensions and features (width, depth, etc.). The experimental results will be discussed in the context of a linearised model which predicts the topography of the feature as a function of experimental parameters.   

Photoinduced Trans-Cis Isomerization of Azobenzene Probes Tagged to Polystyrene in Thin and Ultrathin Films

For the last decade, molecular motion in confined polymer systems have been extensively studied. In the most studied case, polystyrene (PS), a conclusion obtained is that glass transition temperature in the PS thin and ultrathin films is lower than the corresponding bulk value. Although why the enhanced mobility must be manifested in the thin and ultrathin states is far from clear on molecular level for the moment, it is no wonder the presence of the free surface is one of responsible factors in the active molecular motion. In this study, photoinduced transcis isomerization of azobenzene chromophores in PS films was examined as a function of film thickness. The photoisomerization was composed of fast and slow modes. The fractional amount of the fast mode increased with decreasing thickness, implying that an effect of the surface becomes remarkable with decreasing thickness. Finally, local free volume and molecular motion with a relatively small scale in the surface region were discussed. Such enables us to gain access to information about hierarchical molecular motion in the surface region of polymer films.   

Single chain structure in thin polymer films: corrections to Flory's and Silverberg's hypotheses

Conformational properties of polymer melts confined between two hard structureless walls are investigated by the bond-fluctuation model.Chain extension,bond-bond correlation function and structure factor are computed and compared with recent theoretical approaches attempting to go beyond the Flory's and Silverberg's hypotheses. We demonstrate that for ultrathin films the chain size parallel to the walls diverges logarithmically, $R^{2}/2N \approx b^{2}+d\log N$ with $c\sim 1/H$.The corresponding bond-bond correlation function decreases like a power law $C(s)=d/s^{\omega}$,being $s$ the curvilinear distance between bonds and $\omega=1$.Upon increasing $H$ we find--at variance with Flory--the bulk exponent $\omega=3/2$ and,more importantly,a strongly decreasing amplitude $d(H)$ that gives direct evidence of an {\em enhanced} self-interaction of chain segments reflected at the walls.Systematic deviations from the Kratky plateau as a function of $H$ are found for the single chain form factor parallel to the walls in agreement with the {\em non-monotoneous} behavior predicted by theory. For large $H$ the deviations are linear with the wave vector $q$ but very weak.In contrast,for thin films, very strong corrections are found (albeit logarithmic in $q$) suggesting a possible experimental verification of our results.   

Photoemission studies of the photo-degraded polyethylene and polystyrene ultrathin films

The structural degradation induced by photon radiation in polyethylene and polystyrene ultrathin films has been investigated by ultraviolet photoemission spectroscopy (UPS) and molecular orbital (MO) calculations. The UPS results of pristine and degraded films show very good agreement with the calculated density of states by model MO calculations. The UPS and MO calculations reveal double bond conjugation and cross linking structure. Even though a considerable difference exists in the pristine chemical structure of both polymers, the photodegraded films exhibit very similar UPS spectrum, which indicates that the chemical structure of polyethylene and polystyrene films becomes indistinguishable after the prolong degradation.   

Crystalline polymer thin films characterized with NEXAFS dichroism microscopy

The sensitivity of Near Edge X-ray Absorption Spectroscopy (NEXAFS) to bond orientation holds the promise that it can be used in a conjunction with an x-ray microscope to the study the organization of thin films of semi-crystalline polymers. Linear Medium Density Polyethylene (LMDPE) ($\rho $=0.95 g/cm$^{3})$ has been processed into thin films 20-60 nm thick, which were subsequently recrystallized, and characterized with the 5.3.2 x-ray microscope at the Advanced Light Source. Films thicker than 35 nm show spherulitic crystals with primarily edge-on lamellar orientation. Films 25 nm thick, show feather-like structures with significantly more flat-on lamellar character. The results show that improved sample handling should be implemented to allow for in-situ sample rotation. This would significantly improve the sensitivity to small title angles of the carbon-carbon backbone relative to the surface normal.   

Dynamics of Thin Film Mixtures from Incoherent Neutron Scattering

We examined the influence of film thickness on the segmental dynamics of thin film mixtures of polystyrene (PS) and tetramethylbisphenol-A polycarbonate (TMPC) on Si/SiOx substrates using incoherent elastic neutron scattering. By fitting the elastic scattering intensities to the Debye-Waller factor, a mean square atomic displacement (MSD) was calculated. The MSD was found to decrease with decreasing film thickness. Dissipative motions, such as those associated with the glass transition, are manifested as ``kinks'' in the curve of elastic scattered intensity (or MSD) versus temperature. The glass transition temperature was determined to decrease with decreasing film thickness despite the decrease in the segmental mobility with decreasing film thickness. The values of Tg extracted from the neutron scattering are in quantitative agreement with prior Tg measurements made using ellipsometry. These results are examined in light of existing models on the thin film glass transition.   

Diffuse X-ray Scattering from Polystyrene Films

The diffuse x-ray scattering from a series of thin polystyrene (PS) films spun cast onto Si substrates has been measured. A standing wave technique was used to decompose the measured x-ray scattering into the contribution from the surfaces of the film and the contribution from density fluctuations in the films interior. The scattering from the interior yields the compressibility of the film. For thick films (100 nm) the compressibility is found to equal the bulk value. For thinner films the compressibility is increases with decreasing thickness up to 20{\%} for the thinnest film measured (35 nm). Additional diffuse scattering was found to originate from both the top and bottom interfaces. When the contribution from surface capillary waves is taken into account there is a residual scattering, which is of similar magnitude at each interface. We attribute this scattering to a near surface region in the polymer where there is incomplete chain interpenetration.   

Qualitative Discrepancy Between Motion on Different Length Scales in Thin Polymer Films

Ellipsometry is used to measure the interface healing in two- layer polystyrene films at different annealing temperatures. Since the interface healing involves center of mass motion, it serves as a probe of chain diffusion in thin PS films. Using an appropriate model, the time constant of the interface healing can be obtained. The results indicate that at temperatures above the glass transition temperature, as the thickness of layers is decreased, the time constant of interface healing increases, showing slower chain motion. Ellipsometry is also used to measure the glass transition temperature of the same films. Although the chain motion is slower in these films, the Tg reduction indicates enhanced dynamics. This study shows than not all measures of dynamics can be used to determine the Tg.   

In-Situ Hot Stage Atomic Force Microscopy Study of Poly(E-Caprolactone) Crystal Growth in Ultrathin Films

Morphologies, growth rates and melting of isothermally crystallized ultrathin (200 to 1 nm) poly(e-caprolactone) (PCL) films have been investigated in real-time by atomic force microscopy. The flat-on orientation of the lamellar crystals relative to the substrate was determined by electron diffraction. The truncated lozenge shape PCL crystals observed at low undercooling become distorted for films of thicknesses equal or thinner than the lamellar thickness, which depends on the crystallization temperature but not on the initial film thickness. The melting behavior of distorted crystals differs from that of undistorted ones, and their growth is slower and non-linear. The crystal growth rate decreases greatly with the film thickness. All these observations are discussed in terms of the diffusion of the polymer chains from the melt to the crystal growth front.   

Polymer Crystallization in Ultrathin Films

Confinement of a polymer to a thin film can dramatically alter the morphology and crystallinity. In this study, Brewster Angle Microscopy (BAM) is used to follow the dendritic crystallization of poly ($\varepsilon $-caprolactone) in Langmuir monolayers at the air/water interface. In a separate study, atomic force microscopy (AFM) and reflection absorption infrared spectroscopy (RAIRS) on Langmuir-Blodgett (LB) films show poly($_{L}$-lactic acid) form nearly 100{\%} crystalline single chain helices. These studies identify two model systems for studying crystallization and enzymatic degradation in ultrathin systems.   

Session U31: Polymers and Filaments for the Cytoskeleton

The response to point forces in cytoskeletal networks

Networks of semiflexible polymers that are cross-linked densely on the scale of their thermal persistence length form the structural basis of the cytoskeleton. These cytoskeletal networks, together with various cross-linking and other associated proteins largely determine the (visco-)elastic response of cells. We have found that semiflexible networks show a much more complex elastic response than traditional gels constructed of flexible polymers. In particular the both geometry of the deformation field under uniformly imposed shear stress and the effective shear modulus depend sensitively on the length of the constituent filaments relative the ``nonaffinity length'' that is a function of both the filament bending modulus and cross-linker density. In this talk I discuss the elastic Greens function in semiflexible networks, i.e. the response of these networks to localized forces. These investigations further highlight the role of the nonaffinity length and will improve our understanding of the action of molecular motors in the cytoskeleton. They will also facilitate the interpretation of microrheology data in semiflexible networks and in the cell.   

Mechanical Response Study of Collagen by means of Molecular Simulation

We developed a coarse-grained model to study mechanical behavior of collagen fibrils as a function of their degree of cross-linking. A collagen molecule is represented by Lennard-Jones beads, which intra-molecularly are connected through harmonic springs on both bond length and angle. In this model each bead represents a helical turn in a collagen molecule. Triple-helical collagen molecules, which are 300 \textit{nm} long, are packed within fibrils in a staggered fashion with an axial spacing of 67 \textit{nm} in the absence of a load on the tendon. We treat the outer layer or shell different from the core by assuming the shell has the maximum amount of available cross-links. The core has a variable amount of cross-links by allowing cross-link formation and breakage depending on a reaction-type criterion. We study the stress-strain behavior of a single fibril through tensile deformation along the principal axis and a three-point bend perpendicular to the principal axis.   

Forced unfolding of protein domains determines cytoskeletal rheology

Cells have recently been shown to have a power-law dynamic shear modulus over wide frequency range; the value of the exponent being non-universal, varying from 0.1-0.25 depending on cell type. This observation has been interpreted as evidence for the Soft Glassy Rheology (SGR) model, a trap-type glass model with an effective granular temperature. We propose a simple, alternative model of cytoskeletal mechanics based on the thermally activated, forced unfolding of domains in proteins cross-linking a stressed semi-flexible polymer gel. It directly relates a cell’s mechanical response to biophysical parameters of the cytoskeleton’s molecular constituents. Simulations indicate that unfolding events in a random network display a collective self-organization, giving rise to an exponential distribution of crosslink stress that can reproduce cell viscoelasticity. The model suggests natural explanations for the observed correlation between cell rheology and intracellular static stress, including those previously explained using the tensegrity concept. Moreover, our model provides insight into potential mechanisms of mechanotransduction as well as cell shape sensing and maintenance.   

Structure and Interactions in Neurofilament Networks

Neurofilaments (NFs) are a major constituent of nerve cell axons that assemble from three subunit proteins of low (NF-L), medium (NF-M), and high molecular weight (NF-H) to form a 10 nm diameter rod with radiating sidearms. The sidearm interactions result in an oriented network of NFs running parallel to the axon. Here, we reassemble NFs \textit{in vitro} from varying weight ratios of two of the subunit proteins, NF-L and NF-M, purified from bovine spinal cord. We demonstrate the formation of the NF network where synchrotron x-ray scattering (SSRL) reveals a well-defined interfilament spacing, while the defect structure in polarized optical microcopy shows the liquid crystalline nature. The interfilament spacing varies depending on NF-M sidearm density and we relate this change to sidearm interactions. We show that at a low density of sidearms, repulsive forces dominate creating a lattice spacing that is regulated by the buffer volume. With an increasing sidearm density, the equilibrium interfilament spacing decreases as a result of competing repulsive and attractive forces. Supported by NIH GM-59288, NSF DMR- 0203755, {\&} CTS-0404444.   

Electrostatic self-assembly between biological polymers \& macroions: Interactions of F-actin \& DNA with lysozyme

The pathological self-assembly of polyelectrolytes such as DNA and F-actin with cationic antimicrobial proteins such as lysozyme may have significant clinical consequences in Cystic Fibrosis (CF) lung infections. Wild-type lysozyme is a compact, cationic, globular protein which carries a net charge of +9e at neutral pH. Our Small Angle X-ray Scattering (SAXS) experiments on F-actin-lysozyme complexes indicate that the wild-type lysozyme close packs into 1-D columns between hexagonally organized F-actin filaments. We will present SAXS results of the interactions of F-actin and DNA with genetically engineered lysozyme mutants that carry a reduced charge of +5e. We have also used fluorescence microscopy to investigate the morphologies and sizes of such bundles induced with divalent cations, wild-type lysozyme, and mutant lysozymes.   

Phase Behavior of F-actin

To better understand the close spatial proximity of F-actin (filamentous actin) bundles to other structures comprised of F-actin in cellular environments, we have measured the phase boundary between F-actin and F-actin bundles as a function of spermine concentration. To do this, we first grew actin filaments by adding MgCl$_{2}$ to G-actin (globular actin). F-actin was then incubated with spermine (a low-binding-energy linker and actin-bundling factor) overnight, and then the samples were spun at low speeds to separate bundles from unbundled F-actin. The relative amounts of actin in the pellet and supernatant were determined via gel electrophoresis, yielding a description of the bundling transition as a function of actin and spermine concentrations. With this approach, we are constructing a phase diagram for the F-actin/spermine system. Surprisingly, the dependence of bundle formation on actin concentration is small to non-existent. At the actin concentrations we studied (4.5, 9, 18 and 36$\mu $M), actin tends to form bundles at the same spermine concentration. This observation calls for the evaluation of the effect of the ambient Mg$^{2+}$ in solution (added to polymerize actin) on actin bundling by spermine.   

Phase behavior of semidilute polyelectrolyte mixtures of F-actin and DNA

We investigate the phase behavior of semidilute mixtures of polyelectrolyte DNA coils and F-actin rods. F-actin has a persistence length of $\sim $10 microns and a linear charge density of -1e/0.25nm. DNA has a persistence length of $\sim $50nm and a linear charge density of --1e/0.17nm. Confocal and polarized microscopy data show that actin-DNA phase separates into ribbon-like birefringent domains of nematic F-actin and a disordered mesh of DNA coils. Synchrotron Small Angle X-ray Scattering (SAXS) show that DNA compresses F-actin into an ultradense dense nematic phase. The spacing between nematic F-actin domains shows a power-law dependence on DNA concentration.   

Fluorescent Speckle Microrheology of F-actin Networks

We present a non-invasive technique to probe the mechanical properties of F-actin cytoskeletal networks at sub-micron to micron length scales by using fluorescent speckle microscopy (FSM) to directly image the thermally-driven strain fluctuations of filaments in the network. In FSM, cytoskeletal polymers are labeled with a low concentration ratio of fluorescent:non-fluorescent cytoskeletal subunits and stochastic, spatial variations in fluorescence intensity result in diffraction-limited intensity peaks in high magnification, high resolution images called `speckles'. Using TIRF microscopy and a fast, sensitive cooled CCD camera with on-chip multiplication gain, we were able to image speckles in \textit{in vitro} F-actin networks cross-linked with \textit{$\alpha $}-actinin at 30 frames/sec for nearly 120 seconds. We then track the thermally driven spatial trajectories of the speckles with subpixel accuracy and cross-correlate the displacements of pairs of speckles to directly map the strain fluctuations of the networks and use a generalized Stokes-Einstein relation to interpret these fluctuations in terms of the mechanical properties. Fluorescent speckle microrheology will be a powerful, highly spatiotemporally resolved method for non-invasively probing cytoskeletal mechanics in living cells during morphogenic processes such as migration or division.   

Entanglement of Semiflexiible Polymers: A Brownian Dynamics Study

We report extensive Brownian dynamics simulations of very tightly entangled solutions of semiflexible rods, of length $L$ comparable to their persistence length $L_{p}$, at concentrations comparable to those in recent experiments on Fd-virus and filamentous actin. We find a clear crossover with increasing number concentration $c$ from a regime of loosely entangled rods, in which rotational diffusion is hindered by topological constraints but transverse bending fluctuations are not, to a tightly entangled regime in which bending fluctuations are also restricted, and can relax only by reptation along a wormlike tube. This crossover occurs at a dimensionless concentration $c^{**}L^{3} \sim 500$ for chains with $L = L_{p}$. The tube radius $R_{e}$ is found to depend upon $c$ and $L_{p}$ with the predicted scaling relation $R_{e}\propto c^{-3/5} L_{p}^{-1/5}$ for $c > c^{**}$. The dynamic modulus $G(t)$ has been obtained from simulations of the relaxation of stress after a small amplitude step extension of the simulation unit cell. An elastic plateau in $G(t)$ that is absent at lower concentrations also appears for $c \geq c^{**}$.   

Polyelectrolyte Bundles: Finite size at thermodynamic equilibrium?

Experimental observation of finite size aggregates formed by polyelectrolytes such as DNA and F-actin, as well as synthetic polymers like poly(p-phenylene), has created a lot of attention in recent years. Here, bundle formation in rigid rod-like polyelectrolytes is studied via computer simulations. For the case of hydrophobically modified polyelectrolytes finite size bundles are observed even in the presence of only monovalent counterions. Furthermore, in the absence of a hydrophobic backbone, we have also observed formation of finite size aggregates via multivalent counterion condensation. The size distribution of such aggregates and the stability is analyzed in this study.   

Growth of Attached Actin Filaments

Actin filaments in cells extend themselves by polymerizing free actin monomers onto their growing ends. The growing filaments can push obstacles and thus do mechanical work. It is known [1] that if the filaments are not attached to the obstacle, new monomers can be added when the obstacle fluctuates away from the growing filament ends. However, experiments [2, 3] show that the growing ends of actin filaments are firmly attached to the obstacle. Based on the idea of the Brownian ratchet model, we develop an energy-based model to investigate the growth of attached actin filaments. In this model, the force field describing the interaction between the actin filament and surface proteins (such as ActA) on the obstacle's surface is given a simplified but plausible analytic form. We use both Brownian-dynamics simulations and analytical approaches to calculate the attachment time and the growth rate. Our results show that a high binding energy ($\sim$28kT) is required for the binding of an actin filament to the obstacle, and the actin filament can remain attached to a 25 nm bead for about 30 s, while still growing at about 50\% of the free-filament growth velocity.\newline *Supported by NSF grant number DMS-0240770.\newline [1] Peskin, Odell and Oster, Biophys. J. \textbf{65}, 316 (1993). [2] Kuo and McGrath, Nature \textbf{407}, 1026 (2000). [3] Gerbal, et al. Eur. Biophys. J. \textbf{29}, 134 (2000).   

Structure and stability of self-assembled actin-lysozyme complexes studied via computer simulation

Using both molecular dynamics and grand-canonical Monte Carlo simulations, we have studied the structure and stability of complexes of filamentous actin (an anionic polyampholyte) and lysozyme (a cationic globular protein) in aqueous solution. We find that lysozyme initially bridges pairs of filaments, which then relax into hexagonally-coordinated bundles comprised of actin rods held together by one-dimensional arrays of lysozyme macroions. In order to connect to small-angle x-ray scattering results, we have examined the role of the concentration of monovalent salt. We find that exclusion of salt from the bundled phase is essential for bundle stability, and we address with our simulation results the different competing effects which could be responsible for this salt repartitioning.   

Hierarchical Self Assembly of Actin Bundle Networks

The network-like structure of actin bundles formed with the cross-linking protein $\alpha $-actinin has been investigated on different length scales via small angle x-ray scattering and confocal fluorescence microscopy. We describe the hierarchical structure of aggregates formed at different ratios of cross-linker using both $\alpha $-actinin and also the non-specific polyelectrolyte, polylysine. The effects of different lengths of F-actin are also discussed. An interesting feature of this system is the formation of a dense layer on the surface of the actin gel. This layer exhibits interesting morphologies and can be formed to have a defined shape. Biologically based structures such as this have the potential to generate interesting biological scaffolds for applications in cell encapsulation and tissue engineering.   

Elastic actin comet tails: shape, stresses and propulsion

Actin based motility is a recurring theme in a variety of biological systems ranging from keratocytes that use their dynamically re-arranging cytoskeleton for motility to bacterial pathogens like Listeria that hijack the host cell's actin machinery and are propelled by actin comet tails. The basic principle behind all these processes is the conversion of free energy of polymerization into a protrusive force. Recent experimental observations have suggested several distinctive features of such propulsion especially in the case of Listeria motion. We model the process by a finite element simulation of the actin comet tail which is treated as a continuum elastic material that is tethered to the rear of the bacterium. We investigate steady state properties such as the shape of the comet tail, stresses generated and also the time dependence of the motion.   

The orientational order parameter of nematic liquid crystalline phase of F-actin

The cytoskeletal protein actin self-assembles to form long and stiff filaments, F-actin, which serves essential functions in cells, such as control of cell shape, division, and motility. Suspensions of F-actin form either entangled isotropic networks or a nematic liquid crystalline phase. Depending on the average filament length, the isotropic-nematic (I-N) liquid crystalline transition occurs at a concentration of 2 mg/ml or above. We have measured the orientational order parameter of F-actin traversing the I-N phase transition using a combination of techniques, including fluorescence microscopy, local birefringence measurement, and x-ray scattering. With actin concentrations above the region of I-N transition, the order parameter approaches a saturated value of 0.75. This value implies significant extent of misalignment or entanglement among long actin filaments even in the nematic phase. At concentrations slightly below the I-N transition, non zero values of the order parameter were detected within a time window on the order of an hour following the sample preparation, which tends to cause unintended initial alignment. This result shows extremely slow rotational kinetics of F-actin in the entangled networks.   

Actin Filamin networks and stress criticality

We study critical behavior in a model biopolymer network comprised of semiflexible polymers crosslinked by extensible proteins with unfolding domains. The domains unfold reversibly at a critical pulling force. The force extension curve of such a crosslinker resembles a sawtooth function, with another domain unfolding and thus adding entropic compliance each time a critical pulling force is reached. Filamin and alpha-actinin are both biological crosslinkers which exhibit this sawtooth behavior. We demonstrate through theory and simulation that our model network exhibits critical pileup in the distribution of crosslinker lengths when it is sheared. This is to say, the population fraction of crosslinkers at a given tension dies exponentially away from the unfolding force of the unfolding domains. This leads to a novel force relaxation time scaling as crosslinkers are thermally excited over the unfolding threshold.   

Order-Order Transition of Size-mismatched Ions on F-actin Polyelectrolytes

Multivalent ions induce condensation of like-charged F-actin polyelectrolytes into close-packed bundles, in which multivalent ions organize into 1-D density waves. We examine the condensation behavior of anionic F-actin using multivalent cations with a large size mismatch, Ba$^{2+}$ and lysozyme(+9), a small globular protein (2.5nm x 2.5nm x 4.5nm). An unexpected first-order phase transition on the F-actin surface between a Ba$^{2+}$ counterion charge density wave state and 1-D close-packed lysozyme chains is found as the lysozyme-actin ratio is varied. By comparing wild-type lysozyme with genetically-engineered lysozyme with reduced charge, we show that this transition shifts with the actin-lysozyme isoelectric point.   

Fingers and Comet Tails--Motility and Morphology in growing actin gels

Actin-based cell motility has proven to be a useful system for studying the dynamic system of polymer gel formation in the cytoskeleton. There is still no consensus regarding the exact method for the transduction of chemical energy to mechanical energy during actin based motility. Also under debate is the ``symmetry breaking'' which occurs in a biomimetic system used to simulate and study actin based motility. An enzyme coated bead immersed in real or synthetic cell extract will first grow a symmetric cloud of actin gel. Then the gel will spontaneously differentiate into one or more ``tails.'' A symmetry breaking stochastic model has been proposed for the formation of one tail. We propose a model based on an elasto-chemical instability at the outer edge of the gel. Our theoretical model allows us predict when one or more tails will form as a function of system parameters, explain the observed shape of an actin ``comet tail'' and predict when the instability will take place. Our model is supported by finite element simulations.   

Session U32: Density Functional Theory

Distorted plane waves - new basis set for the interstitial region

In the LAPW method plane waves are used to describe the wave function and charge density in the interstitial region between the muffin tin (MT) spheres. Plane waves are an excellent basis set for the interstitial region in many aspects - they have a well defined energy and simulate well the charge density of many materials. However in many applications the number of basis functions needed to describe the wave functions and charge density to desired precision is quite large and the diagonalisation of the corresponding Hamiltonian will be time consuming. In order to speed up the diagonalisation and make it possible to perform calculations on larger systems we implement a new set of basis functions, the distorted plane waves which conserves the simplicity of the plane waves, but reduce the number of basis functions needed to describe the wavefunctions to desired precision.   

Time-dependent Kohn-Sham theory with memory

In time-dependent density-functional theory, exchange and correlation (xc) beyond the adiabatic local density approximation can be described in terms of viscoelastic stresses in the electron liquid. In the time domain, this leads to a velocity-dependent xc vector potential with a memory containing short- and long-range components. The resulting time-dependent Kohn-Sham formalism describes the dynamics of electronic systems including decoherence and relaxation. For the example of collective charge-density oscillations in a quantum well, we illustrate the xc memory effects, clarify the dissipation mechanism, extract intersubband relaxation rates for weak and strong excitations, and demonstrate the generation of plasmon sidebands. This work was supported by the ACS Petroleum Research Fund and Research Corporation.   

Test of a nonempirical density functional for short-range van der Waals interaction in rare-gas dimers

It is known that the nonempirical generalized gradient approximation (GGA) of Perdew, Burke, and Ernzerhof (PBE) provides a much more realistic description of the short-range part of the van der Waals (vdW) interaction than does the local spin density (LSD) approximation. In the present work, the ability of the higher-level nonempirical meta-GGA of Tao, Perdew, Staroverov, and Scuseria (TPSS) [Phys. Rev. Lett. {\bf 91}, 146401 (2003)] to describe vdW interaction is tested self-consistently in rare-gas dimers with $Z \le 36$. The one-parameter hybrid version (TPSSh) of the TPSS exchange-correlation functional is also included in this test. Calculations show that both TPSS and TPSSh functionals correctly yield vdW bonds in these dimers and significantly improve the prediction of bond lengths and binding energies over LSD. The rather close agreement of TPSS with PBE for these dimers confirms a principle of the TPSS construction: preservation of the PBE large-gradient behavior. Compared with the PBE GGA, TPSS and TPSSh yield a slightly weaker binding. The typically too-long bond lengths and too-small binding energies of TPSS meta-GGA suggest the need for some long-range vdW interaction correction which is discussed.   

Analytical Solutions for States of the 3D Hooke's Atom in an External B Field

Hooke's atom is a 2-electron model with the nuclear-electron attraction replaced by an isotropic harmonic potential. Closed-form solutions for certain eigenstates are known in 3D at B=0; B $\ne$ 0 solutions are known only for the 2D quantum dot. Both solutions are products of center-of-mass oscillators and relative motion factors. Because the uniform-B confining potential is quadratic in the cartesian coordinates normal to B, we can find related analytical solutions for certain eignvalues of the 3D Hooke's atom at B $\ne$ 0. They are more complicated because of the imposed axial symmetry. We sketch the somewhat tedious solution techniques, then compare the analytical and numerical solutions for a large range of field strengths. We have used these results to obtain exact Kohn-Sham orbitals for current density functional theory (CDFT) [``Exact Current DFT Study of Hooke's Atom in Magnetic Fields,'' W. Zhu and S.B. Trickey to be published]   

Exact Current DFT Study of Hooke's Atom in Magnetic Fields

From exact analytical [1] and numerical solutions for Hooke's atom in a uniform external magnetic field, we construct the exact Kohn-Sham (KS)orbitals for current density functional theory (CDFT). We discuss the effects of the external B field relative to the harmonic confining potential on the exchange-correlation energy and various energy components, as well as exact exchange-correlation scalar and vector potentials. Exact density functional results are compared with results with several widely used approximate DFT functionals. Our exact CDFT results can be used as a check for any proposed CDFT functionals and as guidance for improvement of existing functionals. [1] "Analytical Solutions for States of the 3D Hooke's Atom in an External B Field", W. Zhu and S.B. Trickey to be published.   

Exact-exchange density-functional calculations for large gap materials: A major step forward?

The electronic structure of several large gap insulators is calculated using the exact-exchange functional (EXX) in density functional theory, and the results are compared with those from the local-density approximation (LDA) and experiment. EXX is considered a major step beyond LDA and has already been shown to provide exceptionally accurate results for semi-conductors. In this study, two classes of large gap systems are examined, the noble-gas solids and simple biatomic ionic crystals. For the noble-gas solids, the dominant binding effect is the Van der Waals interaction which is not properly described by the EXX formalism. Ionic crystals, instead, are held together by the Hartree interaction between oppositely charged ions, and the Van der Waals interaction plays a negligible role. It is seen that the EXX method does not reproduce the fundamental energy gaps as well as has been reported for semiconductors; however, the EXX gaps are much closer to the optical gaps than LDA gaps and still represents a significant advance in band theory calculations.   

Relaxation and dissipation in time-dependent current-density functional theory

In a typical relaxation problem a many-particle system evolves from an initial excited state under the action of its own hamiltonian plus a ``thermal bath", until equilibrium (or the ground-state at $T=0$) is reached. Due to the presence of the thermal bath the time evolution of the system is not unitary, and an initially pure state will evolve into a statistical mixture of states. Here we show that the time-dependent current density functional theory$^1$ allows a hamiltonian description of the relaxation process, whereby the quantum state of the system undergoes a unitary time evolution without becoming entangled with a thermal bath. The essential feature that causes the system to eventually settle into a stationary state of the ground-state Kohn-Sham hamiltonian is the presence of an effective electric field, which is determined by the instantaneous values of the current and the density. Our theory is consistent with recent numerical results by Wijewardane and Ullrich$^ 2$.\\ 1. G. Vignale, C. A. Ullrich, and S. Conti, PRL {\bf 79}, 4878 (1997)\\ 2. H. O. Wijewardane and C. A. Ullrich, cond-mat/0411157   

Correlation Effects in Screened-Exchange Density Functional Theory

While it has been demonstrated that the screened-exchange local density approximation (sX-LDA) gives good agreement with experiment for fundamental energy gaps of many semiconducting systems, the underlying physics is not always clear. One particular question is, in semiconductor systems, whether the screening should be short range (e.g., the Thomas-Fermi screening) or long range (e.g., by the semiconductor dielectric function). To investigate this, we have compared the self-energy term in the sX-LDA formalism with the self-energy term in the GW approximation and the exchange-correlation hole of variational quantum Monte Carlo simulations. We have also tested the band gaps and total energy results within the sX-LDA formalism with different screening models. The sX-LDA calculations are done using norm-conserving pseudopotentials and a plane-wave basis.   

Building improved functionals for self-consistent DFT by better treatment of electronic surface regions

We develop a specialized treatment of electronic surface regions which, via the subsystem functional approach [1], can be used in functionals for self-consistent density-functional theory (DFT). Approximations for both exchange and correlation energies are derived for an electronic surface. An interpolation index is used to combine this surface-specific functional with a functional for interior regions. When the local density approximation (LDA) is used for the interior region, the end result is a straightforward density-gradient dependent functional that shows promising results. Further improvement of the treatment of the interior region by the use of a local gradient expansion approximation is also discussed. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000. [1] R. Armiento and A. E. Mattsson, Phys. Rev. B {\bf 66}, 165117 (2002).   

A functional designed to include surface effects into self-consistent density-functional theory calculations.

We present an exchange-correlation functional that enables an accurate treatment of systems with electronic surfaces. The functional is developed within the subsystem functional paradigm [1], combining the local density approximation for interior regions with a new functional designed for surface regions. It is validated for a variety of materials by calculations of: (i) properties where surface effects exist, and (ii) established bulk properties. Good and coherent results are obtained, indicating that this functional may serve well as universal first choice for solid state systems. The good performance of this first subsystem functional also suggests that yet improved functionals can be constructed by this approach. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000. [1] R.~Armiento and A.~E.~Mattsson, Phys.~Rev. B, {\bf 66}, 165117 (2002)   

Empirical Laplacian-based model of the exchange-correlation energy density and potential in Si

We explore density functional theory (DFT) models based on the Laplacian of the local density, derived from a fit to accurate variational Monte Carlo (VMC) data for the adiabatic exchange-correlation energy density of the Si crystal. We find that the rms discrepancy between the local density approximation (LDA) and the VMC data is reduced 70\% using a three-parameter correction to the LDA that incorporates the local Laplacian only, with a similar reduction in error for the Si atom with no modification to the model. Corrections to the LDA exchange-correlation potential generated by Laplacian terms have been implemented within a pseudopotential plane-wave scheme. Self-consistent calculations of the structural properties of Si using this potential reproduce those of the LDA. In contrast, the local gradient of the density provides an insignificant improvement to the fit, while introducing unphysical features into the exchange-correlation potential, and giving a significantly poorer description of structural properties. Application of our model to other semiconductors will be briefly discussed.   

Magnesium Alloy Precipitate Formation Using Mixed Basis Cluster Expansions

Unlike steel and aluminum alloys, the basic science of magnesium alloys is poorly understood. The automotive industry is driving demand for lighter structural material, and readily available magnesium alloys have a higher strength-to-weight ratio than their aluminum counterparts. We seek to predict magnesium alloy properties from first principles, particularly the hardening effect of precipitate formation. Mixed basis cluster expansions (MBCE) have successfully modeled precipitate shapes and growth in aluminum alloys. Unfortunately, this methodology has not been extended to hcp-based materials such as magnesium alloys. In order to model binary magnesium alloys using the MBCE, particularly precipitate morphologies, we have constructed a coherency strain model for hcp structures to correctly represent the long range strain fields around precipitates. Coupling this generalized strain model to an Ising-like expansion methodology we have developed a mixed-basis cluster expansion for hexagonal symmetries. Results for several representative magnesium alloys will be presented.   

Self-Consistent Radiative Scattering in Time Dependent Density Functional Theory

Time Dependent Density Functional Theory (TDDFT) is an ab initio theory that can be used to model time varying electron densities. We propose a semi-classical method for calculating the self-consistent radiation from a time varying electron density in a TDDFT framework. This scheme allows one to simulate a system where a time varying electron density scatters energy both into the lattice as well as into an electro-magnetic field. We will present a description of this technique and describe a few applications in which it has been employed.   

Many-body perturbation theory using the density-functional concept: beyond the GW approximation

Electronic structure calculations based on the Many-Body Perturbation Theory (MBPT), within GW approximation [1], are very reliable for simple materials. Some failures (e.g. for transition metals) ask for better approximations that could be in principle derived through the usual set of MBPT equations. This procedure would be however computationally prohibitive. We propose an alternative formulation of MBPT that relies on density-functional concept [2]. Our equation for the polarizability is a two-point one, that leads to excellent optical absorption and energy loss spectra [3]. The other MBPT quantities are then simply calculated via an integration. The terms beyond GW approximations, that we obtain, are both more simple and more physically intuitive, than the usual ones. We show a direct impact of this formulation on the TDDFT. Numerical results for optical absorption, IXS, and the band gap of bulk silicon and solid argon illustrate the leading corrections beyond RPA for the polarizability and GW for the self-energy. [1] L. Hedin, Phys. Rev. {\bf 139}, A796 (1965). [2] F. Bruneval {\it et al.}, submitted to Phys. Rev. Lett. [3] F. Sottile, V. Olevano, and L. Reining, Phys. Rev. Lett. {\bf 91}, 056402 (2004).   

A new approach to the treatment of uniform electric fields

It has been known for a long time that the treatment of an external uniform electric field in a periodic system presents conceptual and practical difficulties. At the heart of these difficulties lies the fact that, when a uniform electric field is present, the ground state does not exist: thus one is faced with the dilemma of either breaking the periodicity by a scalar potential or making the problem time-dependent by a vector potential. Within the context of density functional theory this seems to imply that the conventional description of many-body effects in terms of the time-dependent density should be abandoned in favor of a description in terms of the current density.$^1$ However, we will show that it is possible to describe the uniform electric field without leaving the framework of ordinary time-dependent DFT, by passing to a non-inertial reference frame. By leaving the distances invariant, this transformation preserves the periodicity of the lattice, and at the same time the appearance of an ``inertial force" compensates for the vector potential. Thus, we end up with a system subjected to a periodic time-dependent external potential -- a perfectly legitimate candidate for the application of TDDFT.\\ 1. N.T. Maitra, I. Souza, and K. Burke, Phys. Rev. B. {\bf 68}, 045019, (2003).   

Session U33: Cold Fusion

Experimental Evidence for LENR in a Polarized Pd/D Lattice

Experimental evidence in support of claims that excess enthalpy production in a polarized Pd/D lattice is of a nuclear origin is questioned on various grounds, eg marginal intensity and difficulty in reproducing. Here, evidence is presented that is $\approx$100$\%$ reproducible and of sufficient intensity to be well outside of experimental errors. In addition to the thermal behavior, the nuclear manifestations include: X-ray emission; tritium production; and, when an operating cell is placed in an external electric field, fusion to create heavier metals such as Ca, Al, Mg, and Zn.   

On Complex Nuclei Energetics in LENR

Swimming Electron Layer (SEL) theory plus fission of ``complex nuclei'' were proposed earlier to explain reaction products observed in electrolysis with multi-layer thin-film metallic electrodes\footnote{1. G.H. Miley, and J.A. Patterson, J. New Energy, Vol. 1, pp.11-15, (1996).}. SEL was then extended to treat gas-diffusion driven transmutation experiments\footnote{G. H. Miley and H. Hora, ``Nuclear Reactions in Solids,'' APS DNP Mtg., East Lansing, MI, Oct (2002).}. It is also consistent with measured charged-particle emission during thin-film electrolysis and x-ray emission during plasma bombardment experiments\footnote{A. Karabut, ``X-ray emission in high-current glow discharge,'' Proc., ICCF-9, Beijing China, May (2002).}. The binding energy per complex nucleon can be estimated by an energy balance combined with identification of products for each complex – e.g. complexes of A~39 have $\sim$ 0.05 MeV/Nucleon, etc, in thin film electrolysis. Energies in gas diffusion experiments are lower due to the reduced trap site potential at the multi-atom surface. In the case of x-ray emission, complexes involve subsurface defect center traps, giving only a few keV/Nucleon, consistent with experiments$^3$.   

Search for radiation signals from electrolytic cells

There have been many reports of observed heat generated above joule heating from electrolytic systems. A high current density electrolytic system was designed to test for radiation signals other than IR from an electrolytic process. The system employs current densities on the order of 100 Amps/cm$^2$ and uses a large Beryllium sulfate heavy water neutron moderator/reflector. The search is planned to look at the visual spectrum, neutron emission, gamma emission, alpha emission and electrical signals in the current flow to the system. The electrolysis is preformed with a heavy water solution, and materials with large neutron cross sections will be employed at the electrodes. Results of the on-going investigation will be reported.   

Reproducibility of Excess of Power and Evidence of $^4$He in Palladium Foils Loaded with Deuterium

Research at ENEA was oriented to material science study, in order to increase the deuterium concentration in palladium foils undergone to electrochemical loading and to triggering, in order to increase the reproducibility of excess of power production. Laser irradiation was used as trigger. Isoperibolic and flow calorimetry operating with electrochemical cells have been developed in order to reveal excess of power production. Nuclear ashes detection has been performed by means of high resolution and high sensitivity mass spectrometer. Material science studies allowed to obtain a palladium showing high solubility for hydrogen isotopes and giving deuterium concentration at equilibrium larger than 0.95 (as D/Pd atomic fraction) with a reproducibility larger than 90$\%$. Excess of power experiments have been successfully carried out at Energetics Laboratory and at SRI. by using materials prepared at ENEA.Preliminary measurements give an $^4$He signal in reasonable agreement with the expected D+D = $^4$He + heat reaction.   

Kinetics and Lumped Parameter Model of Tardive Excess Thermal Power

The time-integral of tardive excess thermal power (TETP) was previously misnamed "heat after death"\footnote{Pons, S., Fleischman, M., Trans Fusion Tech, 26, 4T, Part 2, p. 87 (1994).}. We have examined the kinetics of tardive excess thermal power (TETP) which occurs after driving, fully loaded, activated, spiral wound cold fusion Phusor cathodes (Pd/D2O/Pt;\footnote{Swartz. M., G. Verner, Proc. ICCF-10 (2004).}$^,$\footnote{Swartz. M., Proc.ICCF-10 (2003).}) at their optimal operating point\footnote{Swartz, M., Fusion Technology, 31, 63-74 (1997).}. TETP, after input electrical power produced an excess power (compared to an ohmic joule control) of 165$+/-$15 percent [excess power $\approx$1.3 Watts], had kinetics suggestive of two distinct sources or physical active regions within the lattice\footnote{Swartz. M., G. Verner, ICCF-11 (2004).}. An electrical engineering TETP model had good correlation. The active palladium lattice has a deuteron-loading capacitance of $\approx$64 micromoles per volt*. The lattice admittance for the TETP reactions ($\approx$7 picomoles/[sec-volt*]) is dwarfed by the admittance for outgassing deuteron loss ($\approx$15 nanomoles/[sec-volt*]).   

Models for Anomalies in Metal Deuterides

There have been a great many claims for anomalies in experiments on metal deuterides, including excess heat, heat correlated with helium, slow tritium, low-level dd-fusion, and particle emission not produced by dd-fusion reactions. We have studied models that involve phonon exchange with a highly excited phonon mode in the case of fusion reactions and disintegrations. We have recently generalized the approach to include phonon- mediated nuclear excitations. The resulting models may be applicable to experiments in which evidence for penetrating radiation is found, as well as to some transmutation effects.   

Bloch-Sensitive Nuclides

Documented condensed matter nuclear science includes Fleischmann and Pons radiationless dd fusion reactions, Iwamura alpha-addition transmutations, and Oriani MeV particle showers. All require partitioned coherent matter in which fractions of each single ``wave like" particle are entangled\footnote{T. A. Chubb, ``Bloch Nuclides, Iwamura Transmutations, and Oriani Showers", ICCF11 Abstract}. If the work required to bring side-by-side deuterons into contact is somehow reduced enough, an energy-minimizing 2-body anti-correlation form of wave function replaces the "molecule" configuration, allowing cold fusion. In the Iwamura process, a second fusion step fuses 2 spin-zero $^4$He$^2$$^+$$_B$$_l$$_o$$_c$$_h$ ions to form $^8$Be$^4$$^+$$_B$$_l$$_o$$_c$$_h$. The nuclear ground state energy of the product nucleus is a function of the number of fragments into which it is partitioned. It is ``Bloch sensitive", i.e., its energy level is a function of N$_w$$_e$$_l$$_l$, the number of potential wells into which the $^8$Be$^4$$^+$$_B$$_l$$_o$$_c$$_h$ is partitioned. The dependence of energy on lattice parameter N$_w$$_e$$_l$$_l$ strongly couples nuclear and electromagnetic forces at the boundary of the coherently ordered volume, causing energy transfer to the lattice.   

Experiment and Theory for Nuclear Reactions in Nano-Materials Show e14 - e16 Solid-State Fusion Reactions

Nano-lattices of deuterium loving metals exhibit coherent behavior by populations of deuterons (d's) occupying a Bloch state. Therein, coherent d-overlap occurs wherein the Bloch condition reduces the Coulomb barrier.Overlap of dd pairs provides a high probability fusion will/must occur. SEM photo evidence showing fusion events is now revealed by laboratories that load or flux d into metal nano-domains. Solid-state dd fusion creates an excited $^4$He nucleus entangled in the large coherent population of d's.This contrasts with plasma dd fusion in collision space where an isolated excited $^4$He nucleus seeks the ground state via fast particle emission. In momentum limited solid state fusion,fast particle emission is effectively forbidden.Photographed nano-explosive events are beyond the scope of chemistry. Corroboration of the nuclear nature derives from photographic observation of similar events on spontaneous fission, e.g. Cf. We present predictive theory, heat production, and helium isotope data showing reproducible e14 to e16 solid-state fusion reactions.   

Simultaneous Excess Power And Anomalous Radiation

Experimental studies of a Pd/D$_2$O + LiOD/Pt electrolysis cell displayed the characteristics of the excess power effect during seven occasions over a 22-day period \footnote{ M.H. Miles, J. Electroanal. Chem., 482, 56 (2000).}. These measurements clearly show the anomalous increase in the cell temperature from two thermistors despite the steadily decreasing electrical input power during electrolysis. During this same time period, the cell thermistor located close to the palladium cathode showed strange temperature excursions that suggest electromagnetic radiation emissions from this cathode \footnote{ M.H. Miles, “NEDO Final Report”, March 31, 1998. (see \urllink{http://lenr-canr.org/acrobat/milesmnedofinalr.pdf)}{ http://lenr-canr.org/acrobat/milesmnedofinalr.pdf}}. These sudden temperature excursions ranged from 1 to 16 $^o$C and quickly returned to normal$^2$. The second thermistor in this cell that was located at a more distant position, where any electromagnetic radiation from the cathode would have to pass through the platinum anode, showed only normal temperature behavior. Later studies using a set of five thermistors also showed anomalous temperature excursions for any thermistors placed in close contact with a Cs-137 radioactive source (b-decay, 94$\%$ 0.511 MeV energy). However, the number of such temperature excursions using Cs-137 was much less than the number observed in the active Pd/D$_2$O electrolysis cell for the same time period.   

Low Mass 1.6 MHz Sonofusion Device

We have developed a much improved cavitation system for sonofusion, compared to our initial systems. The new system is a low mass 1.6 MHz unit that produces 40 watts of excess heat with an acoustic input power of 17 watts. The increase in frequency (to 1.6 MHz from 40 KHz) increases the heat, improves the performance, shows reproducible results, and indicates durability. The calorimetry is a simple in flow through system. The difference between output and input temperature at steady-state, times the flow gives the power (calories/s) output of the sonofusion reactor. The energy density of this system is of the order of commercial energy suppliers.   

Cold Fusion, A Journalistic Investigation

Author of the recent book, $The$ $Rebirth$ $of$ $Cold$ $Fusion$, and founder of New Energy Times, Steven B. Krivit presents a summary of cold fusion's, past, present and possible future. This talk will briefly review five highlights of the recent New Energy Times investigation into cold fusion research:\\ 1. Analysis of early studies that supposedly disproved cold fusion.\\ 2. Key early corroborations that supported the claims of Fleischmann and Pons.\\ 3. The evolving understanding of cold fusion reaction paths and by-products.\\ 4. A look at volumetric power density.\\ 5. Brief comparison of the progress in hot fusion research as compared to cold fusion research.\\ New Energy Times, founded in 2000, is an independent communications company which currently specializes in reporting on cold fusion research\footnote{\urllink{References and copies of the presentation are available at www.newenergytimes.com/reports/aps2005.htm}{http://www.newenergytimes.com/reports/aps2005.htm}}. It has no affiliations with any organization, entity or party which invests in these technologies, nor any individual researcher or research facility.   

Why You Should Believe Cold Fusion is Real

Nuclear reactions are now claimed to be initiated in certain solid materials at an energy too low to overcome the Coulomb barrier. These reactions include fusion, accelerated radioactive decay, and transmutation involving heavy elements. Evidence is based on hundreds of measurements of anomalous energy using a variety of calorimeters at levels far in excess of error, measurement of nuclear products using many normally accepted techniques, observations of many patterns of behavior common to all studies, measurement of anomalous energetic emissions using accepted techniques, and an understanding of most variables that have hindered reproducibility in the past. This evidence can be found at \urllink{www.LENR-CANR.org}{www.LENR-CANR.org}. Except for an accepted theory, the claims have met all requirements normally required before a new idea is accepted by conventional science, yet rejection continues. How long can the US afford to reject a clean and potentially cheap source of energy, especially when other nations are attempting to develop this energy and the need for such an energy source is so great?   

Framework for Understanding LENR Processes, Using Ordinary Condensed Matter Physics

As I have emphasized\footnote{\urllink{S.R. Chubb, Proc. ICCF10 (in press). Also, http://www.lenr-canr.org/acrobat/ChubbSRnutsandbol.pdf} {http://www.lenr-canr.org/acrobat/ChubbSRnutsandbol.pdf}, S.R. Chubb, Trans. Amer. Nuc. Soc. 88 , 618 (2003).}, in discussions of Low Energy Nuclear Reactions(LENRs), mainstream many-body physics ideas have been largely ignored. A key point is that in condensed matter, delocalized, wave-like effects can allow large amounts of momentum to be transferred instantly to distant locations, without any particular particle (or particles) acquiring high velocity through a Broken Gauge Symmetry. Explicit features in the electronic structure explain how this can occur$^1$ in finite size PdD crystals, with real boundaries. The essential physics$^1$ can be related to standard many-body techniques\footnote{Burke,P.G. and K.A. Berrington, \underline{Atomic} \underline{and} \underline{Molecular} \underline{Processes:}\underline{an} \underline{R matrix} \underline{Approach} (Bristol: IOP Publishing, 1993).}. In the paper, I examine this relationship, the relationship of the theory$^1$ to other LENR theories, and the importance of certain features (for example, boundaries$^1$) that are not included in the other LENR theories.   

Morphology of fission gas bubbles in fissioning uranium metal closely

We investigate by SEM the micro-structural and basic phenomenological mechanisms governing the fission-gas and fusion-gas behaviour in metals. This comparative study clearly shows the characteristics of fission-gas bubbles (primarily helium and xenon) in uranium fuel metals have the same characteristics as fusion-gas bubbles (helium) in the solid-state fusion metal - palladium. The remarkably similar characteristic morphology clearly identifies the nuclear phenomenological origins of the gas bubbles in the palladium metal which are correllated and explained by the presence of a large amount of DD fusion. Allied evidence of anomalous heat production during cold fusion experiments suggests the nuclear process. Further analysis of these fusion metals by mass spectroscopy clearly identifies anomalous helium isotopes in large quantity were trapped in the palladium metal.   

Session U36: Quantum Optics, Quantum Control, and Ultrafast Processes

Towards the coupling of microsphere resonators and self-assembled semiconductor quantum dots.

Coupling semiconductor quantum dots with microcavities is a challenge which opens the door to many interesting basic and applied experiments. Using microsphere resonators and self-assembled quantum dots has the advantage of allowing mechanical scanning of the resonator position to find a quantum dot in resonance (or far from it). We will show our progress toward that goal, which includes measuring the Q factor of the resonators in different conditions.   

Hoping to get something out of nothing: Searching for hypothetical forces in the Casimir regime

In the first part of this talk our measurements using microelectromechanical systems in the Casimir regime will be discussed. A metrological analysis of these results is used to rule out different models for the expression of the Casimir force at finite temperatures. In the analysis, corrections to the Casimir force were calculated or estimated. These corrections are due to the grain structure of the metal layers (including the variation of optical data and patch potentials), their surface roughness (including nonmultiplicative and diffraction-type effects), and nonlocal effects. In the second part it will be shown how these families of measurements are being used to set more stringent constraints in the search for corrections to the Newtonian gravitational potential. Our latest results and the proposed improvements to obtain better limits will be presented.   

Two-dimensional `photon fluid': Effective photon-photon interaction and physical realizations

We describe a recently developed effective theory for atom-mediated photon-photon interactions in a two-dimensional ``photon fluid'' confined to a Fabry-Perot resonator. The photons in the lowest longitudinal cavity mode will appear as massive bosons interacting via a renormalized delta-function potential with a strength determined by physical parameters such as the density of atoms and the detuning of the photons relative to the resonance frequency of the atoms. We discuss novel quantum phenomena for photons, such as Bose-Einstein condensation and bound state formation, as well as possible experimental scenarios based on Rydberg atoms in a microwave cavity, or alkali atoms in an optical cavity.   

Path Entangled Photons from Parametric Down-Conversion

Path entangled photon states can be used to overcome classical limits on the accuracy of interferometric measurements such as the diffraction limit. These states are superpositions of finding $n$ photons in one out of two (or more) paths. Using stimulated parametric down-conversion (PDC), we propose a method for generating heralded multiphoton path entanglement, without applying post-selection. PDC is relatively easy to produce compared to pure Fock states as demanded by other proposals. By a special coincidence detection at one down-converted arm, the photons of the second arm non-locally bunch into the desired state. Entanglement in photon number is created between two polarization modes rather than two paths. A polarization beam-splitter and a $\lambda $/2 waveplate can translate between the two representations. The experimental generation of a two-photon path entangled state was detected by observing interference at half the photon wavelength. The scheme is generally extendable to higher photon numbers. \newline [1] M.J. Holland and K. Burnett, ``Interferometric detection of optical phase shifts at the Heisenberg limit'', Phys. Rev. Lett. \textbf{71}, 1355 (1993). \newline [2] P. Kok, H. Lee and J.P. Dowling, ``Creation of large-photon-number path entanglement conditioned on photodetection'', Phys. Rev. A \textbf{65}, 052104 (2002).   

Predicting superluminality using Einstein causality

We will show that in any system having population inversion and a sufficiently small spontaneous decay rate there exists soliton solutions having superluminal velocities. The existence of these solitons can be proven using Einstein causality in any system where there is gain and a loss mechanism, provided spontaneous emission may be neglected. The shape of these solitons depends on the details of the electronic system, but their existence does not. Although coupling to this soliton by pulses outside the gain material does not appear to be possible, we propose that superluminal pulses may be observed by setting up a ``seed field'' in the system prior to introducing the population inversion.   

Slow Light Using Electromagnetically Induced Transparency from Spin Coherence in [110] Strained Quantum Wells

The electromagnetically induced transparency from spin coherence has been proposed in [001] quantum wells recently. [1] The spin coherence is a potential candidate to demonstrate semiconductor-based slow light at room temperature. However, the spin coherence time is not long enough to demonstrate a significant slowdown factor in [001] quantum wells. Further, the required transition of light-hole excitons lies in the absorption of heavy-hole continuum states. The extra dephasing and absorption from these continuum states are drawbacks for slow light. Here, we propose to use [110] strained quantum wells instead of [001] quantum wells. The long spin relaxation time in [110] quantum wells at room temperature, and thus more robust spin coherence, [2] as well as the strain-induced separation [3, 4] of the light-hole exciton transition from the heavy-hole continuum absorption can help to slow down light in quantum wells. [1] T. Li, H. Wang, N. H. Kwong, and R. Binder, Opt. Express 11, 3298 (2003). [2] Y. Ohno, R. Terauchi, T. Adachi, F. Matsukura, and H. Ohno, Phys. Rev. Lett. 83, 4196 (1999). [3] C. Y. P. Chao and S. L. Chuang, Phys. Rev. B 46, 4110 (1992). [4] C. Jagannath, E. S. Koteles, J. Lee, Y. J. Chen, B. S. Elman, and J. Y. Chi, Phys. Rev. B 34, 7027 (1986).   

Optical High Harmonic Generation in $\rm \bf C_{60}$

\newcommand{\cm}{$\rm C_{60}$ } \newcommand{\ete}{{\it et al.}} \newcommand{\prl} {Physical Review Letters } \newcommand{\prb} {Physical Review B } High harmonic generation (HHG) requires a strong laser field, but in \cm a relatively weak laser field is sufficient. Numerical results presented here show while its low order harmonics result from the laser field, its high order ones are mainly from the multiple excitations. Since high order harmonics directly correlate electronic transitions, the HHG spectrum accurately measures transition energies. Therefore, \cm is not only a promising material for HHG, but may also present an opportunity to develop HHG into an electronic structure probing tool. References: G. P. Zhang, \prl {\bf 91}, 176801 (2003); G. P. Zhang and T. F. George, \prb {\bf 68}, 165410 (2003); P. B. Corkum, \prl {\bf 71}, 1994 (1993); G. P. Zhang and Thomas F. George, \prl {\bf 93}, 147401 (2004); H. Niikura \ete, Nature {\bf 417}, 917 (2002); {\it ibid.} {\bf 421}, 826 (2003); Y. Mairesse \ete, Science {\bf 302}, 1540 (2003); A. Baltuska \ete, Nature {\bf 421}, 611 (2003).   

Controlling vibrational excitations in $\rm \bf C_{60}$ by laser pulse durations

Two similar off-resonant ultrafast laser experiments [1-3] in $\rm C_{60}$ have reported two different vibrational modes that dominate the relaxation process: one predicts the ag modes while the other the hg modes. A systematic simulation presented here reveals that this experimental discrepancy results from the laser pulse duration. The numerical results show that since each mode $\nu$ has a distinct optimal duration $\tau_o^\nu$, the ag modes are strongly suppressed for durations longer than 40 fs, while the hg modes start to grow. For the off-resonant and low-intensity excitations, the period $\Omega^o_\nu$ of the dominant mode and $\tau_o^\nu$ satisfy the relation $\Omega_\nu^o/{\tau}_o^\nu \approx 3.4$. By carefully scanning the laser frequencies and pulse durations, a comprehensive excitation diagram is constructed, which can be used to guide experiments to selectively excite the ag and hg modes in cm by an ultrafast laser [4,5]. Its potential impact is also discussed. \newcommand{\prl}{{\small \it Phys. Rev. Lett.} } \newcommand{\prb}{{\it Phys. Rev. B} } \newcommand{\et}{{\it et al. }} \newcommand{\ete}{{\it et al.}} [1] S. Dexheimer \ete, {\it Ultrafast Phenomena VIII}, edited by J. L. Martis \ete, {\it Springer Series in Chemical Physics} {\bf 55}, 81 (1993). [2] V. R. Bhardwaj \ete, \prl {\bf 91}, 203004 (2003). [3] H. Hohmann \ete, \prl {\bf 73}, 1919 (1994). [4] G. P. Zhang and T. F. George, \prl {\bf 93}, 147401 (2004). [5] G. P. Zhang, \prl {\bf 91}. 176801 (2003).   

Adaptive engineering of coherent soft-x-rays by temporal and spatial laser-pulse shaping

We demonstrate qualitative amplitude shaping of the coherent soft x-ray spectrum produced in the process of high-harmonic generation. This is accomplished by applying adaptive femtosecond pulse shaping methods. We performed the basic operations of complete spectral control by 1) selective generation of extended parts of the high-harmonic spectra, 2) tunable single harmonic generation and 3) creation of spectral holes (suppression of harmonics) in the plateau region of the spectrum. Our ability to qualitatively ``engineer'' the coherent spectral properties by application of temporal and spatial laser-pulse-shaping methods has immediate consequences for the developing field of attosecond x-ray science. Control over the spectrum is directly related to the control over the attosecond pulse shape as we will show by comparing experiment with simulation. In addition, even more important is the prospect to extend the field of coherent control into the soft x-ray range. In the future, the proposed technique will allow us to directly manipulate electronic motion on its natural attosecond time scale.   

Synchronization of femtosecond laser and electron pulses with sub-ps precision

Sub-picosecond synchronization between the pump laser and probe electron pulses is crucial for conducting time-resolved electron diffraction. We have achieved this synchronization with such precision using real-time ultrafast shadow imaging. In this approach we quantitatively monitored the temporal evolution of electron shadow images induced by the transient space-charge field formed near a metal surface in the wake of excitation laser pulses. In principle, this is a universal approach independent of the structural dynamics under investigation and can be applied to a variety of diffraction setups and using laser pulses with different wavelengths.   

Athwart Proton Migration in Polyatomic Molecules during Strong-Field Laser Pulse

Coulomb-explosion fragmentation of large polyatomic molecules caused by strong non-resonant laser field may involve complex motion of constituent protons during ulrashort laser pulse. We compare kinetic energy distributions of H+ ions produced in the Coulomb-explosion dissociation of anthracene, (C$_{14}$H$_{10} $), 1,2,3,4,5,6,7,8-octahydroanthracene, (OHA, C$_{14}$H$_{18} $), and 9,10-anthraquinone, (AQN, C$_{14}$O$_{2}$H$_{8}$), subjected to 60 fs, 800 nm laser pulses of intensity between 0.4 and 4.0×10$^{14}$ W·cm$^{-2}$. The distributions are signatures of the molecular structure effects on the energy coupling and the proton motion over the pulse duration. Most revealing are the curves of the kinetic energy cutoff as a function of the laser intensity: they reflect prolong nonadiabatic charge localization in the molecular ions. The cutoff curve for AQN differs drastically from those for anthracene and OHA, demonstrating much greater saturation value ($\sim$80 eV compared to $\sim$50 eV). The differences are interpreted in terms of proton migration across the electric field on the initial stage of fragmentation process. This migration is caused by nonadiabatic electron dynamics; our model calculations agree with the experimental data.   

Dynamics of Molecular Fragmentation Mediated by Charge-Transfer States

Dissociative ionization of large organic molecules caused by ultrashort strong-field laser pulses is largely predetermined by nonadiabatic electron dynamics during the pulse. The key element of the nonadiabatic process is the bottleneck transition from the system’s ground state to the charge-transfer doorway state of the excited-state manifold. The induced charge transfer across large distances in polyatomic molecules and ions evolves into a complicated dynamics that can include prolong charge localization and sequential ionization. This electron-charge dynamics affects essentially the ensuing nuclear motion and thus determines the fragmentation pattern and charge distribution among the fragments. We observed manifestations of nonadiabatic electron-nuclear dynamics mediated by charge-transfer states in a series of experiments on related polyaromatic molecules, including study of the fragmentation threshold as a function of the laser intensity, the proton kinetic energy distributions, and the relative yield of ionized fragments as a function of the carrier wavelength. In all these cases our model calculations agree quantitatively with the experimental data.   

Phase Coherent Photorefractivity in ZnSe Quantum Wells

We observe an efficient phase coherent photorefractive (PCP) effect in ZnSe single quantum wells for ultra short light pulses resonant to the excitonic transition. The effect has been investigated by spectral and temperature dependent measurements in a two- and a three-beam four-wave mixing configuration. The observed PCP is attributed to the formation of an electron grating formed by the interference of coherent excitons that enables the storage of four different logic bits in a three-beam configuration. The different bits can be distinguished by the diffracted signal intensities and by their filed polarizations. The high efficiency of this PCP detectable at an average power level of a few $\mu $W makes it attractive for applications in all optical data processing. Also interesting for optical switch applications is the possibility to erase the PCP by a temporal overlap of the exciting pulses. All characteristic features of the effect are explained and reproduced by numerical calculations based on optical Bloch equations for a three-level system.   

Session U39: Non-Fermi Liquids and Spin Liquids

Numerical Evidences of Fractionalization in an Easy-Axis Two-Spin Heisenberg Antiferromagnet

Based on exact numerical calculations, we show that the generalized Kagome spin model in the easy axis limit exhibits a spin liquid, topologically degenerate ground state over a broad range of phase space, including a point at which the model is equivalent to a Heisenberg model with purely two-spin exchange interactions. We present an (to our knowledge the first) explicit calculation of the gap (and dispersion) of ``vison'' excitations, and exponentially decaying spin and vison 2-point correlators. These are hallmarks of deconfined, fractionalized and gapped spinons. The nature of the phase transition from the spin-liquid state to a magnetic ordered state tuned by a negative four-spin ``potential'' term is also discussed in light of the low energy spectrum. These results greatly expand the range and the theoretical view of the spin-liquid phase in the vicinity of the RK exactly soluble point. Numerical indications of the spin-liquid phase in other spin models will also be discussed.   

Hydrodynamic description of correlations in Quantum Fluids

We employ field theory methods to study correlation functions of Spin Chains. We derive asymptotic behaviors of the correlators through a hydrodynamic formulation of the problem. In particular, we are interested in a correlator known as Emptiness Formation Probability (EFP), which measures the probability $P(n)$ of formation of an empty region of length $n$ in the quantum fluid at low temperature. The EFP in the leading order is found as the action of the instanton solution of hydrodynamic equations of motion. This hydrodynamic approach has already been applied in the study of a number of systems, for instance the XXZ Spin Chain, a Bose gas with delta repulsion and free 1D fermions. The EFP for the XY Spin Chain is asymptotically Gaussian in $n$ at the isotropic point and exponential in the anisotropic regime. We study the crossover between these two regimes by calculating the leading intermediate asymptotics of the EFP using a bosonization approach (linearized hydrodynamics). To study the subleading contributions to the EFP, we include gradient corrections to hydrodynamics and study quantum fluctuations around the saddle-point ``instanton'' solution.   

Probing the Pseudogap for an Algebraic Spin Liquid

Algebraic spin liquids [1] are two-dimensional Mott insulators where the spin sector is in an interacting critical state. One such state, the staggered-flux spin liquid, has been argued to play a key role in the pseudogap regime of the underdoped cuprate superconductors [2,3]. We find that the staggered-flux state supports a variety of slowly-fluctuating competing orders, unified by an emergent SU(4) symmetry. Among these orders are the Neel vector and the order parameter for a columnar valence- bond solid. This structure may have important observable consequences for the rather high-temperature physics of the pseudogap regime. 1. W. Rantner and X.-G. Wen, PRL 86, 3871 (2001). 2. X.-G. Wen and P. A. Lee, PRL 76, 503 (1996). 3. T. Senthil and P. A. Lee, cond-mat/0406066.   

Calogero-Sutherland model and Quantum Benjamin-Ono Equation

Collective field theory for Calogero-Sutherland model represents particles with fractional statistics in terms of holomorphic bosonic field made out of the density and velocity fields. We identify an operator equation of motion for this bosonic field with a quantum deformation of a known classical Benjamin-Ono equation. The latter equation is integrable and the same is true for its quantum version. The inverse scattering transform for the classical Benjamin-Ono equation can be extended to its quantum analog. Soliton solutions of quantum Benjamin- Ono equation correspond to particle and hole excitations of Calogero- Sutherland model.   

Berry phases emerging from the $\pi$-flux state

We derive a new effective action describing fluctuations around the Affleck-Marston $\pi$-flux mean-field solution of the 2d Heisenberg antiferromagnet. The 5-dimensional Clifford algebra inherent in the Dirac fermion obtained as the continuum limit of the $\pi$-flux state is found to sustain a bulit-in competition between antiferromagnet (AF) and valence-bond-solid (VBS) orders. This naturally leads us to cast both orderings as components of a 5 component vectorial field $v$, for which we obtain an O(5) nonlinear sigma model with a novel Wess- Zumino (WZ) term proportional to the Mauer-Cartan form $\int_0^1 dt\int d^3 x v dv \wedge dv \wedge dv \wedge dv$, with $t\in[0,1]$ an auxiliary variable which extends $v(x)$ to $v(t,x)$ in such a way that $v(t=0,x)\equiv(0,0,0,0,1)$ and $v(t=1,x)\equiv v(x)$ are satisfied. We study properties of Berry phases extracted from this WZ term, and recover in particular the AF hedgehog Berry phases (with a VBS core) which are central to recent studies on 2D spin liquids.   

Rectification in quantum wires with strong electron interactions

We investigate the rectification of a low-frequency ac bias in quantum wires with strong electron interactions in the presence of a localized asymmetric scattering potential. Electrons of opposite spin form a two-channel Luttinger liquid. We show that the $I-V$ curve significantly differs from that of the one-channel quantum wire\footnote{D. E. Feldman, S. Scheidl, and V. M. Vinokur, \urllink{cond-mat/0410089}{http://arxiv.org/abs/cond-mat/0410089}.} with polarized electrons. The dc current exhibits a non-monotonic dependence on the ac voltage bias, and the dc $I-V$ curve is strongly asymmetric at low voltages.   

A Tale of Two Theories: Quantum Griffiths Effects in Metallic Systems

We show that two apparently contradictory theories on the existence of Griffiths-McCoy singularities in magnetic metallic systems are in fact mathematically equivalent. We discuss the generic phase diagram of the problem and show that there is a non-universal crossover temperature range T* $<$ T where power law behavior (Griffiths-McCoy behavior) is expect. For T $<$ T* power law behavior ceases to exist due to the destruction of quantum effects generated by the dissipation in the metallic environment. We show that T* is an analogue of the Kondo temperature and is controlled by non-universal couplings.   

Renormalization group study of bond order wave phase in the extended Hubbard chain

We study the phase diagram of the half-filled one-dimensional extended Hubbard model at weak coupling. We obtain a finite region of bond charge density wave order near $U = 2V$ using one loop renormalization group (RG) method. We solve a long-standing controversy in this field, explaining why earlier standard g-ology calculations have not found this phase. We introduce a functional generalization of standard g-ology in which effects of the scattering processes involving electrons away from the Fermi points are included in a systematic way. We argue that this is an example in which formally irrelevant terms change the topology of the phase diagram. We discuss other scenarios in which this may occur and this generalized RG method is essential to fully characterize the phase diagram.   

Hole localization in Doped Mott Insulators

A key experimental puzzle surrounding the high-temperature copper oxide materials is the origin of the insulating behaviour in the underdoped regime. Using a self-consistent cluster method, we compute the resistivity of a lightly doped Mott insulator (described by the Hubbard model) using the Kubo formula in the one-loop approximation. We find that at high temperatures the resistivity increases as some power of temperature but at low temperatures diverges as $\exp(T_0/T)^s$ ($s\approx 0.66$) as is seen experimentally in the cuprates. The localization is due to the pseuedogap which is shown to be a ubiquitous feature of a doped Mott insulator. Quite generally, doped holes form magnetic polarons which remain localized in an otherwise antiferromagnetic background as a result of a non-perturbative phase shift. The phase shift is computed explicitly and is shown to vanish as $U\rightarrow \infty$, in agreement with the Nagaoka dilute limit.   

Variational study of triangular lattice Heisenberg spin-1/2 model with ring exchanges and spin liquid state in $\kappa$-(ET)$_2$Cu$_2$(CN)$_3$

We study triangular lattice spin-1/2 system with antiferromagnetic Heisenberg and ring exchanges using variational approach focusing on possible realization of spin liquid states. Trial spin liquid wave functions are obtained by Gutzwiller projection of fermionic mean field states and their energetics is compared against magnetically ordered trial states. We find that in a range of ring exchange coupling upon destroying the antiferromagnetic order, the best such spin liquid state is essentially a Gutzwiller-projected Fermi sea state. We propose this spin liquid with spinon Fermi surface as a candidate for the nonmagnetic insulating phase observed in the organic compound $\kappa$-(ET)$_2$Cu$_2$(CN)$_3$, and describe some experimental consequences of this proposal.   

The infrared catastrophe and tunneling into a correlated conductor

It is well known that the tunneling density of states has anomalies (cusps, algebraic suppressions, and pseudogaps) at the Fermi energy in a wide variety of low- dimensional and strongly correlated electron systems. We propose that the origin of these anomalies is the infrared catastrophe associated with the sudden introduction of a new electron into a conductor during a tunneling event. We introduce an exact functional integral representation for the interacting Green's function, by means of a Hubbard-Stratonovich transformation, single out the field configurations responsible for the infrared catastrophe, and treat them with methods developed for the X-ray edge problem. Applications to a variety of interacting systems will be presented.   

Critical Theory of the Multi-Channel Anderson Impurity Model

We have carried out a nonperturbative analysis of the multi-channel Anderson impurity model, using a combination of Bethe Ansatz and boundary conformal field theory techniques. We present exact, analytical expressions for the zero-temperature entropy, the low-temperature impurity thermodynamics - including the Wilson ratio - and the critical exponents of the Fermi edge singularities characterizing the time-dependent hybridization of conduction electrons and impurity. For the case of two channels we also present exact results for the single-electron Green's function, the impurity self-energy, and the low-temperature resistivity of the model. We compare our results to those obtained from more conventional, approximate methods. Implications for the study of the non-Fermi liquid physics of Uranium-based heavy fermion materials are discussed.   

Non-Fermi liquid behavior in a three-orbital Anderson model with inverted Hund's rule.

We investigate the critical properties of a threefold orbitally degenerate Anderson impurity model in the presence of a generalized Hund's rule coupling. We use in combination conformal field theory and Wilson numerical renormalization group. The fixed points of the model correspond to boundary conformal field theories including spin, orbital and charge sectors together with a three state Potts model sector. Depending on the average occupation of the impurity we find different situations. In particular at particle-hole (p-h) symmetry we find an unstable non-Fermi liquid fixed point (UFP) separating a Kondo screened phase from a non-Fermi liquid stable phase. Away from p-h symmetry the non-Fermi liquid stable phase is replaced by a conventional Fermi liquid one but the UFP remains. The spectrum obtained numerically agrees with the finite size spectrum predicted by conformal field theory. We obtain the spectral function of the impurity across the UFP. Beyond the interest in the single impurity problem, this system can be relevant to understand via Dynamical Mean Filed Theory the behavior of a strongly correlated lattice model close to the Mott transition, where we expect that the UFP instability of the single impurity turns into a superconducting instability of the bulk.   

Quantum creep and variable range hopping of one-dimensional interacting electrons

The variable range hopping results for non-interacting electrons of Mott and Shklovskii are generalized to 1D disordered charge density waves and Luttinger liquids using an instanton approach. In the present paper we calculate the quantum creep of charges at zero temperature and the linear conductivity at finite temperatures for these systems. The hopping conductivity for the interacting electrons acquires the same form as for non-interacting particles if the one-particle density of states is replaced by the compressibility. It turns out that dissipation is crucial for tunneling to happen. Contrary to pure systems the new meta-stable state does not propagate through the system but is restricted to a region of the size of the tunneling region. This corresponds to the hopping of an integer number of charges over a finite distance. A global current results only if tunneling events fill the whole sample. We argue that rare events of extra low tunneling probability are not relevant for realistic systems of finite length. Finally we show that an additional Coulomb interaction only leads to small logarithmic corrections.   

Quasi-linear temperature dependence of the resistivity due to a nested Fermi surface

Non-Fermi liquid (NFL) behavior is often found in the neighborhood of a quantum critical point (QCP). We consider a QCP arising from the nesting of Fermi surfaces of an electron pocket and a hole pocket separated by a wavevector ${\bf Q}$. The nesting gives rise to antiferromagnetism if the interaction between the carriers is repulsive. The order can gradually be suppressed by mismatching the nesting and a QCP is obtained as $T_N \to 0$. The specific heat $\gamma$ coefficient and the magnetic susceptibility increase with the logarithm of the temperature as $T$ is lowered.$^1$ The electrical resistivity and the linewidth of the neutron scattering quasi-elastic peak acquire a quasi-linear temperature dependence, as a consequence of the nesting of the Fermi surface.$^2$ This deviation from the usual Fermi liquid $T^2$ dependence is a manifestation of NFL behavior. The results are discussed in the context of NFL behavior observed in many heavy fermion compounds.$^3$ \vskip 0.1in \par\noindent Work supported by the National Science Foundation under grant No. DMR01-05431 and the Department of Energy under grant No. DE-FG02-98ER45797. \vskip 0.1in \par\noindent $^1$ P. Schlottmann, Phys. Rev. B {\bf 68}, 125105 (2003). \par\noindent $^2$ A. Virosztek and J. Ruvalds, Phys. Rev. B {\bf 42}, 4064 (1990). \par\noindent $^3$ G.R. Stewart, Rev. Mod. Phys. {\bf 73}, 797 (2001).   

Influence of thermal fluctuations on quantum phase transitions in 1D disordered systems: CDWs and Luttinger liquids

The low temperature phase diagram of 1D weakly disordered quantum systems like charge or spin density waves and Luttinger liquids is studied by a \emph{full finite temperature} renormalization group (RG) calculation. For vanishing quantum fluctuations this approach is amended by an \emph{exact} solution in the case of strong disorder and by a mapping onto the \emph{Burgers equation with noise} in the case of weak disorder, respectively. At \emph{zero} temperature we reproduce the quantum phase transition between a pinned (localized) and an unpinned (delocalized) phase for weak and strong quantum fluctuations, respectively, as found previously by Fukuyama or Giamarchi and Schulz. At \emph{finite} temperatures the localization transition is suppressed: the random potential is wiped out by thermal fluctuations on length scales larger than the thermal de Broglie wave length of the phason excitations. The existence of a zero temperature transition is reflected in a rich cross-over phase diagram of the correlation functions. In particular we find four different scaling regions: a \emph{classical disordered}, a \emph{quantum disordered}, a \emph{quantum critical} and a \emph{thermal} region. The results can be transferred directly to the discussion of the influence of disorder in superfluids. Finally we extend the RG calculation to the treatment of a commensurate lattice potential. Applications to related systems are discussed as well.   

Nonlinear ac conductivity of interacting 1d electron systems

We consider low energy charge transport in one-dimensional (1d) electron systems with short range interactions under the influence of both periodic and random potentials. Combining RG and instanton methods, we calculate the nonlinear ac conductivity and discuss the crossover between the nonanalytic field dependence of the electric current at zero frequency and the linear ac conductivity at small electric fields and finite frequency.   

Breakdown of Strong Coupling Expansions for doped Mott Insulators

We show that doped Mott insulators, such as the copper-oxide superconductors, are asymptotically slaved in that the quasiparticle weight, $Z$, near half-filling depends critically on the existence of the high energy scale set by the upper Hubbard band. In particular, near half filling, the following dichotomy arises: $Z\ne 0$ when the high energy scale is integrated out but $Z=0$ in the thermodynamic limit when it is retained. Slavery to the high energy scale arises from quantum interference between electronic excitations across the Mott gap.   

Breakdown of One-Paramater Scaling in Quantum Critical Scenarios for the High-Temperature Copper-oxide Superconductors

We show that if the excitations which become gapless at a quantum critical point also carry the electrical current, then a resistivity linear in temperature, as is observed in the copper-oxide high-temperature superconductors, obtains only if the dynamical exponent, $z$, satisfies the unphysical constraint, $z<0$. At fault here is the universal scaling hypothesis that, at a continuous phase transition, the only relevant length scale is the correlation length. Consequently, either the electrical current in the normal state of the cuprates is carried by degrees of freedom which do not undergo a quantum phase transition, or quantum critical scenarios must forgo this basic scaling hypothesis and demand that more than a single correlation length scale is necessary to model transport in the cuprates.   

Session U40: Focus Session: Transport Properties of Nanostructures V: Molecules

Theory of electron hopping through atomic-scale contacts

With the rapid progress of constructing atomic-scale nanostructure devices, understanding of electron transport between electrodes becomes an important problem. In such systems, the transfer of an electron is achieved {\it via} tunneling or ballistically through atomic-scale contacts. Here, we consider the junction system with atomic-scale contacts and evaluate the current-voltage characteristics as a function of the distance between electrodes. We use the recursion transfer matrix (RTM) method with plane-wave basis sets to obtain accurate scattering states between electrodes under finite bias voltages. Then we construct the nonequilibrium Green's function (NEGF) to evaluate charge densities and current-voltage characteristics from the obtained scattering states. Based on the density-functional method, we perform self-consistent calculations of the charge density and effective potential for the junctions systems. This method enables us to treat accurate scattering states from tunneling to ballistic transport regimes. As the distance between electrodes becomes large, we find a strong nonlinear behavior in the current-voltage (I-V) characteristics and correspondingly a gap structure appears in conductance. We consider the mechanism to appear such nonlinear behavior in I-V characteristics and compare the experimental observations.   

Encapsulated Organic Molecules in Carbon Nanotubes: Novel p-n junctions

Recently Lu et al [1] have shown that both p- and n-type doping can be realized on single-walled carbon nanotubes (SWCNTs) by encapsulating various organic or organometallic molecules, such as TCNQ, F$_{4}$TCNQ, TTF, and TDAE, with different electron affinities or ionization potentials. Using a method that combines density functional theory and Keldysh nonequilibrium Green's functions, we have investigated the charge transport properties of such encapsulated SWCNTs. We have found that the current-voltage characteristics of p-type and n-type doped SWCNTs are very different, which can be explained in terms of their individual band structures. More importantly, if these p-type and n-type doped SWCNTs are joined together, p-n junctions can be achieved. Current-voltage curves for such p-n junctions will be presented. \begin{enumerate} \item J. Lu, S. Nagase, D. Yu, H. Ye, R. Han, Z. Gao, S. Zhang, and L. Peng, Phys. Rev. Lett. \textbf{93,} 116804 (2004). \end{enumerate} *Supported through the NSF grant No. DMR-0097187   

Nonequilibrium Kondo

We consider a quantum dot connected to Fermi liquid leads in the Kondo regime, when ac voltages are applied to the leads and also to the dot. For temperatures well-below the Kondo temperature ($T_K$) we identify an effective time-dependent Hamiltonian that contains all the relevant (in the renormalization group sense) operators. With this as the starting point, we solve the ac transport problem by including the role of leading irrelevant operators. The theoretical results are compared to the recent experimental observation by Kogan {\it et al.}~[Science~\bf{304}, 1293 (2004)] of satellite peaks in the differential conductance of a single electron transistor in the Kondo regime.   

Superconductivity in Long and Short Molecules

We present the results of experimental study of superconductivity in individual molecules of carbon nanotubes [1,2], DNAs [3] and metallofullerenes [4]. Critical currents of supeconductor-molecule-superconductor junctions were extensively studied as a function of temperature and magnetic field. The mechanism of current induced superconductor-normal state transition for a long molecule (carbon nanotubes and DNAs) is the creation of phase slip centers and for a short molecule (metallofullerens) -- multiple Andreev reflections. We observe an influence of spin state of encapsulated atom on the induced superconductvity in a metallofullerene molecule. \begin{enumerate} \item A.Yu.Kasumov, et.al, Science 284, 1508 (1999). \item A.Yu.Kasumov et al., Phys. Rev. B 68, 214521 (2003). \item A.Yu.Kasumov et al., Science 291, 280 (2001). \item A.Yu.Kasumov et al., cond-mat/0402312, submitted to Phys.Rev.Lett. \end{enumerate}   

Using molecular transistors to study the Kondo effect in the presence of ferromagnetism

I will describe a technique to study spin-polarized electron transport through nm-sized molecular transistors. We create nm-sized gaps in nanofabricated ferromagnetic wires by electromigration, into which organic molecules can be incorporated. We can tailor the shapes of the ferromagnetic electrodes using electron-beam lithography so that the relative magnetization orientation can be switched from parallel to antiparallel. We use this technique to make contact to C$_{60}$ molecules using nickel electrodes. We are able to observe signatures of the Kondo effect in low- resistance devices. The ferromagnetism in the electrodes splits the Kondo resonance, resulting in two symmetric peaks in the differential conductance as a function of bias voltage. This splitting is decreased (even to zero) when the electrode magnetizations switch from parallel to antiparallel. Our measurements are in good agreement with theories that predict an exchange splitting of the Kondo resonance. The Kondo effect leads to values of magnetoresistance that are several times larger than the Julliere value.   

Inelastic effects on the transport properties of alkanethiols

We discuss inelastic scattering effects in alkanethiols of different lengths sandwiched between metal electrodes. In particular, we examine local heating and the inelastic contribution to the current. We observe that the intensities of certain peaks in the inelastic scattering profile alternate depending on odd and even number of alkyl carbons. The cause of the odd-even effect is the alternating strength of the coupling between electrons and the longitudinal component of CH3-group motion. We also find that in the absence of heat dissipation into the bulk electrodes the local temperature of alkanethiols is relatively insensitive to the length of the wires. This is due to the fact that the rates of heating and cooling processes scale similarly with length. On the other hand, when considering heat dissipation into the bulk electrodes, the local temperature of alkanethiols decreases as their length increases.   

Electron Transport Through Molecules: Gate Induced Polarization and Potential Shift

We analyze the effect of a gate on the conductance of molecules by separately evaluating the gate-induced polarization and the potential shift of the molecule relative to the leads. The calculations use {\it ab initio} density functional theory combined with a Green function method for electron transport. For a general view, we study several systems: (1) atomic chains of C or Al sandwiched between Al electrodes, (2) a benzene molecule between Au leads, and (3) (9,0) and (5,5) carbon nanotubes. We find that the polarization effect is small because of screening. The effect of the potential shift is significant, providing a mechanism for single-molecule transistors. This work was supported in part by the NSF (DMR-0103003).   

Franck-Condon Blockade and Giant Fano Factors in Transport Through Single Molecules

We show$^1$ that Franck-Condon physics leads to a significant current suppression at low bias voltages (termed {\it Franck-Condon blockade}) in transport through single molecules with strong coupling between electronic and vibrational degrees of freedom. We find that transport in this regime is characterized by remarkably large Fano factors ($10^2$ - $10^3$ for realistic coupling parameters), which arise due to avalanche-like transport of electrons. Avalanches occur in a self-similar manner over a wide range of time scales, as reflected in power-law dependences of the current noise on frequency and vibrational relaxation rate. \\ \\ $^1$ Jens Koch and Felix von Oppen, cond-mat/0409667   

Controlled Fabrication of Nanogaps for Molecular Electronics

We have developed a controlled and highly reproducible method of making nanometer-spaced electrodes using electromigration in ambient lab conditions. This advance has several advantages over the typical method at liquid-helium temperatures. One advantage is that it will make feasible electrical measurements of molecules that do not survive a sub-freezing environment. A second advantage is that it yields nanogaps of desired tunneling resistance, as opposed to the random formation at liquid-helium temperatures. We discuss how the nanogap evolves through three regimes -- a bulk-neck regime where electromigration is triggered at constant temperature, then a few-atom regime characterized by quantized plateaus in the conductance, and finally to a tunneling regime across the nanogap once the conductance falls below the conductance quantum ($G_o=2e^2/h$). We end with a discussion on the electronic properties of molecules measured using the new electrodes.   

Orbital resolved electron transport in single molecule contacts

We have performed a theoretical study of the phase-coherent electron transport in three different molecular junctions consisting of hydrogen, CO and bi-pyridine connected to Pt and Au leads. In all three cases we find good agreement with experimental results. We use a Green's function transport scheme in combination with density functional theory and a Wannier function basis set. The transport characteristics of the three molecules differ substantially ranging from complete transparency in the case of hydrogen to resonant tunneling through the LUMO state of the bi-pyridine molecule. By reconstructing the molecular orbitals of the contacted molecule and subsequently calculating the transmission with different orbitals removed from the basis set, we can directly test the individual contributions to the transmission from each molecular orbital and thereby obtain a detailed picture of the conduction mechanism.   

Tunneling mechanisms of transport in single organic molecules

Electron transport in one-dimensional organic molecular structures exhibits unique and intriguing properties that cannot be explained using traditional concepts of solid state physics and/or quantum chemistry of organic molecules. We present a new theoretical approach to study tunneling phenomena in single organic molecular systems that provides an explanation of the experimental observations within a conceptually simple and unified framework. The tunneling in metal/organic-molecule/metal systems is considered as a sub-barrier scattering of tunneling electrons off the electrons and nuclei of the molecular wire and is described using the powerful technique of scattering operators. We will discuss the unique features of the tunneling electron spectrum, the combined mechanisms of ordinary and resonant tunneling, and other phenomena that are important for understanding and interpreting experiments.   

Session U41: Correlated Electrons: Superconductivity and Magnetism

Superconductivity and non-Fermi liquid behavior near antiferromagnetic quantum critical points in CeRh$_{1-x}$Co$_x$In$_5$

Single crystals of CeRh$_{1-x}$Co$_x$In$_5$ have been investigated via measurements of specific heat, $C(x,T)$, and electrical resistivity under hydrostatic pressure, $\rho(x,P,T)$, up to $28$ kbar. Specific heat measurements for samples with cobalt concentrations of $x = 0.65, 0.71, 0.77, 0.87,$ and $0.93$ confirm the existence of antiferromagnetism (AFM) for $0\leq x \leq0.7$ and suggest the existence of a quantum critical point (QCP) at $x_c \sim 0.8$. Entropy vs $x$ isotherms below $\sim 5$ K and the normalized residual resistivity $\rho(0$ K$ )/\rho(290$ K) vs $x$ curve both display maxima near $x_c \sim 0.8$, suggesting further evidence for the existence and location of the QCP. Electrical resistivity measurements under pressure for samples with $x = 0.1, 0.2, 0.4$, and $0.6$ reveal AFM, pressure-induced superconductivity (SC), and the coexistence of AFM and SC. The $\rho(0$ K$ )/\rho(290$ K) vs $P$ curves favor the existence of QCP's at critical pressures $P_c \sim 24$ kbar for the $x = 0.1$, and $0.2$ samples and $P_c \sim 6$ kbar for the $x = 0.4$ sample. This research was supported by the U.S. DOE, NSF, and NNSA under the SSAA program.   

Field dependence of thermal conductivity in the superconducting state of

The thermal conductivity of heavy-fermion superconductor CeCoIn5 reveals a notable hysteresis between up and down sweeps of magnetic field, observed at low temperatures, slightly below the upper critical field Hc2. We study systematically this effect as a function of temperature and magnetic history. A possible relation to a coexisting magnetic order is discussed.   

Low Temperature Susceptibility of the Noncentrosymmetric Superconductor Ce$_{1-x}$La$_{x}$Pt$_{3}$Si

We report ac susceptibility measurements of polycrystalline Ce$_{1-x}$La$_x$Pt$_3$Si down to 60 mK and in applied fields up to 9 T. In zero applied field, a full Meissner state emerges for pure CePt$_3$Si at temperatures $T/T_{c}$$<$ 0.3, where $T_{c}$ = 0.65 K is the onset transition temperature. Though transport measurements show a relatively high upper critical field $B_{c2}\sim$ 4 - 5 T, the low temperature susceptibility, $\chi \prime$, is quit fragile to applied field, with $\chi \prime$ diminishing rapidly in fields of a few kG. Interestingly, the field dependence of $\chi \prime$ is well described by the power law, 4$\pi$$\chi \prime$ + 1 = $(B/B_{c})^{1/2}$, where $B_{c}$ is the field at which the onset of resistance is observed in transport measurements. The effects of La doping on the superconductivity will also be presented.   

2D fermionic systems near a magnetic $2k_F$ instability: application to electron-doped cuprates

We study spin-mediated pairing in 2D itinerant electron systems near antiferromagnetic instability in a situation when the antiferromagnetic vector $(\pi,\pi)$ connects nodal points on the Fermi surface. This scenario is related to the electron-doped high- $T_{c}$ materials where the variation of the Fermi surface with doping drives the hot spots towards zone diagonals. We obtain fermionic self-energy at strong coupling and analyze the $d-$wave pairing problem.   

Spin excitations in cuprates with fluctuating stripe order

We present a phenomenological Landau-Ginzburg-Wilson quantum lattice model with both anti-ferromagnetic and charge/bond order that is able to capture the physics of static stripes as well as fluctuating stripe order. We focus on the spin response of this model for the case of fluctuating stripe order and compare it with the results for the static case. We claim that our results are relevant to recent neutron scattering experiments in the cuprates.   

Quantum Monte Carlo Study of ``Fictive Impurity'' to Half-Filled Hubbard Model

Quantum Monte Carlo (QMC) and analytical approximations are used to show that the two (and higher) impurity dynamical mean field approximation (both in the real space and DCA version) to the square lattice Hubbard model do not reproduce the paramagnetic Mott insulating phase at particle density n=1.Moreover the real space version strongly overestimates the Neel transition temperature at large U. Systematic comparison of QMC and semiclassical analytical approximation results are also presented. Research supported by DAAD, SFB 608, DFG-SPP 1073, JSPS, NSF DMR 0431350.   

Phonons in Hubbard Ladders

The effects of phonons are studied in N-leg Hubbard ladders within the framework of one-loop renormalization group. In particular, we explicitly demonstrate that the role of phonons changes qualitatively even in the simplest two-leg ladder, as compared to the single-chain system. Our numerical results suggest that in the spin-gapped phase of the two-leg ladder, the opening of the spin gap by electron-electron interaction also drives the electron-phonon interaction to strong coupling in a subdominant fashion. Therefore, even though the inclusion of phonons does not alter the phase, their subdominant relevance renormalizes some physical properties strongly below the energy scale of the spin gap. This might shine some light on the recent experiments showing an anomalous isotope effect in high-temperature superconductors.   

Theoretical Evidence for the Equivalence between the Ground States of the Strong-Coupling BCS Hamiltonian and the Antiferromagnetic Heisenberg Model

By explicitly computing wavefunction overlap via exact diagonalization, we show that, in the limit of strong coupling, the ground state of the Gutzwiller-projected BCS Hamiltonian (accompanied by proper particle-number projection) is identical to the exact ground state of the 2D antiferromagnetic Heisenberg model on the square lattice. This identity is adiabatically connected to a very high overlap between the ground states of the projected BCS Hamiltonian and the $t$-$J$ model at moderate doping.   

Possible Observation of a 'Zhang-RiceTriplet': RIXS Study of Li2CuO2

The electronic structure of cuprates has been of considerable interest since the discovery of high T$_{c}$ superconductivity in such materials. Li$_{2}$CuO$_{2}$ consists of edge-sharing Cu-O chains, with a quasi-one dimensional crystal structure. The electronic structure, however, is regarded as quasi-zero dimensional, since the resistivity is similar along all three crystal axes, and the insulating gap is estimated at 3.2eV. We have measured the O K edge and Cu L edge XAS, SXE, and RIXS spectra of Li$_{2}$CuO$_{2}$. The results indicate the presence of Cu d-d excitations, as well as a higher energy charge transfer excitation that is tentatively identified as a two hole final state ``Zhang-Rice Triplet.'' This identification is supported by energetic and magnetic arguments, as well as more general theoretical considerations. Supported in part by the U.S. DOE under DE-FG02-98ER45680.   

Pronounced enhancement of the lower critical field and critical current deep in the superconducting state of PrOs$_{4}$Sb$_{12}$

We have observed an unexpected enhancement of the lower critical field \textit{H}$_{c1}$(\textit{T}) and the critical current \textit{I}$_{c}$(\textit{T}) deep in the superconducting state below \textit{T}$\approx$0.6~K (\textit{T}/\textit{T}$_{c}$$\approx$0.3) in the filled skutterudite heavy fermion superconductor PrOs$_{4}$Sb$_{12}$. From a comparison of the behavior of \textit{H}$_{c1}$(\textit{T}) with that of the heavy fermion superconductors U$_{1-x}$Th$_{x}$Be$_{13}$ (\textit{x}=0.027) and UPt$_{3}$, we speculate that the enhancements in PrOs$_{4}$Sb$_{12}$ reflect a transition into another superconducting phase that occurs below \textit{T}/\textit{T}$_{c}$$\approx$0.3. An examination of the literature reveals unexplained anomalies in other physical properties of PrOs$_{4}$Sb$_{12}$ near \textit{T}/\textit{T}$_{c}$$\approx$0.3 that correlate with the features we have observed in \textit{H}$_{c1}$(\textit{T}) and \textit{I}$_{c}$(\textit{T}). On the other hand, the lack of obvious features in the heat capacity at \textit{T}/\textit{T}$_{c}$$\approx$0.3 is somewhat reminiscent of the transition between the A and B phases of superfluid $^{3}$He. Vortices in PrOs$_{4}$Sb$_{12}$ are very strongly pinned. They relax from a metastable state following a logarithmic law with decay rates smaller than 0.5\%.   

RPES of the Electron-Doped Cuprates Studied by the Variational Monte-Carlo Method

We use a variational approach to gain insight into low-energy states of extended $t-J$ model in the electron-doped regime. Compared with the recent results on $Nd_{1.87}Ce_{0.13}CuO_{4}$ obtained by ARPES, we show that strong correlations lead to qualitatively similar trends in ARPES spectra and Fermi surface topology. Additionally, the results about Fermi surface evolution as a function of doping density will be discussed.   

Enhancement of pairing in a boson-fermion model for coupled ladders

Motivated by the presence of various charge inhomogeneities in strongly correlated systems of coupled ladders, a model of spatially separated bosonic and fermionic degrees of freedom is numerically studied. In this model, bosonic chains are connected to fermionic chains by two types of generalized Andreev couplings. It is shown that for both types of couplings the long-distance pairing correlations are enhanced. Near quarter filling, this effect is much larger for the splitting of a pair in electrons which go to the two neighboring fermionic chains than for a pair hopping process. It is argued that the pairing enhancement is a result of the nearest neighbor Coulomb repulsion which tunes the competition between pairing and charge ordering.   

Regular Perturbation Theory About Generalized Self-Consistent Field Hamiltonian

Strongly correlated electrons require non-traditional approaches to describe their unexpected properties. The perturbation theory about non-interacting electron gas first developed by Abrikosov and Khalatnikov has a zero convergence radius for the resulting perturbation series. The analytical theory of Gell-Mann and Bruckner, now known as random phase approximation (RPA), gives exact values for the correlation energy in the high density-weak coupling regime. However, this method also runs into difficulties due to the insufficient treatment of fluctuations. We give a formulation of a regular perturbation theory within the repulsive Hubbard model for interacting quasi-particles about exactly solvable generalized self-consistent field (GSCF) Hamiltonian for studying the intermediate range of interaction strength $U/t$, where there is no small parameter. Proposed perturbation series for interacting quasi-particles in entire parameter space of $U/t$ and electron concentration $n$ do not diverge. Performed analytical calculations of the ground state properties in extreme conditions of one dimensionality provide good numerical agreement with the Bethe-{\it ansatz} results and reasonable interpolation scheme for intermediate range of $U/t$ and $n$. The method can be used also for studies electron correlations in finite size clusters.   

Enhanced Superconductivity in Bilayered Systems

We investigate superconducting correlations in bilayered systems. The planes are described by a two-dimensional $t_{\parallel} - J_{\parallel} - U$ with $t_{\perp}$ and $J_{\perp}$ denoting the interplanar parameters. These interplanar couplings when coupled with hopping anisotropies in the planes may account for a host of unusual superconducting properties. Our main focus is to calculate superconducting correlations (using BCS theory) for various regions of the parameter space formed by the interplanar variables. For $t_{\perp} = 0$ (confining the carriers in planes) and $J_{\perp} < J_{\parallel}$, the pairing correlations are found to be purely planar. Further we generalize to $t_{\perp} \ne 0$ and $J_{\perp}/J_{\parallel} = \left (t_{\perp}/t_{\parallel}\right )^{2}$ and find that pairing correlations are enhanced at lower densities with increasing $t_{\perp}$. The most dramatic effect sets in when, additionally the planar hopping frequencies are made highly anisotropic ($t_{y} \ll t_{x}$) and $J_{\perp} \sim J_{\parallel}$. A small $t_{\perp}$ increases $T_{c}$ as much as {\it {four}} times when compared with the calculations performed for a single layer (PRB, {\bf {66}}, 144507 (2002)). Straightforward generalizations to more number of layers is discussed with a goal to study crossover to the bulk 3D limit.   

Nodal-antinodal dichotomy in doped Mott insulators

The cuprate high-Tc superconductors are anisotropic in momentum space as observed by a variety of experiments. In hole underdoped samples the pseudogap regime becomes of preponderant importance and the excitations around the nodal points [$\vec{k}=(\pm \frac{\pi}{2},\pm \frac{\pi}{2})$] are well described as Landau's quasiparticles while those near the antinodal points [$\vec{k}=(\pi,0),\, (0,\pi)$] show no signs of quasiparticle-like behavior. We employ the exact diagonalization and the self-consistent Born approximation techniques to study the single hole $tt't''J$-model in order to address how excitations with different momentum can be so disparate. We find that the single hole states can be understood as the superposition of two distinct states, namely a state with hole-like quasiparticle features and a spin-charge separated state, and explain how the different properties of these states underlie the observed nodal-antinodal dichotomy in the pseudogap regime.   

Higher order corrections to effective low-energy theories for strongly correlated electron systems

There is a significant recent interest in higher-order corrections to effective low-energy theories for a broad range of strongly-correlated electronic problems. We use the Hubbard model as an example to show how, to fourth order in hopping $t$, well-known perturbative approaches lead to the same effective theory, namely the $t$-$J$ model with ring exchange and various correlated hoppings. Several new issues appear in deriving higher-order low-energy effective theories. One may find amusing that the low-energy Hamiltonians obtained from different methods appear to be {\it different} in each case. However, we show that they are all connected by an additional unitary transformation that leaves the block-diagonal form invariant. We also emphasize the importance of transforming all the operators along with the Hamiltonian and demonstrate the equivalence of their transformed structure within the different approaches.   

Session U42: Magnetism

Low temperature physical properties of R$_{3}$Co$_{4}$Sn$_{13}$ (R = La, Ce, Pr, Nd and Gd)

We report the low temperature physical properties of the series of compounds R$_{3}$Co$_{4}$Sn$_{13}$ where R=La, Ce, Pr, Nd and Gd. They crystallize in a cubic Yb$_{3}$Rh$_{4}$Sn$_{13}$ type structure, space group Pm-3n, which has 40 atoms per unit cell. Measurements of magnetic susceptibility, electrical resistivity, and low temperature heat capacity were carried out on single crystals grown from Sn-flux. These compounds order antiferromagnetically at low temperature (T$_{N}$ $<$ 15 K) for R = Nd and Gd, while Pr$_{3}$Co$_{4}$Sn$_{13}$ and Ce$_{3}$Co$_{4}$Sn$_{13}$ are paramagnetic down to 2K. In addition Ce$_{3}$Co$_{4}$Sn$_{13}$ display heavy fermion behavior and La$_{3}$Co$_{4}$Sn$_{13}$ is a Pauli paramagnetic which superconducts at 2.3 K. The present data are compared to the magnetic properties of the isostrucutural R$_{3}$(Rh,Ir)$_{4}$Sn$_{13}$ compounds, and the validity of de Gennes scaling as a function of rare earth for these materials is discussed.   

Properties of Heusler alloy Co$_{2}$Cr$_{1-x}$Fe$_{x}$Al epitaxial thin films

We have studied properties of thin films of the new compound Heusler alloy Co$_{2}$Cr$_{1-x}$Fe$_{x}$Al. Recently, calculations have shown ordered compounds with small amounts of Fe doping to be half-metallic, and a magnetoresistance of approximately 30{\%} has been measured in bulk polycrystalline samples by others. Using physical vapor deposition, we have grown Co$_{2}$Cr$_{1-x}$Fe$_{x}$Al epitaxially on MgO. Our results show that the films are highly disordered and have a reduced magnetization as compared to the bulk and to theoretical predictions. Studies of films incorporated into current in plane spin valves show relatively large giant magnetoresistances, especially for a Heusler alloy. Recent results also indicate the presence of a large spin orbit coupling, which is unusual for a transition metal system.   

Pressure-temperature magnetic phase diagram of Au$_4$V investigated by electrical resistivity using Designer Diamond Anvils

The electrical resistivity of Au$_4$V has been measured up to a pressure of 20~GPa between room temperature and 15~K. These measurements were performed using designer diamonds, which consist of lithographically deposited tungsten micro-leads embedded within a single crystal of diamond. The electrical resistivity of Au$_4$V has a kink in its slope at 45~K under ambient pressure, which is associated with a ferromagnetic transition. Designer diamonds can be used with a diamond anvil cell to track the pressure dependence of this ferromagnetic transition, which is found to increase under the application of pressure.   

Synthesis and device applications of doped perovskite manganite nanowires

Doped perovskite manganite films such as LaCaMnO$_{3}$ and LaSrMnO$_{3}$ have been studied intensively over the past years due to their colossal magnetoresistance (CMR) properties. It is expected that these doped manganite in the form of nanowires may offer great opportunities to explore intriguing physics and also practical applications in the emerging field of spintronics. We have developed a ``nanocasting'' technique to produce high-quality single-crystalline La$_{0.67}$Ca$_{0.33}$MnO$_{3 }$and La$_{0.67}$Sr$_{0.33}$MnO$_{3}$ nanowires [1]. This was achieved by using pulsed laser deposition to epitaxially grow the desired manganite on single-crystalline MgO nanowire templates. The core-shell structures of these novel nanowires were clearly revealed using transmission electron microscopy (TEM). Following the material characterization, systematic transport studies have been performed based on devices consisting of individual manganite nanowires. We observed the metal-insulator transition with pronounced colossal magnetoresistance and also anisotropic effects related to the high aspect ratio of these nanowires. These devices could find application for spin injection and magnetic data storage. \newline [1] Nano Letters 4, 1241 (2004).   

Epitaxial growth of half metal thin films on GaAs(100) for spin injection

We report epitaxial thin films of half metal Fe$_{3}$O$_{4}$ and Fe$_{3}$Si on the GaAs(100) buffer layer grown by \textit{in-situ} MBE method. With only one spin band at $E_{F}$, half metals are 100{\%} spin polarized and are considered as an ideal candidate for spin injection. Fe$_{3}$Si is a ferromagnet with a T$_{c}$ of 840K, and a cubic DO$_{3}$ structure almost perfectly lattice matched to GaAs (100) surface. Preliminary RHEED studies showed the attainment of (100) FeO$_{x}$ thin films epitaxially grown on (100) GaAs with an in-plane 45\r{ } rotation in matching the major crytstallographic axes. The crystallinity of FeO$_{x}$ depends significantly on oxygen partial pressure during growth, film thickness, and the surface may undergo decomposition during cooling process. The chemical composition of the FeO$_{x}$ film was determined by XPS analysis by fitting the Fe 2p spectrum with two components of Fe$^{2+}$ and Fe$^{3+}$. Low temperature magnetic and electrical transport measurements are now underway.   

Magnetic Response of Weakly Coupled Ladders: A Model for Stripes in the Cuprates

We study the spin response of pairs of weakly coupled Hubbard ladders, comparing the results to a recent neutron scattering experiment on stripe-ordered $L_{1.875}B_{0.125}CuO_4$ (J. Tranquada et al., Nature 429 (2004) 534.) We assume the doping is segregated: for every pair of ladders, we treat one as half-filled, the other as a 3/8-filled Hubbard ladder. The magnetic response of the half-filled ladder is treated straightforwardly using a single mode approximation. In contrast, the spin response of the doped ladder is computed using the RG-equivalent, low-energy effective field theory, the SO(6) Gross-Neveu model. The combined response of the coupled ladder pairs is then calculated under a RPA approximation. Good agreement is found to the aforementioned experimental observations at all relevant energy scales. In addition, we find that the spin response of the ladder system below the transistion to full magnetic order resembles neutron scattering studies of slightly overdoped $L_{1.82}Sr_{0.18}CuO_4$.   

Orbital Paramagnetism of Strongly Confined Micron Width 2DEG Strips in the Extreme Quantum Limit

The possibility of orbital paramagnetism in a confined degenerate electron gas arising from surface corrections was pointed out by F.S. Ham over fifty years ago [1]. Several theoretical studies of such surface effects have since been published, including confinement effects in mesoscopic systems [2,3]. Experiments have also revealed the presence of size-dependent orbital paramagnetism [4]. In this work I report the results of calculations on the orbital magnetism of strongly confined micron-width strips of 2DEG systems in the extreme quantum limit. A maximum in orbital paramagnetism is predicted at achievable steady magnetic fields for electron areal densities of 10$^{10}$ cm$^{-2}$. It is suggested that such strips, when configured parallel to each other in a plane, with similar appropriately spaced plane layers, may constitute a novel paramagnetic material. 1. F.S. Ham, Phys. Rev. \underline {92} 1113 (1953) 2. B.L. Altshuler, Y. Gefen, and Y. Imry, Phys. Rev. Lett. \underline {66} 88 (1991) 3. B.L. Altshuler, Y.Gefen, Y.Imry, and G. Montambaux, Phys. Rev.B \underline {47 } 10335 (1993) 4. L.P. Levy, D.H. Reich, L. Pfeiffer, and K. West, Physica B \underline {189 204 (1993).}   

Anisotropic photo-induced magnetism of a sequentially deposited thin films of Rb$_{j}$Co$_{k}$[Fe(CN)$_{6}$]$_{l}$$\cdot$$n$H$_{2}$O

Using sequential deposition methods, we have generated two different films of Rb$_{j}$Co$_{k}$[Fe(CN)$_{6}$]$_{l}$$\cdot$$n$H$_{2}$O for magneto-optical studies. The synthesis protocol was intentionally varied in order to generate samples with different degrees of surface homogeneity. As a consequence, film \textbf{1} possessed a powder-like rough surface, while film \textbf{2} was a smooth, quasi-two-dimensional film. Upon irradiation at 5 K with an external magnetic field of 200 G perpendicular to the film surface, the magnetization of film \textbf{1} increased, whereas the magnetization of film \textbf{2} decreased. This contrasting behavior is consistent with dipolar field model describing the phenomena* and is related to the novel anisotropy of the photoinduced magnetism in film \textbf{2}, where the photoinduced magnetization increases or decreases depending on the orientation of the film with respect to the external magnetic field.\linebreak *J.-H. Park \emph{et al}., Appl. Phys. Lett. \textbf{85}, 3797 (2004).   

From itinerant ferromagnetism to insulating antiferromagnetism: A magnetic and transport study of single crystal SrRu$_{1-x}$Mn$_{x}$O$_{3}$ (0$\le $ x$<$0.60)

We report results of a magnetic and transport study of SrRu$_{1-x}$Mn$_{x}$O$_{3}$ (0$\le $ x$<$0.60), i.e., Mn doped SrRuO$_{3}$. The Mn doping drives the system from the itinerant ferromagnetic state (T$_{C}$=165 K for x=0) through a quantum critical point at x$_{c}$=0.39 to an insulating antiferromagnetic state. The onset of antiferromagnetism is abrupt with a Néel temperature increasing from 205 K for x=0.44 to 250 K for x=0.59. Accompanying this quantum phase transition is a drastic change in resistivity by as much as 8 orders of magnitude as a function of x at low temperatures. The critical composition x$_{c}$=0.39 sharply separates the two distinct ground states, namely the ferromagnetic metal from the antiferromagnetic insulator.   

Charge ordering transition in La$_{1.67}$Sr$_{0.33}$NiO$_4$ studied by Angle Resolved Photoemission Spectroscopy

The La$_{1.67}$Sr$_{0.33}$NiO$_{4 }$compound has attracted a lot of interested due to the report of static spin-charge stripe ordering, as well as for being isostructural to the high temperature superconductor La$_{2-x}$Sr$_{x}$CuO$_{4}$. While several theoretical and experimental studies have been reported to investigate the stripe phase in this compound, a full analysis of its electronic properties and how this evolves in the stripe phase is still missing in the literature. Here we present the first high-resolution angle resolved photoemission study of a single crystal of La$_{1.67}$Sr$_{0.33}$NiO$_{4}$. Data below and above the charge ordering temperature are presented. The evolution of the electronic structure as well as the changes observed in quasiparticle lineshapes through the stripe phase are discussed.   

Magneto-Optic Properties of Small Atomic Clusters of Ga and In with As and V

The magneto-optic properties of small, virtually (i.e., fundamental theory-based, computationally) pre-designed and vacuum pyramidal clusters of Ga-As-V and In-As-V atoms have been investigated by means of the Hartree-Fock (HF) method. The optic transition energies (OTEs) of these clusters are about 3 times smaller than those specific to small Ga-As-P and In-As-P clusters of the same structure and numbers of Ga and In atoms studied earlier. The HF analysis of the spin density distributions for In-based clusters suggests that these clusters possess noticeable magnetic properties: their total spin density distributions (SDDs) expand beyond the space occupied by the cluster's atoms and can be considered as collective features of the entire corresponding clusters, rather than individual atoms. In the case of the Ga-based clusters with V atoms the absolute values of the SDDs are an order of magnitude lesser than those specific to similar In-based clusters. Unfortunately, stabilization of such In-based clusters grown experimentally may involve chemical means, both in vacuum and in confinement.   

Surface-Kondo Effect Observed for Single Gd and Ho Atoms Embedded in Lu(0001)

By using scanning tunneling spectroscopy at low temperature, we found narrow antiresonances at the Fermi energy in the vicinity of single magnetic defect atoms (Gd and Ho) that are embedded within the surface layer of Lu(0001). The effect is explained by interaction of the magnetic defect atoms with the narrow surface-state band of Lu (0001) that crosses the Fermi energy. It can therefore be interpreted as a surface-Kondo effect.   

First-principles calculations of the magnetic interactions in Fe dimers

We present the results of first-principles calculations of the magnetic interactions between the Fe(III) ions in Fe2 dimers, and those within the larger Fe8 cluster which interact by superexchange through two oxygen ions. The magnitude and sign of interaction is found to be strongly dependent on the Fe-O-Fe angle $\theta$. We rationalize the obtained behavior analyzing the valence charge and spin densities calculated versus the angle. For the experimentally relevant range $100 \le \theta \le 105^{\circ}$, in addition to the sign and magnitude of the isotropic Heisenberg exchange interaction constant, we obtain the intramolecular global and local spin anisotropy interaction constants.   

Mott-Hubbard Type Transition and Thermodynamic Properties in Nanoscale Clusters

Thermodynamic and magnetic properties of clusters of various geometries, sizes, etc. are calculated using exact diagonalization and quantum Monte Carlo simulations. Studies of electron correlations in clusters, with respect to the interaction strength $U$, number of electrons $n$, temperature $T$ and magnetic field $h$ are fundamental for understanding the nature of ferromagnetism. Small clusters contain also important technical features necessary for monitoring the Mott-Hubbard (MH) transition in thermodynamic systems in higher dimensions. Grand canonical ensemble approach for the two site Hubbard cluster gives insight into the nature of MH transition at half filling ($n=1$) with respect to variations of $U$, $h$ and $T$. At $n=1$ the developed ``pseudo gap'' at infinitesimal temperature decreases with increasing $T$. This pseudo gap in the spectrum disappears at characteristic $T_c$ similar to MH critical temperature. A four peak structure in the density of states at finite $U$ indicates the existence of a pseudo gap at $n=1$, quarter $n=1/2$ and three $n=3/2$ fillings at relatively low $T$. The differences between the spin and charge energy gaps for finite clusters are also analyzed. A comparison of the exact results for the two atomic cluster with those calculated from quantum Monte Carlo shows reasonable agreement in a wide range of $T$ and $h$.   

High Coercivity FePt Nanoparticles Synthesized Using the Particle Gun

Chemically ordered FePt nanoparticles pose a potential solution to the superparamagnetic limit for magnetic recording media. This paper presents the synthesis and characterization of high coercivity face centered tetragonal nanoparticles with sizes in the range of 4 to 6 nanometers. As deposited samples are face centered cubic as shown by selected area diffraction. Magnetic measurements made using SQUID and vibrating sample magnetometry confirm this as the nanoparticles are soft ferromagnets. Upon heat treatment the nanoparticles transform to the chemically ordered face centered tetragonal (FCT) phase. Magnetic measurements of the annealed samples reveal a high coercivity in excess of 10 kOe for the appropriate annealing conditions commensurate with the high anisotropy FCT phase. Some unwanted agglomeration is inevitable with post synthesis annealing as indicated by bright field electron micrographs. The use of an in-situ heater stage in dynamically annealing the nanoparticles will be discussed as relates to the L10 transformation and its limits.   

Session U43: Focus Session: Phase Complexity and Enhanced Functionality in Magnetic Oxides V

X-ray Absorption spectroscopic investigation of Novel Magnetic oxides

Magnetic oxides have attracted so much attention as candidates of magnetic materials for next generation magnetic devices such as spintronics device, tera-bit magnetic storages, high efficient magnetic sensors, etc. The magnetic oxides display variety of magnetic behaviors at a function of doping, pressure, external fields, etc., and those behaviors can be even controllable. X-ray absorption spectroscopy including the magnetic circular dichroism is a powerful microscopic probe to characterize and elucidate the origin and the mechanism for the magnetic behaviors. Here I will discuss recent findings in the spectroscopic studies on various magnetic oxides, including magnetic oxide nano-particles, a multiferroic oxide, and spin- orbit-lattice coupled magnetic oxide films.   

Anomalous electronic state in CaCrO$_3$ and SrCrO$_3$

Measurements of thermal conductivity, thermoelectric power, electrical conductivity, magnetization and the equation of state have been carried out on ceramic samples of CaCrO$_{3}$ and SrCrO$_{3}$ that were synthesized under high pressure. Contrary to earlier reports, both compounds have been found to be a spin-glass insulator. While the magnetic susceptibility $\chi $(T) of SrCrO$_{3}$ becomes completely incompatible with the Curie-Weiss law, the $\mu _{eff}$=3.4 $\mu _{B}$ obtained in CaCrO$_{3}$ is close to the spin-only moment of a localized electronic state. Suppression of the thermal conductivity in both compounds indicates that orbital fluctuations are present, which confirms further the ``localized'' electronic state. Factors such as a higher $\kappa $(T) and weaker temperature dependence of $\chi $(T) for SrCrO$_{3}$ than CaCrO$_{3}$ suggest that SrCrO$_{3}$ is close to the crossover from the localized to the itinerant electronic state. More importantly, the Cr-O bond length in SrCrO$_{3}$ is much smaller than that calculated from the ionic radii. An anomalous small bulk modulus found for SrCrO$_{3}$ at P $>$ 40 kbar confirms unambiguously that the electronic state transition is induced under high pressure. The bulk modulus of SrCrO$_{3}$ below 40 kbar and CaCrO$_{3}$ falls in line with other perovskite oxides.   

Phase separation and Jahn-Teller effect in spinels

Inter-relationship between phase separation and the Jahn-Teller effect has been investigated in various spinel compounds containing Cu2+ and Mn3+ ions. We have employed comprehensive experiments of resistivity and magnetic susceptibility measurements, x-ray diffraction, and TEM. Particular attention was given to study the evolution of physical properties with the substitution of non Jahn-Teller ions to the Jahn-Teller-active Cu or Mn sites. The results of x-ray and TEM clearly show nanometer-scale chemical/structural phase separation and our phase diagram demonstrates a close relationship between phase separation and the Jahn-Teller effect.   

Tuning of magnetic and electronic states by control of oxygen content in lanthanum strontium cobaltates

We report on the magnetic, resistive, and structural studies of perovskite La$_{1/3}$Sr$_{2/3}$CoO$_{3-\delta}$. By using the relation between the temperature, partial oxygen pressure and the oxygen content from the thermogravimetric analysis, we have to synthesized a series of samples with precisely controlled $\delta=0.00-0.49$. The samples show significant coupling among the structural, magnetic and transport properties as a function of $\delta$. The stoichiometric material with $\delta=0.00$ is a cubic ferromagnetic metal with the Curie temperature $T_C=274$ K. The increase of $\delta$ to 0.15 is followed by a linear decrease of $T_C$ to $\approx$ 160 K and a metal- insulator transition within the cubic structure range. Further increase of $\delta$ results in formation of orthorhombic $a_p\times a_p \times 2a_p$ phase for $\delta\approx 0.25$ and brownmillerite phase for $\delta\approx 0.5$. Those phases are weak ferromagnetic insulators with $T_C=230$ K and 120 K, respectively. The present data show that the control of oxygen stoichiometry in lanthanum strontium cobaltates allows to modify the crystal structure and physical properties of these materials.   

Electronic Structure at Mott Insulator/Band Insulator Interfaces

A conceptual dilemma arises in the study of oxide thin film heterostructures. The question is: how should one consider the correlated equivalent of band bending and quantum confinement? These semiconductor concepts are based on rigid single particle band diagrams, which are known to be an inadequate description for strongly correlated electrons. In addition to presenting an interesting scientific challenge, this underlies attempts to develop new applications for doped Mott insulators in device geometries. They further offer a unique approach to creating new two-dimensional states in artificial structures. Modern thin film growth techniques can be used to fabricate highly idealized oxide heterostructures on the atomic scale, allowing a direct examination of these issues. Here we present our recent study of three examples of a Mott insulator/band insulator interface. By measuring the electronic properties and using spatially-resolved electron energy-loss spectroscopy, we can deduce the distribution and response of the correlated electrons. In LaTiO$_3$/SrTiO$_3$, we find the charge distribution length is dominated by the strong polarizability of the lattice, arising from a nearby ferroelectric instability. In LaTiO$_3$/LaAlO$_3$, we demonstrate a dramatic narrowing of the charge distribution by quantum confinement. Finally, in LaMnO$_3$/SrTiO$_3$ we find the charge distribution can be modified by a magnetic field, reflecting the strong charge-spin coupling arising from the double-exchange mechanism.   

Field-Induced Orbital and Magnetic Phases in the Layered Ruthenates

Magnetic-field- and Temperature-dependent Raman scattering studies have been performed on Ca$_{3}$Ru$_{2}$O$_{7}$, which undergoes an antiferromagnetic transition at 56K and a metal-insulator transition at 48K. Although no appreciable changes in the magnon or phonon spectra are observed for fields oriented along the c-axis, a field applied in the a-b plane reveals dramatic magnetic-induced changes to both the magnon and phonon spectra. For fields aligned along the magnetic easy-axis (a-axis), a splitting in the magnon mode occurs with increasing field, as well as evidence for a metamagnetic transition above 5 T, based upon which we can deduce several magnetic parameters for this material. Furthermore, by monitoring the field-induced changes in the Ru-O phonon frequency at various temperatures, we are able to map-out various field-induced orbital and magnetic phases for fields applied along both the hard and easy axis directions. *Work supported by NSF DMR02-44502, DOE DEFG02-91ER45439, and the Sony Scholar Fund.   

Structural and Magnetic Behavior of a New Low Dimensional Cobalt Oxide

The study of transition metal oxide physics has been dominated by octahedral coordination of the transition metal, such as in perovksite manganites and cobaltites. A less common coordination geometry is the tetrahedron, whose weaker crystal field [10 \textit{Dq} (tetrahedron) = 4/9 10 \textit{Dq} (octahedron)] favors high-spin complexes across the periodic table. Here we discuss the crystal and magnetic structure of a recently-identified class of tetrahedrally coordinated mixed-valent cobalt oxides, RBaCo$_{4}$O$_{7}$ (R=Y, Tm, Yb, Lu). The structure of these compounds consists of planes of corner-sharing CoO$_{4}$ tetrahedra that form a Kagome net when considering only the Co ions. These planes are connected in the third dimension by yet another CoO$_{4}$ tetrahedral layer with a density 1/3 that of the Kagome plane. A full temperature-dependent neutron diffraction study on the Yb compound reveals a structural phase transition from trigonal ($P31c)$ to monoclinic (\textit{Cc}) on cooling through T=180 K. This first order transition is accompanied by an anomaly in the magnetization and a pronounced increase in resistivity. Below 75 K, broad superlattice lines appear. We discuss these findings in terms of Co spin states, the possibility of charge order of the Co$^{2+} $ and Co$^{3+}$ ions (formally a 3:1 ratio Co$^{2+}$/Co$^{3+})$, and low-dimensional magnetism engendered by the crystal structure.   

Magnetic Field Induced Phases of the Strontium Ruthenates

Magnetic-field- and temperature-dependent Raman scattering has been used to investigate the magnetic-field-induced structural and magnetic phases of the triple-layer ruthenate system Sr$_{4}$Ru$_{3}$O$_{10}$ (Sr4310), which is a low temperature ferromagnet with T$_{C }$= 105 K. Magnetic-field-induced changes in the phonon spectra reveal dramatic spin-reorientation transitions and strong magnetoelastic coupling in this material. Further, the highly anisotropic field-induced effects observed for magnetic fields along the c-axis (magnetic easy axis) and ab-plane provide insight into the complex magnetic and structural (H,T) phase diagram of this material. We compare our magnetic-field dependent Raman results in Sr4310 with those of the quantum-critical bilayer material Sr$_{3}$Ru$_{2}$O$_{7}$, as well as with magnetic and transport measurements of Sr4310.   

Coexistence of superconductivity and antiferromagnetism in multilayered high-$T_{\rm c}$ cuprates

We propose a mechanism for high critical temperature ($T_c$) in the coexistent phase of superconducting- (SC) and antiferromagnetic (AF) CuO$_2$ planes in multilayered cuprates. The Josephson coupling between the SC planes separated by an AF insulator (Mott insulator) is calculated perturbatively up to the fourth order in terms of the hopping integral between adjacent CuO$_2$ planes. The perturbative processes comprises two parts: The first provides a positive value of Josephson coupling called {\it 0-Josephson coupling}, while the second makes a negative contribution called {\it $\pi$-Josephson coupling}. We find that the AF exchange interaction suppresses the latter process, and allows the Cooper pair to tunnel through the Mott insulator. The fluctuations of the SC phase are suppressed by this long-ranged Josephson coupling, and it is this which enables the coexistence and a rather high value of $T_c$.   

Superconducting Clusters and Colossal Effects in Underdoped Cuprates

Phenomenological models for the antiferromagnetic vs. $d$-wave superconductivity competition in cuprates are studied[1] using conventional Monte Carlo techniques. The analysis suggests that cuprates may show a variety of different behaviors in the very underdoped regime: local coexistence, stripes, or, if disorder is present, states with nanoscale superconducting clusters. The transition from an antiferromagnetic to a superconducting state does not seem universal. In particular, inhomogeneous states lead to the possibility of colossal effects in some cuprates, analogous of those in manganites. Under suitable conditions, non-superconducting Cu-oxides could rapidly[2] become superconducting by the influence of weak perturbations that align the randomly oriented phases of the superconducting clusters in the mixed state. Consequences of these ideas for angle resolved photoemission and scanning tunneling microscopy experiments[3] will also discussed. {[}1{]} Alvarez et al., cond-mat/0401474, to appear in PRB. {[}2{]} I. Bozovic et al., PRL 93, 157002, (2004) {[}3{]} A. Ino et al., PRB 62, 4127 (2000); K. Lang et al, Nature 415, 412 (2002). Research performed in part at the Oak Ridge National Laboratory, managed by UT-Battelle, LLC, for the U.S. Department of Energy under Contract DE-AC05-00OR22725.   

Magnetic coupling near the metal-insulator transition in RNiO3

In the temperature versus geometric tolerance factor t phase diagram of the RNiO$_{3}$ perovskite family, the N\'{e}el temperature T$_{N}$ increases with t, $i.e.$ the (180\r{ }- $\phi )$ Ni-O-Ni bond angle, until it is intercepted by an insulator-metal transition occurring at T$_{IM}$ that decreases with increasing t. Recent XAS data reveal that as T$_{N}$ approaches T$_{IM}$ in the insulator phase, large and small NiO$_{6/2}$ octahedra emerge locally although neutron and x-ray diffraction are fit well by an orthorhombic rather than monoclinic space group. The bonding has been shown to be vibronic where T$_{N}$ approaches T$_{IM}$. In order to probe how the interatomic exchange interactions evolves where the bonding is vibronic with T$_{N} \quad <$ T$_{IM}$, we have carried out a systematic measurement of the pressure dependence of T$_{N}$. This dependence was determined by tracking under pressure an anomaly of the resistivity $\rho $(T) that occurs at T$_{N}$. The coefficient dlnT$_{N}$/dP of GdNiO$_{3}$ falls into the range of values for magnetic insulators well-described by superexchange theory. However, this coefficient increases dramatically as t increases, reaching a maximum at lower pressure in SmNiO$_{3}$ before falling to zero; in Nd$_{0.5}$Sm$_{0.5}$NiO$_ {3}$ it is zero in the pressure interval where T$_{N} \quad <$ T$_{IM}$. A schematic magnetic phase diagram of T$_{N}$ versus the Ni-O-Ni electron transfer integral is presented.   

Session U44: Focus Session: Interfaces, Characterization, and Fabrication

Combined Surface Analytical Methods to Characterize Degradative Processes in Anti-Stiction Films in MEMS Devices

The performance and reliability of microelectromechanical (MEMS) devices can be highly dependent on the control of the surface energetics in these structures. Examples of this sensitivity include the use of surface modifying chemistries to control stiction, to minimize friction and wear, and to preserve favorable electrical characteristics in surface micromachined structures. Silane modification of surfaces is one classic approach to controlling stiction in Si-based devices. The time-dependent efficacy of this modifying treatment has traditionally been evaluated by studying the impact of accelerated aging on device performance and conducting subsequent failure analysis. Our interest has been in identifying aging related chemical signatures that represent the early stages of processes like silane displacement or chemical modification that eventually lead to device performance changes. We employ a series of classic surface characterization techniques along with multivariate statistical methods to study subtle changes in the silanized silicon surface and relate these to degradation mechanisms. Examples include the use of spatially resolved time-of-flight secondary ion mass spectrometric, photoelectron spectroscopic, photoluminescence imaging, and scanning probe microscopic techniques to explore the penetration of water through a silane monolayer, the incorporation of contaminant species into a silane monolayer, and local displacement of silane molecules from the Si surface. We have applied this analytical methodology at the Si coupon level up to MEMS devices. This approach can be generalized to other chemical systems to address issues of new materials integration into micro- and nano-scale systems. * This work was supported by the United States Department of Energy under Contract DE-AC04-94AL85000. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy's National Nuclear Security Administration.   

Role of interface in nanocrystalline diamond film internal friction

Nanocrystalline diamond films are an emergent material for use in the fabrication of nanoscale mechanical devices. Because the performance of many mechanical structures is limited by the internal friction of the material from which they are made, understanding the origin of the internal friction of these materials is essential for developing high-quality films and devices. Recent experiments suggest that the mechanical loss of a nanocrystalline diamond film is dominated by the so-called transition region, the initial growth surface of a film that is composed of growing crystallites before they coalesce into a film. This partially amorphous layer is thought to have a substantially higher internal friction than the fully dense nanocrystalline film that subsequently grows. To investigate this, we have prepared several 0.5 $\mu$m thick nanocrystalline diamond films in which the thickness of the transition region varies over a large range. The internal friction and shear modulus of the films were measured between 0.4 K and room temperature. The films are grown on silicon double paddle oscillator substrates, which have an extremely low background internal friction to permit sensitive measurements of film mechanical properties. Subsequent to measurement, the substrate is etched away so that the transition layer of the film can be examined and its characteristics correlated with the film measurements.   

TEM-STM for Novel Nanotechnological Experimentation

We present the design and construction of a miniature scanning tunneling microscope (STM) to be used inside a transmission electron microscope (TEM). In our system, the entire STM head is fitted inside the TEM sample holder, which allows for both TEM imaging/diffraction and STM-tip indentation experiments. The positioning of STM-tip over the desired sample locations can be guided through the real time TEM images. In addition to the nano indentation experiments, the STM program also allows the state-of-the-art control of atom/molecule manipulation procedures [1]. This hybrid TEM-STM system can be used for nanoscale manipulation, electrical characterization and mechanical strength examination of various nanomaterials including nanowires, nanotubes and quantum dots. [1]. S.-W. Hla, K.-F. Braun, V. Iancu, A. Deshpande, Nano Lett. 4 (2004) 1997-2001. This work is financially supported by the NSF-NIRT grant no. DMR- 0304314 and the US-DOE grant no. DE-FG02-02ER46012.   

Photoacoustic Characterization of Nanoelectromechanical Systems

A photoacoustic microscopy system has been developed to study the nanomechanical properties of Nanoelectromechanical Systems (NEMS). In these experiments, the fundamental flexural resonances of doubly-clamped nanomechanical beams are excited photo-thermally and the resulting displacements are detected using optical interferometry. Our system uses an amplified electroabsorption modulated laser source, and allows excitation at frequencies up to 5 GHz. Femtometer scale displacements of NEMS are detectable using a path-stabilized Michelson interferometer and narrowband phase sensitive detection techniques. Our measurements have enabled the determination of resonance parameters such as resonance frequencies and mechanical quality ($Q)$ factors, elastic constants and mode shapes. The results are compared to a theoretical model for photothermal excitation of doubly clamped beams. Our measurements indicate that photoacoustic microscopy is well suited for the nondestructive evaluation and opto-mechanical operation (actuation and transduction) of NEMS. This project is supported by the NSF under grant No. 0304446.   

Observation of the skin-depth effect on the Casimir force between metallic surfaces

We have measured the attractive Casimir force between a metallic plate and a transparent sphere covered with a palladium thin film. When the thickness of the coating is less than the skin-depth of the electromagnetic modes that mostly contribute to the interaction, the force is significantly smaller than that measured with a thick bulk-like film. Our results are in agreement with theoretical predictions and are the first direct evidence of the skin-depth effect on the Casimir force between metallic surfaces.   

On the torque on birefringent plates induced by quantum fluctuations

We present detailed numerical calculations of the mechanical torque induced by quantum fluctuations of the electromagnetic field on two parallel birefringent plates with in plane optical anisotropy, separated by either vacuum or a liquid (ethanol). The torque is found to vary as sin(2$\theta )$, where $\theta $ represents the angle between the two optical axes, and its magnitude rapidly increases with decreasing plate separation d. For a 40 $\mu $m diameter disk made out of calcite which is kept parallel to a Barium Titanate plate at a distance d=100 nm, the maximum torque (at $\theta =\pi $/4) is on the order of $\sim 10^{-18}$ N$\cdot $m. We propose an experiment to observe this torque when the Barium Titanate plate is immersed in ethanol and the other birefringent disk is placed on top of it. In this case the retarded van der Waals (or Casimir-Lifshitz) force between the two birefringent slabs is repulsive. The disk would float parallel to the plate at a distance where its net weight is counterbalanced by the retarded van der Waals repulsion, free to rotate in response to very small driving torques.   

Nonlinear dynamics of micro-opto-mechanical cavities

We present a detailed theoretical analysis of the nonlinear dynamics of a cantilever moving under the influence of radiation pressure, as it carries one of the mirrors of a Fabry-Perot cavity. We will discuss the existence and the properties of multiple stable dynamical attractors, the influence of noise, the possibility of tailoring the effective potential via multi-color laser input, and the effects of the dynamics on the output light. We will comment on the relevance of this analysis for existing and planned implementations of micro-opto-mechanical cavities.   

Novel Method for Gas Sampling Nanoliter MEMS Packages to Determine Hermeticity

Microelectromechanical systems (MEMS) are attractive for applications requiring complicated, small electrically operated ``machines.'' These complicated devices typically contain many moving parts and very large voltage gradients can exist across material interfaces. Thus, proper control of the internal atmosphere is a crucial requirement. The most definitive way to assess the hermeticity of the package is to sample and analyze the gas. MEMS packages are of various sizes and have internal volumes that range from about a milliliter down to tens of nanoliters. As the MEMS package size decreases, characterization of the internal atmosphere becomes increasingly difficult. Analysis of gases within milliliter-sized volumes is challenging enough with conventional technology; however, nanoliter-sized volumes are impossible. In this paper, we present a newly developed method for sampling a variety of MEMS packages, including those that have an internal volume of 30 nanoliters. The approach that was developed is radically different from standard techniques because of the custom hardware used and the pulsed method for gas introduction into the residual gas analyzer. This change enables not only the analysis of these small MEMS packages, but also a rapid way to analyze the gases repetitively in a statistically significant manner (e.g., gas from each package was analyzed \textit{dozens of times during a 20 minute time period}). Challenges resulting from this paradigm shift include calibration, and sample and manifold preparation (will also be discussed).   

Bottom-up Fabrication of Nanoelectromechanical Systems by Two-layer Nanoimprint Lithography

Nanoelectromechanical Systems (NEMS) are being developed for a variety of applications as well as for accessing new regimes of fundamental research. NEMS are electromechanical systems --- much like Microelectromechanical Systems (MEMS) --- mostly operated in their resonant modes, with dimensions in the deep submicron. Up to now, for the most part, researchers\textbf{ }have employed ``\textit{top-down}'' techniques to create NEMS devices from semiconductor materials --- i.e., high-resolution lithography followed by various etching techniques. Here, we describe a ``\textit{bottom-up}'' imprint lithographic approach to fabricate freely suspended nanomechanical beam resonators. In this approach, we first fabricate an anchor layer upon the wafer using nanoimprint lithography and film deposition. A subsequent step of imprint upon the anchors followed by thin film deposition and lift-off creates the suspended nanomechanical devices. We have used optical displacement detection techniques to characterize the electromechanical properties of our devices.   

Piezoelectric Aluminum Nitride Thin Films Deposited onto Metal Layer for Micromachined Ultrasonic Transducers

Aluminum nitride (AlN) thin films were deposited onto metal on silicon substrates by plasma source molecular beam epitaxy (PSMBE) system. The low deposition temperature of 300-450 deg. C was chosen to make the process compatible with standard Si IC technology. X-ray diffraction (XRD) data show highly textured c-axis oriented films with strong (0002) AlN peaks. Micromachined ultrasonic transducers (MUT) have been successfully fabricated using Al/AlN/Al sandwich structure on silicon resonator. Electrical properties of AlN thin films and MUT devices were systematically characterized. The resonance of the flexural acoustic mode of our MUTs was determined at about 200kHz from the impedance measurements. The effective couple factor was derived from the resonant frequency and anti-resonant frequency of MUT devices. The development of this technology would have a great potential in the integration of acoustic sensing/transducing with MEMs technology.   

Nanostructured electron beam deposited resonator combined with nanoparticles for mechanical single-electron transport

We present an integrated approach to build nano- electromechanical systems for single electron transport devices. By combining electron beam lithography with nanoparticles and direct three-dimensional electron-beam induced carbon growth, we have developed a scheme for fabricating an electromechanical single-electron transistor (emSET) for ultra-high frequencies. This process commands a size reduction of the metallic island and an improvement of the mechanical forces involved. We demonstrate how to combine two fabrication techniques for the realization of NEMS-SET devices. This will finally leads to Coulomb blockade effects and mechanical resonances in the GHz range.