Showing papers in "International Journal of Modern Physics B in 2001"

The discrete nonlinear schrödinger equation: a survey of recent results

Abstract: In this paper we review a number of recent developments in the study of the Discrete Nonlinear Schrodinger (DNLS) equation. Results concerning ground and excited states, their construction, stability and bifurcations are presented in one and two spatial dimensions. Combinations of such steady states lead to the study of coherent structure bound states. A special case of such structures are the so-called twisted modes and their two-dimensional discrete vortex generalization. The ideas on such multi-coherent structures and their interactions are also useful in treating finite system effects through the image method. The statistical mechanics of the system is also analyzed and the partition function calculated in one spatial dimension using the transfer integral method. Finally, a number of open problems and future directions in the field are proposed.

Key concepts in time-dependent density-functional theory

Abstract: We give an overview of the underlying concepts of time-dependent density-functional theory. The basic relations between densities, potentials and initial states, for time-dependent many-body systems are discussed. We obtain some new results concerning the invertability of response functions. Some fundamental difficulties associated with the time-dependent action principle are discussed and we show how these difficulties can be resolved by means of the Keldysh formalism.

Quantum holonomies for quantum computing

Abstract: Holonomic Quantum Computation (HQC) is an all-geometrical approach to quantum information processing. In the HQC strategy information is encoded in degenerate eigen-spaces of a parametric family of Hamiltonians. The computational network of unitary quantum gates is realized by driving adiabatically the Hamiltonian parameters along loops in a control manifold. By properly designing such loops the nontrivial curvature of the underlying bundle geometry gives rise to unitary transformations i.e., holonomies that implement the desired unitary transformations. Conditions necessary for universal QC are stated in terms of the curvature associated to the non-abelian gauge potential (connection) over the control manifold. In view of their geometrical nature the holonomic gates are robust against several kind of perturbations and imperfections. This fact along with the adiabatic fashion in which gates are performed makes in principle HQC an appealing way towards universal fault-tolerant QC.

Electroactive and electrostructured elastomers

Abstract: Electroactive elastomers are composites made of solid particles embedded in an elastomeric network whose mechanical or optical properties can be changed by the application of an electric or a magnetic field. These materials have obviously a strong connection with ER and MR fluids and can be more appropriated for some applications. We present recent results concerning two kinds of filled elastomer, one based on carbonyl iron particles and the second one on silica particles. In the first case we show that that change of elastic properties obtained by the application of a magnetic field depend dramatically on the way we have structured the suspension before the polymerization. We explain quantitatively these experimental results with the help of finite element calculation to predict the magnetic forces between the particles. In the second case we show how it is possible to modulate the transmission of a laser beam by shearing a thin elastomeric film whose particles have been initially aligned with the help of an electric field. Some applications related to the organization of the filler particles by the application of a field or a combination of a field and a flow before polymerization will be discussed.

Realistic modeling of strongly correlated electron systems: an introduction to the lda+dmft approach

Abstract: The LDA+DMFT approach merges conventional band structure theory in the local density approximation (LDA) with a state-of-the-art many-body technique, the dynamical mean-field theory (DMFT). This new computational scheme has recently become a powerful tool for ab initio investigations of real materials with strong electronic correlations. In this paper an introduction to the basic ideas and the set-up of the LDA+DMFT approach is given. Results for the photoemission spectra of the transition metal oxide La1-xSrxTiO3, obtained by solving the DMFT equations by quantum Monte Carlo (QMC) simulations, are presented and are found to be in very good agreement with experiment. The numerically exact DMFT(QMC) solution is compared with results obtained by two approximative solutions, i.e. the iterative perturbation theory and the non-crossing approximation.

Quantum computation with ballistic electrons

Abstract: We describe a solid state implementation of a quantum computer using ballistic single electrons as flying qubits in 1D nanowires. We show how to implement all the steps required for universal quantum computation: preparation of the initial state, measurement of the final state and a universal set of quantum gates. An important advantage of this model is the fact that we do not need ultrafast optoelectronics for gate operations. We use cold programming (or pre-programming), i.e. the gates are set before launching the electrons; all programming can be done using static electric fields only.

Metal atoms in carbon nanotubes and related nanoparticles

Abstract: The paper reviews the present state of research in the field of metal-carbon nanocomposites and the interaction of metal atoms with graphitic structures. Metal crystals can be encapsulated within graphitic shells of cylindrical, spherical, or other geometry. Various chemical and physical production methods to generate metal containing carbon nanotubes and possible microscopic formation mechanisms axe presented. In this context, the role of metals as catalysts in the formation of single-walled carbon nanotubes is discussed. The interaction of metal atoms with the graphitic lattice is of particular interest. In situ electron microscopy is used to study the behaviour of individual metal atoms in a graphitic lattice. Furthermore, novel nanostructures can be generated under electron irradiation. Finally, an overview of theoretical studies using molecular dynamics and tight binding calculations of the carbon-metal interaction is given.

Synthesis and Electrorheological Characterization of Polyaniline and NA + -MONTMORILLONITE Clay Nanocomposite

Abstract: Polyaniline-Na+-montmorillonite nanocomposite particles were synthesized using an emulsion intercalation method, and electrorheological (ER) fluids were produced by dispersing the synthesized nanocomposite particles in an electrically-insulating silicone oil. The emulsion of an aniline monomer with dodecyl benzenesulfonic acid was inserted into the layers of clay, and polymerization was processed by adding the oxidant initiator solution. DBSA as a emulsifier and a dopant took a important role for polyaniline clay nanocomposite. This insertion of polyaniline was confirmed by X-ray diffraction. To observe its ER properties, we measured the shear viscosity and the shear stress by controlling shear rate. Furthermore, we conducted dynamic tests to investigate the viscoelastic properties of the ER fluid under an electric field in the linear viscoelastic region.

Quasi-long range order in glass states of impure liquid crystals, magnets, and superconductors

Abstract: We consider glass states of several disordered systems: vortices in impure superconductors, amorphous magnets, and nematic liquid crystals in random porous media. All these systems can be described by the random-field or random-anisotropy O(N) model. Even arbitrarily weak disorder destroys long range order in the O(N) model. We demonstrate that at weak disorder and low temperatures quasi-long range order emerges. In quasi-long-range-ordered phases the correlation length is infinite and correlation functions obey power dependencies on the distance. In pure systems quasi-long range order is possible only in the lower critical dimension and only in the case of Abelian symmetry. In the presence of disorder this type of ordering turns out to be more common. It exists in a range of dimensions and is not prohibited by non-Abelian symmetries.

Orbital effects in manganites

Abstract: We review some aspects related to orbital degrees of freedom in manganites. The Mn3+ ions in these compounds have double orbital degeneracy and are strong Jahn-Teller ions, causing structural distortions and orbital ordering. We discuss ordering mechanisms and the consequences of orbital order. The additional degeneracy of low-energy states and the extreme sensitivity of the chemical bonds to the spatial orientation of the orbitals result in a variety of competing interactions. This quite often leads to frustration of classical ordered states and to the enhancement of quantum effects. Quantum fluctuations and related theoretical models are briefly discussed, including the occurrence of resonating orbital bonds in the metallic phase of the colossal magnetoresistance manganites.

Excitonic photoluminescence in pentacene single crystal

Abstract: Photoluminescence of pentacene single crystals is studied in the temperature range of 7 K to 200 K under excitation with He-Ne laser light. Photoluminescence spectra consist of four broad bands, L1 to L4. The highest energy band, L1, located close to the lowest exciton absorption band mainly appears for //b-polarization. The intensity of the second one, L2, with Stokes-shift of about 1500 cm-1, decreases as temperature rises above 30 K and disappears at 100 K. The bands, L3 and L4, which are located at lower energy, are observed at higher temperatures up to 200 K. Based on their energy positions, the band L2 is assigned to a shallow self-trapped exciton luminescence band, and the bands L3 and L4 to deep self-trapped exciton luminescence bands. By comparing this result with reported result on tetracene crystals, self-trapped excitons are considered to be more stable in pentacene.

Large Deviations and the Random Energy Model

Abstract: We present a simple proof of the formula for the free energy of the random energy model using a large deviation property which holds almost surely with respect to the randomness. This proof is extended to the case with external magnetic field leading to the solution of a model with higher-order ferromagnetic term. It is shown that this model is useful for Sourlas' application to error-correcting codes as was already pointed out in a recent letter by the authors.

The Mbx Challenge Competition:. a Neutron Matter Model

Abstract: In this paper I report my solution to MBX Challenge Competition. Namely, the Bertsch, nonparametric model of neutron matter is analyzed and strong indications are found that, in the infinite system limit, the ground state is a Fermi liquid with an effective mass.

Effective Low-Energy Theories and QCD Dirac Spectra

Abstract: We analyze the smallest Dirac eigenvalues by formulating an effective theory for the Dirac spectrum. We find that in a domain where the kinetic term of the effective theory can be ignored, the Dirac eigenvalues are distributed according to a Random Matrix Theory with the global symmetries of the QCD partition function. The kinetic term provides information on the slope of the average spectral density of the Dirac operator. In the second half of this lecture we interpret quenched QCD Dirac spectra (with eigenvalues scattered in the complex plane) in terms of an effective low energy theory.

NEUTRON SCATTERING SIGNATURE OF d-DENSITY WAVE ORDER IN THE CUPRATES

Abstract: An ordered d-density wave (DDW) state has been proposed as an explanation of the pseudogap phase in underdoped high-temperature superconductors. The staggered currents associated with this order have signatures which are qualitatively different from those of ordered spins. We apply the order parameter theory to an orthorhombic bilayer system and show that the expected magnitude as well as the momentum, energy, and polarization dependence of the consequent neutron scattering is consistent with the findings of a recent experiment.

Generalized Intelligent States for Nonlinear Oscillators

Abstract: The construction of Generalized Intelligent States (GIS) for the x4-anharmonic oscillator is presented. These GIS families are required to minimize the Robertson–Schrodinger uncertainty relation. As a particular case, we will get the so-called Gazeau–Klauder coherent states. The properties of the latters are discussed in detail. Analytical representation is also considered and its advantage is shown in obtaining the GIS in an analytical way. Further extensions are finally proposed.

SYNTHESIS AND OPTICAL PROPERTIES OF WATER SOLUBLE ZnSe NANOCRYSTALS

Abstract: ZnSe nanocrystals are prepared in water by a wet chemistry method. By selecting an appropriate pH value and surface-capping agents, a whitish blue fluorescence peaking at 470 nm is observed under ZV irradiation. The intensity of this fluorescence increases dramatically under reflux and saturates after ~ 40 hrs. The final mean size of the ZnSe nanocrystals measured by transmission electron microscopy is aboyt 2 nm in diameter. The quantum efficiency of the fluorescence form the final solution is estimated to be ~1%, although the preparation conditions have not yet been completely optimized. These properties are discussed in comparison with those of similarly prepared CdTe and differently prepared ZnSe nanocrystals.

Muscular contraction mimiced by magnetic gels

Abstract: The ability of magnetic-field-sensitive gels to undergo a quick controllable change of shape can be used to create an artificially designed system possessing sensor- and actuator functions internally in the gel itself. The peculiar magneto-elastic properties may be used to create a wide range of motion and to control the shape change and movement, that are smooth and gentle similar to that observed in muscle. Magnetic field sensitive gels provide attractive means of actuation as artificial muscle for biomechanics and biomimetic applications.

Structures of a Magnetorheological Fluid

Abstract: Molecular dynamics simulations were carried out to find the underlying structures of a Magnetorheological (MR) fluid while taking into account dipolar forces, viscous drag, and the Brownian force. Three different structures were found: the bct lattice, chains, and a liquid state. The conditions under which these structures are found is based on two parameters A and B which are the ratios of the dipolar force to the viscous drag force and the Brownian force to the dipolar force respectively.

Size and surface structure of diamond nano-crystals

Abstract: High-resolution transmission electron microscopy and electron nano-diffraction patterns provide useful information concerning the size distribution and surface condition of nano-crystalline diamond powder produced by shock detonation. Analysis of X-ray powder diffractometer data, using the Debye equation to predict the powder profile, starting with a set of spherical diamond ball models, allows the mean size to be obtained for bulk quantities of the same powders. The electron powder diffraction patterns for some diamond spheres, and also spherical shell, models of the surface structures were calculated using the Debye equation, and compared with experiment. It is suggested that the surface structure may become accessible to diffraction methods, if a single nano-crystal is irradiated and both the single crystal pattern plus the diffuse scattering are recorded, as a function of the orientation of that single crystal.

Directed percolation and other systems with absorbing states: impact of boundaries

Abstract: We review the critical behavior of nonequilibrium systems, such as directed percolation (DP) and branching-annihilating random walks (BARW), which possess phase transitions into absorbing states. After reviewing the bulk scaling behavior of these models, we devote the main part of this review to analyzing the impact of walls on their critical behavior. We discuss the possible boundary universality classes for the DP and BARW models, which can be described by a general scaling theory which allows for two independent surface exponents in addition to the bulk critical exponents. Above the upper critical dimension dc, we review the use of mean field theories, whereas in the regime d

LUMINESCENCE PROCESS IN ANATASE TiO2 STUDIED BY TIME-RESOLVED SPECTROSCOPY

Abstract: We have studied time response of self-trapped exciton (STE) luminescence in anatase phase of TiO2 as functions of excitation photon energy and temperature. The decay curves, which consist of two components obeying exponential and power-law decay, are found to depend significantly on the two parameters. The exponential component is stimulated efficiently under excitation near the absorption edge. This fact suggests that under such an excitation condition, photogenerated electron-hole pairs directly form STE's. It is found from temperature dependence of the decay curves that the exponential component is thermally unstable compared with the power-law one. It is inferred that the electron-hole pairs are thermally separated, and thus the direct formation of STE's is prevented.

Static yield stress in magnetorheological fluid

Abstract: A method and a device for measuring a true static yield stress in magnetorheological (MR) fluids are proposed. The data obtained by means of this device are compared with the measured values of the dynamic yield stress for similar compositions as well as with the quantities calculated by the reported models. It is shown that the dynamic yield stress exceeds the static one. The experimental data better agree with Rosensweig's model.

Chaos in a Simple Boolean Network

Abstract: We study the complex dynamics of a simple stochastic Boolean network. The investigated system is equivalent to a randomly connected Boolean cellular automaton. The dynamical evolution of the cellular automaton is exactly described by a polynomial map with binomial coefficients. We show that the map is chaotic and the route to chaos is period-doubling bifurcations.

Exact Partition Function for the Potts Model with Next-Nearest Neighbor Couplings on Arbitrary-Length Ladders

Abstract: We present exact calculations of partition function Z of the q-state Potts model with next-nearest-neighbor spin–spin couplings, both for the ferromagnetic and antiferromagnetic case, for arbitrary temperature, on n-vertex ladders with free, cyclic, and Mobius longitudinal boundary conditions. The free energy is calculated exactly for the infinite-length limit of these strip graphs and the thermodynamics is discussed. Considering the full generalization to arbitrary complex q and temperature, we determine the singular locus ℬ in the corresponding space, arising as the accumulation set of partition function zeros as n → ∞.

Mechanical Properties of AN ER Fluid in Tensile, Compression and Oscillatory Squeeze Tests

Abstract: In this work, the mechanical properties of an anhydrous electrorheological fluid made of carbonaceous particles dispersed in silicone oil were determined in tensile, compression and oscillatory squeeze tests. The mechanical tests were carried out on a Mechanical Testling Machine and the device developed for measuring the ER properties was composed of two parallel steel electrodes between which the ER fluid was placed. The mechanical properties were measured for different DC electric field strengths, velocity and initial gap between the electrodes, and the ERF was tested in two different ways: (a) the fluid was placed between the electrodes (configuration 1) and (b) the electrodes were immersed inside the ERF (configuration 2). The results showed that the ER fluid is more resistant to compression than to tensile, and that the shape of the tensile stress-strain curve and the tensile strength varies with the electric field strength and the initial gap between the electrodes. The compressive stress increased with the increase of the electric field strength and with the decrease of the gap size and upper electrode velocity. In oscillatory test, for both configurations 1 and 2, increasing the oscillation frequency f and the number of cycles N produced a decrease of the damping performance of the ER fluid. Besides this, the damping force of each cycle in oscillatory tests increased with N. The electric field also played an important role on the shape of the hysteresis loop (stress as a function of fluid strain) for both configurations.

Nano-Crystals of c-Diamond, n-Diamond and i-Carbon Grown in Carbon-Ion Implanted Fused Quartz

Abstract: Combined high-resolution transmission electron microscopy, selected area electron diffraction and parallel electron energy loss spectroscopy are used to characterise carbon nano-phases found embedded in fused quartz. These appear after implantation of 1 MeV carbon ions, followed by annealing in argon, oxygen and forming gas for 1 hour at 1100°C. For Ar, virtually all of the carbon diffuses out of the substrate with no observable carbon clusters for all doses studied. After annealing in oxygen, a crystalline COx phase is identified at the end of range, following a dose of 5×1017C/cm2. Three nano-crystalline carbon phases, including diamond, appear after annealing in forming gas: these form a layer 170 nm beneath the fused quartz surface for all ion doses. The average size of these clusters and the corresponding phases depend on the ion dose; the smallest size of 5–7 nm diameter crystallise as fcc diamond following a dose of 0.5× 1017C/cm2, whereas clusters of 8–13 nm diameter, for a higher dose of 2× 1017C/cm2, have a modified phase of diamond known as n-diamond. The largest clusters, diameter 15–40 nm, for a dose of 5× 1017C/cm2, have the cubic P213 (or P4232) structure known as i-carbon. These buried layered diamond and diamond-related materials may have applications for field emission and optical waveguide type devices.

Candidates for μ<0, ∊<0 Nanostructures

Abstract: Recent interest has been generated in composite materials for which both the dielectric constant and the magnetic permeability are negative. These composites behave as if they possess a negative index of refraction, although the broader, less specific adjective "left-handed" has also been applied to them. Such composites possess two sets of structures which are separately responsible for the negative ∊ and μ. However, materials having a negative μ are common in the microwave frequency range and such a material can replace one set of structures in the composite. An example of a simple composite is a YIG medium (μ<0) penetrated by sets of metallic strips or wires which give rise a negative dielectric constant in the appropriate frequency range. The index of refraction of the composite can be modulated by adjusting the applied magnetic field.

Cooperative phase dynamics in coupled nephrons

Abstract: The individual functional unit of the kidney (the nephron) displays oscillations in its pressure and flow regulation at two different time scales: Relatively fast oscillations associated with the myogenic dynamics of the afferent arteriole, and slower oscillations related with a delay in the tubuloglomerular feedback. Neighboring nephrons interact via vascularly propagated signals. We study the appearance of various forms of coherent behavior in a model of two such interacting nephrons. Among the observed phenomena are in-phase and anti-phase synchronization of chaotic dynamics, multistability, and partial phase synchronization in which the nephrons attain a state of chaotic phase synchronization with respect to their slow dynamics, but the fast dynamics remains desynchronized.

Studies of electronic properties of medium and large molecules oriented in a strong uniform electric field

Abstract: Polarization spectroscopy of oriented gas phase medium and large molecules achieved via a uniform DC electric field provides a means to determine the direction of transition dipoles. In this article, the theoretical background of this orientation method, its characterization, and its application in studies of electronic transitions, will be presented. Mature gas phase spectroscopic methods have been developed for studies of small molecules, but studies of medium to large sized species are faced with special challenges. These challenges arise from differences between large and small molecules: large systems typically exhibit fast internal conversion, slow dissociation, and low translational energy release upon dissociation. Thus conventional gas phase spectroscopic techniques are not applicable to derive the direction of the transition dipole. DC induced orientation offers a solution to this problem. It is ideal for studies of systems with small rotational constants and large permanent dipoles, even when a detailed knowledge of the molecular structure, such as the direction of the permanent dipole in the molecular frame, is unknown. The degree of orientation can be calculated using the linear variation method, given the rotational temperature and the size of the permanent dipole. The associated experimental observables can be used to confirm the effect of orientation, or to determine the direction of a transition dipole. These observables include the ratio of excitation probabilities under different polarization directions and spectroscopic features. In some cases, the direction and size of the permanent dipole of the excited electronic state can also be determined. Examples of this type of polarization spectroscopy are presented for asymmetric tops such as diazines, acetelye-HF clusters, nitroaromatics and butyl nitrite. Illustrations of pendular states and its application in linear and diatomic molecules are also briefed. Applications of this method for studies of large molecules and potential pitfalls will be discussed.