Mean Field Theory of Self-Organizing Memristive Connectomes

Biological neuronal networks are characterized by nonlinear interactions and complex connectivity. Given the growing impetus to build neuromorphic computers, understanding physical devices that exhibit structures and functionalities similar to biological neural networks is an important step toward this goal. Self-organizing circuits of nanodevices are at the forefront of the research in neuromorphic computing, as their behavior mimics synaptic plasticity features of biological neuronal circuits. However, an effective theory to describe their behavior is lacking. This study provides for the first time an effective mean field theory for the emergent voltage-induced polymorphism of circuits of a nanowire connectome, showing that the behavior of these circuits can be explained by a low-dimensional dynamical equation. The equation can be derived from the microscopic dynamics of a single memristive junction in analytical form. The effective model is tested on experiments of nanowire networks and show that it fits both the potentiation and depression of these synapse-mimicking circuits. It is shown that this theory applies beyond the case of nanowire networks by formulating a general mean-field theory of conductance transitions in self-organizing memristive connectomes.     

Kinetic temperature and electron density measurement in?an?inductively coupled plasma torch using degenerate four-wave mixing

Laser wave mixing is presented as an effective technique for spatially resolved kinetic temperature measurements in an atmospheric-pressure radio-frequency inductively coupled plasma. Measurements are performed in a 1 kW, 27 MHz radio-frequency plasma using a continuous-wave, tunable 811.5 nm diode laser to excite the 4s³P₂→4p³D₃ argon transition. Kinetic temperature measurements are made at five radial steps from the center of the torch and at four different torch heights. The kinetic temperature is determined by measuring simultaneously the line shape of the sub-Doppler backward phase-conjugate degenerate four-wave mixing and the Doppler broadened forward-scattering degenerate four-wave mixing. The temperature measurements result in a range of 3,500 to 14,000±150 K. Electron densities measured range from 6.1 (±0.3)×10¹⁵ cm⁻³ to 10.1 (±0.3)×10¹⁵ cm⁻³. The experimental spectra are analyzed using a perturbative treatment of the backward phase-conjugate and forward-geometry wave-mixing theory. The Stark width is determined from the collisional broadening measured in the phase-conjugate geometry. Electron density measurements are made based on the Stark width. The kinetic temperature of the plasma was found to be more than halved by adding deionized water through the nebulizer.     

Renormalization-group improvement of effective actions beyond summation of leading logarithms

Invariance of the effective action under changes of the renormalization scale μ leads to relations between those (presumably calculated) terms independent of μ at a given order of perturbation theory and those higher-order terms dependent on logarithms of μ. This relationship leads to differential equations for a sequence of functions, the solutions of which give closed form expressions for the sum of all leading logs, next to leading logs, and subsequent subleading logarithmic contributions to the effective action. The renormalization group is thus shown to provide information about a model beyond the scale dependence of the model's couplings and masses. This procedure is illustrated using the φ₆³ model and Yang–Mills theory. In the latter instance, it is also shown by using a modified summation procedure that the μ dependence of the effective action resides solely in a multiplicative factor of g²(μ) (the running coupling). This approach is also shown to lead to a novel expansion for the running coupling in terms of the one-loop coupling that does not require an order-by-order redefinition of the scale factor ΛQCD. Finally, logarithmic contributions of the instanton size to the effective action of an SU(2) gauge theory are summed, allowing a determination of the asymptotic dependence on the instanton size ρ as ρ goes to infinity to all orders in the SU(2) coupling constant.     

Stability of two-dimensional,controlled, Bose-Einstein coherent states

Two-dimensional stability of a controlled Bose-Einstein condensation state, in the form of a nonlinear Schr?dinger soliton [JETP Lett. 80 535 (2004)], is studied for the condensations with both repulsive and attractive inter-atom interactions. The Gross-Pitaevski equation is solved numerically, taking initialy a controlled soliton whose “effective mass” is several times bigger than the critical value for a weak collapse in the absence of a potential well, and allowing for reasonably large errors in the experimental realization of the trapping potential required by the theory. For repulsive and sufficiently weak attractive interactions, the controlled state is shown to remain stable inside a breathing potential well, for a time that is an order of magnitude longer than the characteristic periods of the forced and eigenoscillations of the soliton. The collapse is observed only for attractive interactions, when the nonlinear attraction exceeded the appropriate threshold. Electronic supplementary material  Supplementary Online Material     

A heuristic screening approach for atoms includingl-splitting

Summary  TheZ-expansion theory of Atomic Structure is supplemented with the inclusion of the external screening concept, calculated from a simplified version of the works of M. Kregar, where the two-body potential energy operators are split into the sum of effective one-body operators. The use ofZ- andN-dependent screening parameters instead of screening constants, gives a simplified Screened Hydrogenic Model which compares favourably with more sophisticated methods, specially for ionized atoms. The author of this paper has agreed to not receive the proofs for correction. Member of the Consejo Nacional de Investigaciones Cientificas y Tecnicas, Argentina, and Associate Member ICTP (Trieste, Italy).     

Design and Simulation of an Ultra-Broadband Ku-BandGyro-TWT for Radar Applications

In this paper, the design of an ultra-broadband Ku-band gyro-TWT amplifier is presented in detail. The preliminary parameters of the interaction structure derived from the dispersion relationship and the linear theory of the gyro-TWT with distributed wall losses is optimized by the self-consistent nonlinear code. The performance of the designed gyro-TWT is simulated by the nonlinear code. The simulation results show that the gain and the bandwidth of the designed Ku-band gyro-TWT is about 36.5 dB and 2.1 GHz with 3 dB bandwidth (about 12.7%) respectively at the input power 19.0 W.     

Correlation effects,image charge effects and finite size in the macro-ion-electrolyte system: A field-theoretic approach

We consider a model of a macro-ion surrounded by small ions of an electrolyte solution. The finite size of ionic charge distributions of ions, and image charge effects are considered. From such a model it is possible to construct a statistical field theory with a single fluctuating field and derive physical interpretations for both the mean field and two-point correlation function. For point-like charges, at the level of a Gaussian (or saddle point) approximation, we recover the standard Poisson-Boltzmann equation. However, to include ionic correlation effects, as well as image charge effects of individual ions, we must go beyond this. From the field theory considered, it is possible to construct self-consistent approximations. We consider the simplest of these, namely the Hartree approximation. The Hartree equations take the form of two coupled equations. One is a modified Poisson-Boltzmann equation; the other describes both image charge effects on the individual ions, as well as correlations. Such equations are difficult to solve numerically, so we develop an (a WKB-like) approximation for obtaining approximate solutions. This, we apply to a uniformly charged rod in univalent electrolyte solution, for point like ions, as well as for extended spherically symmetric distributions of ionic charge on electrolyte ions. The solutions show how correlation effects and image charge effects modify the Poisson-Boltzmann result. Finite-size charge distributions of the ions reduce both the effects of correlations and image charge effects. For point charges, we test the WKB approximation by calculating a leading-order correction from the exact Hartree result, showing that the WKB-like approximation works reasonably well in describing the full solution to the Hartree equations. From these solutions, we also calculate an effective charge compensation parameter in an analytical formula for the interaction of two charged cylinders. Electronic supplementary material  Supplementary material in the form of a doc file available from the journal web page at and are accessible for authorised users.     

Nonlinear photoionization and laser-induced damage in silicate glasses by infrared ultrashort laser pulses

We show that photoionization of wide band gap silicate glasses by infrared ultrashort laser pulses can occur without laser-induced damage. Two glasses are studied, fused silica and a multi-component silicate photo-thermo-refractive (PTR) glass. Experiments are performed by low numerical aperture focusing of ultrashort laser pulses (100 fsec<τ<1.5 psec) at the wavelengths 780 nm, 1430 nm, and 1550 nm. Filaments form inside both glasses and are visibly observable due to intrinsic luminescence. Keldysh’s theory of nonlinear photoionization is used to model the formation of filaments and values of about 10¹³ W cm⁻² for the laser intensity and 10¹⁹ cm⁻³ for the free electron density are estimated for stable filaments to arise. Laser-induced damage is studied by the generation of a third harmonic from an interface created between a damage site and the surrounding glass matrix. It is found that third harmonic generation occurs only after several thousands of laser shots indicating that damage is not a single-shot phenomena. The ability to photoionize PTR glass without damage by ultrashort laser pulses offers a new approach for fabricating diffractive optical elements in photosensitive glass.     

Quantitative determination of magnetic fields from iron particles of oblong form encapsulated by carbon nanotubes using electron holography

Using electron holography (interference electron microscopy) we have made measurements of the magnetic flux and magnetic field distribution around a carbon nanotube filled with iron. At the surface of the carbon nanotube, an iron particle with a radius of 30 nm and a length of 200 nm created a magnetic flux of 10⁻¹⁵ Wb (Weber) and a magnetic field of 0.3–0.4 T (Tesla). The theory developed in this work is constrained to the case of cylindrical symmetry of the investigated ferromagnetic particles, but, in general, such studies can be made for ferromagnetic particles of any shape.     

Design of a polymer directional coupler electro-optic switch using two-section reversed electrodes

By using the coupled mode theory, electro-optic modulation theory, conformal transforming method and image method, the structure is designed, the parameters are optimized, and the characteristics are analyzed for a polymer directional coupler electro-optic switch with two-section reversed electrodes. Simulation shows that the designed device exhibits excellent switching functions. Under the operation wavelength of 1550 nm, the electro-optic coupling region length is 4751 μm, the cross-state and bar-state voltages are about 1.22 and 2.65 V, and the insertion loss and crosstalk are less than 2.21 and −30 dB, respectively. By slightly adjusting the state voltages, the blight of the fabrication errors on the switching characteristics can be easily eliminated. The calculation results of the presented technique are in good agreement with those of the beam propagation method (BPM).     

Post-growth tailoring of quantum-dot saturable absorber mirrors by chemical etching

We have designed and grown a resonant, low-finesse quantum-dot saturable absorber mirror and subsequently modified the important parameters using chemical etching. The modulation depth and saturation fluence at the design wavelength of 1064 nm were modified by etching the sample to tune the cavity resonance. The device properties were characterised using normal incidence spectroscopic reflectivity measurements, intensity dependent reflectivity measurements and modelled using a transfer matrix approach. The saturable absorber mirror was used to facilitate self-starting, passively mode locked pulses in a neodymium vanadate laser operating at 1064 nm. The etching was found to affect the duration of the pulses, leading to temporal width tuning over a range of 94 ps. The shortest pulse duration of 84 ps was achieved for the cavity resonance close to 1064 nm, with an output power of 3 W. This method is an effective technique for post-growth engineering of the properties of semiconductor saturable absorber mirrors (SESAMs) with nanometre precision.     

One-loop effective action for non-local modified Gauss–Bonnet gravity in de Sitter space

We discuss the classical and quantum properties of non-local modified Gauss–Bonnet gravity in de Sitter space, using its equivalent representation via string-inspired local scalar-Gauss–Bonnet gravity with a scalar potential. A classical, multiple de Sitter universe solution is found where one of the de Sitter phases corresponds to the primordial inflationary epoch, while the other de Sitter space solution—the one with the smallest Hubble rate—describes the late-time acceleration of our universe. A Chameleon scenario for the theory under investigation is developed, and it is successfully used to show that the theory complies with gravitational tests. An explicit expression for the one-loop effective action for this non-local modified Gauss–Bonnet gravity in the de Sitter space is obtained. It is argued that this effective action might be an important step towards the solution of the cosmological constant problem.     

Curvilinear Fresnel-Zone Plate Lens Antenna: Vector Radiation Theory

A generalized vector diffraction theory of the half-open curvilinear Fresnel zone plate (FZP) tens antenna that is valid for any lens profile shape is presented. It is an extension to the vector Kirchhoff diffraction theory for the plane half-open FZP lens antenna and is based on the conical-segment lens profile approximation. An equation for the electric far-field vector is derived from which follow the expressions for the co- and cross-polarization radiation patterns and directive gain. The proposed theory is utilized for a numerical analysis and comparison of 140-GHz curvilinear half-open FZP lens antennas grouped in two distinct sets:

On the size dependence of surface tension in the temperature range from melting point to critical point

The problem of size dependence of surface tension was investigated in view of a more general problem of the applicability of Gibbs’ thermodynamics to nanosized objects. For the first time, the effective surface tension (coinciding with the specific excess free energy for an equimolecular dividing surface) was calculated within a wide temperature range, from the melting temperature to the critical point, using the thermodynamic perturbation theory. Calculations were carried out for Lennard-Jones and metallic nanosized droplets. It was found that the effective surface tension decreases both, with temperature and particle size.     

Quantum confinement and surface-state effects in bismuth nanowires

Bulk bismuth is an efficient thermoelectric material. Assuming intrinsic conditions, the theory of quantum confinement of bismuth nanowires by Hicks and Dresselhaus predicts a semimetal-to-semiconductor transformation for critical diameters of around 50 nm. For nanowires of diameters below the critical diameter, electronic states can be considered to be one dimensional and therefore the thermopower can be very large. However, angle-resolved photoemission spectroscopy (ARPES) studies of Bi planar surfaces present direct evidence of heavy mass surface states that can inhibit the semimetal-to-semiconductor transformation. We present a study of the Fermi surface of Bi nanowires of diameters ranging between 200 and 30 nm employing the Shubnikov–de Haas method. Our results can be understood in terms of the model of surface states. For 30 nm nanowires we find that the Fermi surface is spherical, that the carriers have high effective mass, and that the number of carriers corresponds to that inferred from ARPES measurements.     

Diffusion of magnetic field lines in a confined RFP plasma

Summary  A volume-preserving symplectic map is proposed to describe the magnetic field lines when the Taylor equilibriumis perturbed in a generic way. The standard scenario is observed by varying the perturbation strength, but the statistical properties in the chaotic regions are not simple due to the presence of boundaries and remnants of invariant structures. Simpler models of volume-preserving maps are proposed. The slowly modulated standard map captures the basic topological and statistical features. The diffusion is analytically described for large perturbations (above the break-up of the last KAM torus) in terms of correlation functions and for small perturbations using the adiabatic theory, provided that the modulation is sufficiently slow.     

CdS plume composition and dynamics of neutral species upon ablation with 532 nm laser light

The neutral species present in CdS ablation plumes upon nanosecond 532 nm laser irradiation at a moderate fluence of 0.5–0.75 J cm⁻² have been studied. Neutral Cd _n S _m clusters have been identified, some as large as (CdS)₃₃₋₃₄ (1–2 nm in diameter). The analysis of the dynamics of neutral species shows an expansion with two components that differ both in composition and dynamics. A fast, high kinetic energy component, dominated by S₂ which acquires free-flow conditions at short distances from the target, is followed by a slower component characterized by similar speeds for all species. This slower component shows dynamic features that are expected to favor aggregation processes leading to effective cluster formation.     

Ab-initio calculation of optical properties of AA-stacked bilayer graphene with tunable layer separation

Density functional theory based calculations revealed that optical properties of AA-stacked bilayer graphene are anisotropic and highly sensitive to the interlayer separation. In the long wave length limit of electromagnetic radiation, the frequency dependent response of complex dielectric function becomes vanishingly small beyond the optical frequency of 25.0 eV. Besides, static dielectric constant shows a saturation behaviour for parallel polarization of electric field vector when interlayer spacing is greater than 2.75 Å. As a consequence, an appropriate modification of effective fine structure constant is observed as a function of layer separation. Moreover, the bilayer systems are highly transparent in the optical frequency range of 7.0–10.0 eV. The electron energy loss function exhibits two different in-plane collective (plasmon) excitations and a single out of plane plasmon excitation. The spectral nature of different frequency dependent optical properties is observed to be very similar to that of the monolayer pristine graphene apart from their exact numerical values.     

SPDM: light microscopy with single-molecule resolution at?the?nanoscale

Far-field fluorescence techniques based on the precise determination of object positions have the potential to circumvent the optical resolution limit of direct imaging given by diffraction theory. In order to use localization to obtain structural information far below the diffraction limit, the ‘point-like’ components of the structure have to be detected independently, even if their distance is lower than the conventional optical resolution limit. This goal can be achieved by exploiting various photo-physical properties of the fluorescence labeling (‘spectral signatures’). In first experiments, spectral precision distance microscopy/spectral position determination microscopy (SPDM) was limited to a relatively small number of components to be resolved within the observation volume. Recently, the introduction of photoconvertable molecules has dramatically increased the number of components which can be independently localized. Here, we present an extension of the SPDM concept, exploiting the novel spectral signature offered by reversible photobleaching of fluorescent proteins. In combination with spatially modulated illumination (SMI) microscopy, at the present stage, we have achieved an estimated effective optical resolution of approximately 20 nm in the lateral and 50 nm in the axial direction, or about 1/25th–1/10th of the exciting wavelength.