Memory effects in complex materials and nanoscale systems

Memory effects are ubiquitous in nature and are particularly relevant at the nanoscale where the dynamical properties of electrons and ions strongly depend on the history of the system, at least within certain time scales. We review here the memory properties of various materials and systems which appear most strikingly in their non-trivial, time-dependent resistive, capacitative and inductive characteristics. We describe these characteristics within the framework of memristors, memcapacitors and meminductors, namely memory-circuit elements with properties that depend on the history and state of the system. We examine basic issues related to such systems and critically report on both theoretical and experimental progress in understanding their functionalities. We also discuss possible applications of memory effects in various areas of science and technology ranging from digital to analog electronics, biologically inspired circuits and learning. We finally discuss future research opportunities in the field.     

Theoretical studies of mutual diffusivities and surface properties in Cd-Ga liquid alloys

The mutual diffusion coefficients, activation energy for diffusion, surface concentration and surface tension of the liquid Cd-Ga alloys have been calculated throughout the concentration range using energetics obtained from the theoretical model fits of the experimental thermodynamic data. Our calculations show that Cd-Ga liquid alloys showed immiscibility at a temperature of 723 K between 0.4 and 0.5 atomic fraction of Cd. Within this concentration region and temperature, the mutual diffusion coefficients approached zero values and the activation energy for diffusion reached high peak values of 220 J/mol. It was also observed that within the region of immscibility, the diffusion coefficients do not obey Arrhenius relation over a wide range of temperature.     

Neutrons for fuel cell membranes: Structure, sorption and transport properties

A molecular level understanding of structure and transport properties in fuel cell ionomer membranes is essential for designing new electrolytes with improved performance. Scattering techniques are suited tools for this purpose. In particular, neutron scattering, which has been extensively used in hydrogen-containing systems, is well adapted to investigate water-dependent complex polymeric morphologies. We report Small-Angle Neutron Scattering (SANS) studies on different types of fuel cell polymers: perfluorinated, radiation-grafted and sulfonated polyphosphazene membranes. We show that contrast variation methods can be efficiently employed to provide new insights on membrane microstructure and reveal ionic condensation effects. Neutrons have been used also as non-intrusive diagnosis tool to probe water properties and distribution inside membranes. Recently, in-situ neutronography and SANS experiments on operating fuel cells have been reported. In-plane cartography of water distribution at the surface of bipolar plates and water profiles across membrane thickness have been obtained and studied as a function of operating conditions. The last section of the article is devoted to the use of Quasi-Elastic Neutron Scattering to study water dynamics at molecular scale. We show that analysis with an appropriate sophisticated diffusion model allows to extract diffusion coefficients, characteristic times and length-scales of molecular motions. This quantitative information is fruitfully integrated in multi-scale modelling and usefully compared with numerical simulations. QENS also permits to compare alternative polymers and relate dynamical properties to chemical composition and membrane nanostructure.     

Al^q＋（q=0～12）的电子碰撞电离截面和速率系数

使用扭曲波玻恩交换(DWBE)近似计算Al^q (q=0～12)的电子碰撞电离截面和速率系数,其中电离截面的高能行为由Bethe系数决定。分析了计算数据随电离度的变化规律,并且与可得到的实验的和其他计算的数据进行了比较,表明结果可靠。     

Data and modeling of negative ion transport in gases of interest for production of integrated circuits and nanotechnologies

We review techniques to prepare, evaluate and apply sets of cross section and transport data for negative ions that are required for the modeling of collisional non-equilibrium plasmas used for processing of microelectronic circuits. We collect and discuss the transport coefficients and cross section sets.We have compiled data for negative ions in CF₄ and CF₄-related negative ions in rare gases. In addition, we consider data for F⁻ and CF₃⁻ in rare gases. Furthermore, we analyze the cross sections of halogen negative ions in rare gases and other molecules. This is followed by the data for SF₆ related ions in SF₆ and in rare gases. The cross section for scattering of O⁻ in O₂ has been derived from the transport data and used to make calculations of the transport properties. Finally we give a brief discussion of the availability of the data for H⁻ ions in H₂. We have derived cross sections in several cases but the basic aim is to show the basic features of transport coefficients. In particular we discuss the need to represent properly some details such as the non-conservative nature of transport coefficients and the anisotropy of diffusion. Application of approximate theories and representations of cross sections are also discussed.     

Enhanced diffusion by reciprocal swimming

Purcell's scallop theorem states that swimmers deforming their shapes in a time-reversible manner ("reciprocal" motion) cannot swim. Using numerical simulations and theoretical calculations we show here that, in a fluctuating environment, reciprocal swimmers undergo, on time scales larger than that of their rotational diffusion, diffusive dynamics with enhanced diffusivities, possibly by orders of magnitude, above normal translational diffusion. Reciprocal actuation does therefore lead to a significant advantage over nonmotile behavior for small organisms such as marine bacteria.     

Statistical calculations of tracer and intrinsic diffusion coefficients in concentrated alloys and estimates of microscopic parameters of diffusion from experimental data

The earlier-developed statistical theory of diffusion in concentrated alloys based on the master equation approach is generalized to treat tracer diffusion in binary alloys. The theory developed is used to describe concentration dependencies of both tracer and intrinsic diffusion coefficients and to estimate microscopic parameters of diffusion in alloys CuNi, CuZn and AgCd for which necessary experimental data are available. We show that all main features of strong and peculiar concentration dependencies of diffusion coefficients observed in these alloys are naturally explained by the theory. Signs and scales of interatomic interactions important for diffusion in these alloys are found to strongly depend on the ratio of atomic sizes of alloy components, and types of these dependencies agree with simple physical considerations. We also discuss physical reasons for sharp concentration dependencies of diffusion coefficients characteristic of real alloys.     

Regression Analysis of Confocal FRAP and its Application to Diffusion in Membranes

In most biological processes, diffusion plays a critical role in transferring various bio-molecules to transfer desirable locations in an effective and energy-efficient manner. How fast molecules are transferred is measured by diffusion coefficients. Since each bio-molecules, in particular, signaling molecules have their unique diffusion coefficients and quantifying the diffusion coefficients help us to understand various time scales of both physiological and pathological processes in biological systems. Moreover, since diffusion profiles of a diffusant vary in different micro-environments of cell membranes, accurate diffusion coefficient also can provide a good picture of membrane landscapes as well as interactions of different membrane constituents. Currently, only a few experimental methods are available to assess the diffusion coefficient of a biomolecule of interest in live cells including Fluorescence Recovery After Photobleaching (FRAP). FRAP was developed to study diffusion processes of biomolecules in the cell membranes in the 1970s. Albeit its long history, the main principle of FRAP analysis has remained unchanged since its inception: fitting FRAP data to a theoretical diffusion model for the best fitting diffusion coefficient or using the relation between the half time of recovery and ROI size. In this study, we developed a flexible yet versatile confocal FRAP data analysis framework based on linear regression analysis which allows FRAP users to determine the diffusion from either single or multiple FRAP data points without data fitting. We also validated this approach for a series of fluorescently labeled soluble and membrane-bound proteins and lipids.

     

Phase separating bulk metallic glass: a hierarchical composite

Phase separating systems present a unique opportunity for designing composites with hierarchical microstructure at different length scales. We report here our success in synthesizing phase separating metallic glasses exhibiting the entire spectrum of microstructural possibilities expected from a phase separating system. In particular, we report novel core shell and hierarchical structures of spherical glassy droplets, resulting from critical wetting behavior and limited diffusion. We also report synthesis of a bulk phase separating glass in a metallic glass system. The combination of unique core shell and hierarchical structures in metallic glass systems opens a new avenue for the microstructure design of metallic glasses.     

Scattering of electrons with formyl radical

A detailed theoretical study is carried out for electron interactions with formyl radical (HCO) with incident energies ranging from 0.01 to 5000 eV. This wide range of energy has allowed us to investigate a variety of processes and report data on vertical electronic excitation energies, dissociative electron attachment (DEA) and total cross-section along with scattering rate coefficients. We observed Ramsaur–Townsend minimum at 0.59 and 0.74 eV using DZP and cc-pDVZ basis sets, respectively. HCO has large number of low-lying excited states and the present study finds an overall good agreement with earlier reported data. In order to compute total cross-section, we have employed ab initio R-matrix method (0.01 to ～ 20 eV) and the spherical complex optical potential method (～ 10 to 5000 eV) employing static-exchange plus polarisation potential. The R-matrix calculations are performed using a close coupling method with the aid of 21 target states, 1191 configuration state functions and 195 channels. The DEA cross-sections of fragmentation of H^?, excitation cross- sections and scattering rate coefficients are reported for the first time. Total cross section presented here will provide a reference data set over an extensive impact energy range.     

On the porosity of coldly condensed sers active Ag films: II. Comparison of adsorption and Raman scattering of pyridine

The adsorption of pyridine on coldly deposited Ag films annealed at temperatures ranging from 58 to 330 K, the porous surface topography of which has been investigated in part I of this work, has been studied by means of UPS, work function change and thermal desorption measurements. Pyridine induced work function changes have been employed to follow the surface diffusion of pyridine molecles into the pores of these Ag films. The surface diffusion is very slow below 60 K, but readily takes place at 130 K with an estimated activation energy of surface migration of E_m ≈ 4 kcal/mol. Preadsorption of Xe into the pores of the films causes inhibition of pyridine diffusion into the pores. The onset of pyridine desorption from porous films is detected at ≈ 200 K while from flat films the desorption begins already at 150 K. The careful analysis of our data on the structure of the coldly deposited Ag films and the adsorption behavior of pyridine on these films as well as a survey of published SERS data lead us to conclude that the SERS active sites of coldly deposited Ag films are within the pores. This conclusion is in agreement with recent theoretical calculations.     

Zero-point energy,tunnelling, and vibrational adiabaticity in the Mu + H2 reaction

Isotopic substitution of muonium for hydrogen provides an unparalleled opportunity to deepen our understanding of quantum mass effects on chemical reactions. A recent topical review in this journal of the thermal and vibrationally state-selected reaction of Mu with H₂ raises a number of issues that are addressed here. We show that some earlier quantum mechanical calculations of the Mu + H₂ reaction, which are highlighted in this review, and which have been used to benchmark approximate methods, are in error by as much as 19% in the low-temperature limit. We demonstrate that an approximate treatment of the Born–Oppenheimer diagonal correction that was used in some recent studies is not valid for treating the vibrationally state-selected reaction. We also discuss why vibrationally adiabatic potentials that neglect bend zero-point energy are not a useful analytical tool for understanding reaction rates, and why vibrationally non-adiabatic transitions cannot be understood by considering tunnelling through vibrationally adiabatic potentials. Finally, we present calculations on a hierarchical family of potential energy surfaces to assess the sensitivity of rate constants to the quality of the potential surface.     

Determination of bulk and surface transport properties by photocurrent spectral measurements

Reliable minority carrier diffusion length and surface recombination velocity values have been obtained from stationary photocurrent measurements. A modified surface photovoltage method has been used to determine diffusion lengths longer than the wafer thickness in high-purity Si, whereas the spectral variation of the photocurrent has been employed to measure the surface recombination velocity. The novelty presented in this paper is that a Schottky diode has been employed in both the methods to collect generated charged carriers. Moreover the same Schottky diode has been employed in both the methods in order to avoid any a priori assumptions on the material transport parameters. This combined application of the two methods at the same device enables the determination of highly reliable results. Received: 17 February 2000 / Accepted: 28 March 2000 / Published online: 30 June 2000     

Microscopic transport theory of heavy-ion collisions

Analytical expressions for the diffusion coefficients for the transfer of neutrons and protons in a heavy-ion collision are derived. The mass diffusion coefficient implied by these two coefficients is compared with previous theoretical calculations and the values extracted from experiment for a variety of systems. Drift coefficients are related to the diffusion coefficients and the driving potential by the usual Einstein relations. On approximating the driving potential by a second-order form, the Fokker-Planck equation is solved analytically by the moment expansion method. A comparison of the predictions of this theory with a variety of recent experimental data gives generally good agreement. Thus we conclude that the experimental data currently available can be interpreted in purely statistical terms. Some deviations between theory and experiment indicate a possible influence of shell effects and/or direct transfer processes in the early stages of the reaction.     

Influence of entropy-fluctuation on the Rayleigh spectrum

We discuss the light scattering cross section due to entropy-fluctuation on the region of the central peak of the spectrum. A back-scattering oblique incidence geometry is used in our theoretical calculations which are based on the linear response function approach. Our results show that the power spectrum varies significantly with the incident angle and frequency and so does the light scattering cross section. The results are illustrated with numerical calculations for silicon.Work partially supported by the Brazilian Research Agencies CNPq and FINEP     

A Depolarized Dynamic Light Scattering Method to Calculate Translational and Rotational Diffusion Coefficients of Nanorods

A new analysis of depolarized dynamic light scattering data is presented, which allows the unambiguous determination of rotational and translational diffusions coefficients of nanorods in suspension. By visualizing data scaling, purely translational diffusive motions can be isolated from vertically polarized scattering, allowing the unique determination of rotational diffusion from the depolarized scattering. The method is applied to nanorods with four different aspect ratios, and compared with theoretical predictions. Diffusion coefficients obtained show good agreement with calculations based on the direct measurements of rod length and diameter. Where the theories are shown to be valid, the method allows the measurement of statistically meaningful particle sizes and aspect ratios.