Electrical measurements of nanoscale bismuth cluster films

A range of percolating atomic cluster films, with nanoscale overall dimensions, have been studied using a combination of in situ and ex situ electrical transport measurements, together with field emission electron microscopy and atomic force microscopy. Bismuth clusters with mean diameter 20 nm were deposited between electrical contacts defined by electron beam lithography. The morphology of the films can be understood within percolation theory, and the electrical measurements show complex behaviour characteristic of both percolation effects and modification of the cluster films by current flow and by oxidation.     

无规分形衬底上银膜逾渗系统的交流特性

通过对沉积在具有无规自相似结构的α-Al₂O₃断裂面上的银薄膜逾渗系统交流特性的测量,证明了在逾渗阈值p_c附近这一系统的交流电导率和介电常数满足指数临界规律和普适标度关系。还给出了这种逾渗系统的电容与直流电阻的关系,并对其意义作了讨论。 关键词：     

Percolation of polyatomic species on a square lattice

In this paper, the percolation of (a) linear segments of size k and(b) k-mers of different structures and forms deposited on a square lattice have been studied. In the latter case, site and bond percolation have been examined. The analysis of results obtained by using finite size scaling theory is performed in order to test the universality of the problem by determining the numerical values of the critical exponents of the phase transition occurring in the system. It is also determined that the percolation threshold exhibits a exponentially decreasing function when it is plotted as a function of the k-mer size. The characteristic parameters of that function are dependent not only on the form and structure of the k-mers but also on the properties of the lattice where they are deposited.Received: 3 September 2003, Published online: 23 December 2003PACS: 64.60.Ak Renormalization-group, fractal, and percolation studies of phase transitions - 68.35.Rh Phase transitions and critical phenomena - 68.35.Fx Diffusion; interface formation     

Nonlinear resistance of polymer composites with carbon nanotube additives in the percolation state

The electrical properties of a polymer composite with carbon nanotube additives have been analyzed. The state of the system near the percolation threshold, when charge is transferred along a single percolation path, has been considered. For this state, the current–voltage characteristics of a percolation chain made up of carbon nanotubes have been calculated under the assumption that the contact resistance between neighboring nanotubes is much higher than the intrinsic resistance of the nanotubes. According to recent data, the distance between neighboring (contacting) nanotubes has been assumed to be randomly distributed. It has been shown that, under the given conditions, the current–voltage characteristic is essentially nonlinear. This indicates the nonohmic conductivity of the composites. The dependence of the current–voltage characteristic on the spread of the contact distribution over distances has been discussed.     

Correlated percolation in a film-deposition model

Clustering phenomenon has been studied in a film-deposition model in which a monolayer of particles is deposited onto a substrate. The occupation of a given site is assumed to depend on the occupation states of its nearest neighbouring sites as well as on the temperature of the particles being deposited. It is found that the percolating clusters remain ramified in that the number of their boundary sites are proportional to the number of particles in the cluster. The percolation threshold, however is lowered to (54±2)% as compared to 59% density for the uncorrelated case.     

Ag-MgF2复合纳米金属陶瓷薄膜的渗透阈研究

介绍了几种计算纳米金属颗粒镶嵌于陶瓷基体中而形成的复合纳米金属陶瓷薄膜渗透阈的理论方法，分析了理论方法中所运用模型的特点及其精度.并将Landauer有效介质理论和Priou渗透阈理论应用于Ag-MgF₂复合纳米金属陶瓷薄膜的渗透阈计算，所得值分别为0.08和0.14(Ag的体积分数)，按Priou渗透阈理论计算的结果与实验结果相符.最后讨论了影响复合金属陶瓷薄膜体系渗透阈的主要因素. 关键词： 2复合纳米金属陶瓷薄膜')" href="#">Ag-MgF₂复合纳米金属陶瓷薄膜 渗透阈理论 渗透阈     

Threshold model of cascades in empirical temporal networks

Threshold models try to explain the consequences of social influence like the spread of fads and opinions. Along with models of epidemics, they constitute a major theoretical framework of social spreading processes. In threshold models on static networks, an individual changes her state if a certain fraction of her neighbors has done the same. When there are strong correlations in the temporal aspects of contact patterns, it is useful to represent the system as a temporal network. In such a system, not only contacts but also the time of the contacts are represented explicitly. In many cases, bursty temporal patterns slow down disease spreading. However, as we will see, this is not a universal truth for threshold models. In this work we propose an extension of Watts’s classic threshold model to temporal networks. We do this by assuming that an agent is influenced by contacts which lie a certain time into the past. I.e., the individuals are affected by contacts within a time window. In addition to thresholds in the fraction of contacts, we also investigate the number of contacts within the time window as a basis for influence. To elucidate the model’s behavior, we run the model on real and randomized empirical contact datasets.     

Nonequilibrium phase transition on a randomly diluted lattice

We show that the interplay between geometric criticality and dynamical fluctuations leads to a novel universality class of the contact process on a randomly diluted lattice. The nonequilibrium phase transition across the percolation threshold of the lattice is characterized by unconventional activated (exponential) dynamical scaling and strong Griffiths effects. We calculate the critical behavior in two and three space dimensions, and we also relate our results to the recently found infinite-randomness fixed point in the disordered one-dimensional contact process.     

Percolation of polyatomic species on a simple cubic lattice

In the present paper, the site-percolation problem corresponding to linear k-mers (containing k identical units, each one occupying a lattice site) on a simple cubic lattice has been studied. The k-mers were irreversibly and isotropically deposited into the lattice. Then, the percolation threshold and critical exponents were obtained by numerical simulations and finite-size scaling theory. The results, obtained for k ranging from 1 to 100, revealed that (i) the percolation threshold exhibits a decreasing function when it is plotted as a function of the k-mer size; and (ii) the phase transition occurring in the system belongs to the standard 3D percolation universality class regardless of the value of k considered.     

硅衬底GaN基LED N极性n型欧姆接触研究

在Si衬底GaN基垂直结构LED的N极性n型面上,利用电子束蒸发的方法制作了Ti/Al电极,通过了I-V曲线研究了有无AlN缓冲层对这种芯片欧姆接触的影响.结果显示,去除AlN缓冲层后的N极性n型面与Ti/Al电极在500到600 ℃范围内退火才能形成欧姆接触.而保留AlN缓冲层的N极性n型面与Ti/Al电极未退火时就表现为较好的欧姆接触,比接触电阻率为2×10^-5 Ω·cm²,即使退火温度升高至600 ℃,也始终保持着欧姆接触特性.因此,AlN缓冲层的存在是Si衬底GaN基垂直结构LED获得高热稳定性n型欧姆接触的关键. 关键词： 硅衬底 N极性 AlN缓冲层 欧姆接触     

Aspect ratio control of acid modified multiwalled carbon nanotubes

This study examined the effects of acid treatments on the length of multiwalled carbon nanotubes (MWCNTs) as well as the influence of the aspect ratio of the MWCNTs on the electrical percolation threshold. In particular, the length distribution, intensity ratio of the G and D Raman peaks, BET surface area, and maximum decomposition temperatures of the MWCNTs were investigated. The MWCNTs showed different electrical percolation thresholds depending on the aspect ratio. The higher aspect ratio MWCNTs had lower electrical percolation thresholds than those with smaller ratios due to their ease of contact with other MWCNTs. In terms of the electrical behavior of a MWCNT film, many more short MWCNTs were necessary to reach the electrical percolation threshold than long MWCNTs. These results demonstrate the need to control the aspect ratio of MWCNTs in order for them to be used efficiently in electrical applications.     

Threshold effects for two pathogens spreading on a network

Diseases spread through host populations over the networks of contacts between individuals and a number of results about this process have been derived in recent years by exploiting connections between epidemic processes and bond percolation on networks. Here we investigate the case of two pathogens in a single population, which has been the subject of recent interest among epidemiologists. We demonstrate that two pathogens competing for the same hosts can both spread through a population only for intermediate values of the bond occupation probability that lie above the classic epidemic threshold and below a second higher value, which we call the coexistence threshold, corresponding to a distinct topological phase transition in networked systems.     

Evolution of a Magnetoresonance Line Near to a Limit of Percolation in Dilute Magnetics

In paper the results of numerical modeling of a magnetic resonance in dilute magnetics near to a threshold of a percolation are discussed. The classical equation of motion of magnetic moments is used in view of an exchange interaction such as RKKI and imitation of spin-phonon interaction by Monte-Carlo method. It is shown, that cluster structure of a magnetic and threshold of percolation are determined by critical distance, on which there is a change of a sign of an exchange interaction. In an examination of percolation phase transition the jump change of breadth of a line of a magnetic resonance is set, that can form the basis for experimental definition of a threshold percolation and parameters of an exchange interaction by methods of a radiospectroscopy.     

Ohmic contacts to GaAs epitaxial layers

Formation of ohmic contacts onto GaAs epitaxial layers was reviewed. Because the Fermi energy of GaAs is pinned at the surface near the middle of the bandgap, it is impossible to choose a metal with the proper work function to make an ohmic contact. Instead, it is necessary to create a heavily doped surface layer and using field emission or thermionic field emission to achieve an ohmic contact. The oldest known contact metallization for GaAs is sequentially deposited thin films of Au, Ge, and Ni. It was shown that the ‘dopant diffusion model’, widely accepted to date to explain the formation of an n⁺ layer on GaAs to form the ohmic contact, is incorrect. Instead, the “solid phase regrowth model” was discussed in detail and shown to describe formation of ohmic contacts in this system. Based on this result, general rules for forming ohmic contacts to compound semiconductors with pinned Fermi levels or large values of electron affinities plus bandgap were expressed.     

Broadband dielectric spectroscopy of inhomogeneous and composite weak conductors

ABSTRACT

In this paper, we discuss broadband dielectric spectroscopy from mHz up to the infrared range mainly for materials with inhomogeneous weak conductivity, including conductor-dielectric nanocomposites. Our discussion is based on the effective medium approximation (EMA) and experiments modeled by this approach are reviewed. We discuss core–shell composites modeled by coated-spheres (Hashin–Shtrikman model) and normal composites with a possible percolation of the conductor component resulting in sharp or smeared percolation threshold of the DC conductivity and diverging static permittivity in the former case. The sharp percolation threshold is modeled by the Bruggeman EMA or by general EMA with arbitrary percolation threshold and arbitrary critical exponents of the DC conductivity and static permittivity. For the case of smeared percolation threshold in the case of complex topologies, we use the Lichtenecker model allowing for partial percolation of both the components. Finally, numerous papers reporting negative permittivity in weakly conducting materials are criticized and concluded to be due to spurious effects.     

Long-range connections, quantum magnets and dilute contact processes

In this article, we briefly review the critical behaviour of a long-range percolation model in which any two sites are connected with a probability that falls off algebraically with the distance. The results of this percolation transition are used to describe the quantum phase transitions in a dilute transverse Ising model at the percolation threshold p_c of the long-range connected lattice. In the similar spirit, we propose a new model of a contact process defined on the same long-range diluted lattice and explore the transitions at p_c. The long-range nature of the percolation transition allows us to evaluate some critical exponents exactly in both the above models. Moreover, mean field theory is valid for a wide region of parameter space. In either case, the strength of Griffiths McCoy singularities are tunable as the range parameter is varied.     

Metastability Thresholds for Anisotropic Bootstrap Percolation in Three Dimensions

In this paper we analyze several anisotropic bootstrap percolation models in three dimensions. We present the order of magnitude for the metastability thresholds for a fairly general class of models. In our proofs, we use an adaptation of the technique of dimensional reduction. We find that the order of the metastability threshold is generally determined by the ‘easiest growth direction’ in the model. In contrast to anisotropic bootstrap percolation in two dimensions, in three dimensions the order of the metastability threshold for anisotropic bootstrap percolation can be equal to that of isotropic bootstrap percolation.     

Description of the degree of reinforcement of polymer/carbon nanotube nanocomposites in the framework of percolation models

A radically new percolation model for describing the extremal dependence of the degree of reinforcement of polymer/carbon nanotube nanocomposites on the nanofiller content has been proposed. It has been shown that, for this nanofiller, the percolation threshold almost coincides with the aggregation threshold on the concentration scale. From the structural point of view, the extremum of this dependence is caused by the change in the type of the reinforcing component (from interphase regions to the skeleton of carbon nanotubes). From the mathematical point of view, the behavior of the degree of reinforcement is described by the general percolation relationship with replacement of the critical exponents near the percolation threshold. Neither the functionalization of the nanofiller nor the preliminary ultrasound treatment qualitatively change the dependence under study.     

Neutralization of the effect of surface doping at a metal-semiconductor contact

A study is made of the volt-ampere and volt-faraday characteristics of metal-semiconductor contacts formed by the deposition of a number of metals (Al, In, Sn, Cu, Au) onto the surface of a semiconductor (GaAs, Si) subjected to preliminary doping by indium from a molecular beam in a vacuum. It is established that surface doping (SD), while considerably altering the charge and the work function of the free surface of the semiconductor, at the same time has hardly any effect on the properties of the contacts investigated. The deposition of a sublayer of SiO with a thickness of 6–30 Å onto the doped surface before the formation of the contact makes it possible to retain the SD effect even after the application of the contact. The disappearance of the SD effect when the metal is deposited in the absence of a sublayer of SiO is explained by the action of mirror-image forces and the field of the contact potential difference.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 131–134, March, 1977.