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Numerical investigation of non-ohmic hopping conduction in macroscopically non-homogeneousthin layers. Weak electron-phonon interaction
No full textThe current-field and the conductivity-field characteristics of random hopping non-uniform systems in the limit of a weak electron-phonon coupling (band-like hopping transport in disordered solids) have been calculated numerically within the Bottger-Bryksin model. In particular, we consider thin layers placed between two planar Ohmic contacts, with exponential spatial variations in the total centre concentration over the layer thickness. We find that firstly for r-hopping transport in spatially uniform layers the current decreases with increasing field, reaching a constant saturation value, and on increasing the degree of non-uniformity the saturation current becomes lower and secondly both uniform and non-uniform r- epsilon -hopping systems are almost exactly Ohmic in the investigated field range; conductivity decreases on increasing the degree of non-uniformity of the centre distribution. Degree of the layer non-uniformity in the limit of weak electron-phonon coupling are quite different from the dependences in the case of strong electron-phonon coupling. The dependences on the degree of the layer non-uniformity of the centre distribution. The dependences on the degree of the layer non-uniformity in the limit of weak electron-phonon coupling are quite different from the dependences in the case of strong electron-phonon coupling
Numerical investigation of non-ohmic hopping conduction in macroscopically non-homogeneousthin layers. Strong electron-phonon interaction
No full textThe current-field and conductivity-field characteristics of random r hopping as well as r-E hopping systems with a strong electron-phonon coupling have been calculated numerically and discussed by Bottger and co-workers (1985-86). They have assumed a random but macroscopically homogeneous distribution of centres over the sample. However, in most real cases an extremely thin layer can hardly be thought to be macroscopically homogeneous and the local hopping-centre density should depend on the distance from the electrodes. In the present work the authors apply the Bottger-Wegener procedure to random r hopping and r-E hopping systems with macroscopic average density exponentially dependent on the distance from the contact. Only a strong electron-phonon coupling will be considered, i.e. the authors shall consider small-polaron transport in a disordered solid. The influence of the inhomogeneity in the centre distribution on current field and conductivity-field characteristics may be summarized as follows. Firstly, for r-hopping transport in homogeneous layers, Bottger and Wegener observe a decrease in the differential conductivity with increasing field, its local minimum being followed by an exponential increase; the authors confirm the results for homogeneous systems, whereas for inhomogeneous systems they find that, on increasing the degree of the site distribution inhomogeneity the local conductivity minimum is no longer followed by an exponential conductivity increase, but the system becomes ohmic (conductivity saturation), at least up to the fields consistent with the assumption of constant carrier concentration. Secondly, for r-E hopping in homogeneous layers, Bottger et al. observe for not too high an energy spread in the hopping centres that the local conductivity maximum occurring just after the ohmic region is followed by a local minimum, that latter being followed by an exponential conductivity increase for still higher fields
Non Isothermal Dispersive Transient Currents in Non-Homogeneous Insulating Layers
No full textIn the present work the authors have investigated the influence of spatial non-homogeneity of trap distribution on thermally stimulated currents (TSC) due to transport of carriers in insulating layers. It has been assumed that at zero time carriers are generated only in a thin layer near one of the contacts. They have proved that for both non-dispersive and dispersive transport the initial increase of TSC is strongly dependent on the spatial distribution of traps. On the other hand, the dependence of TSC maximum on electric field strength, layer thickness and heating rate, and in the case of dispersive transport also on the final current decay, is almost the same as for a homogeneous trap distribution. The analytical results have been found to agree satisfactorily with the results of Monte Carlo simulation of non-isothermal carrier transport for all the model spatial and energetic trap distributions considered. The possibility of determining spatial trap distribution on the basis of the measured TSC has been discussed
R-hopping transient currents in thin dielectric surface layers with macroscopicallynon-homogeneous spatial distribution of hopping centres
No full textThe work is a contribution to the understanding of transient currents, which are usually discussed in the context of the classical time-of-flight (TOF) experiment. In particular, r-hopping pulse injection transient currents in dielectric layers with a macroscopically nonhomogeneous distribution of hopping centres have been investigated with the aid of the Monte Carlo simulation, according to a new algorithm. Both the shape of the transient currents and the value of the effective TOF are found to depend strongly on the degree of macroscopic variations in the hopping-centre concentration over the layer thickness. For a smooth change in space centre concentration, the transient currents may be described by simple analytical expressions, suitable for the estimation of the spatial distribution of hopping centres from experimental data
A Monte-Carlo simulation of the time-of-flight experiment in non-uniformly defected thincrystalline layers
No full textIn the present paper we report on the results of the Monte Carlo simulation of the time of flight (TOF) experiment for r and r- epsilon hopping transport in highly defected very thin crystalline layers. The defects are considered as localized states, between which a hopping motion of the injected carriers is possible. The total (integrated over energy) density of defects is assumed to be spatially non-uniform on the macroscopic scale, i.e. the scale comparable with the layer thickness. In particular, we consider an exponential dependence of the total density of hopping centres on the distance from the layer contacts. The results of simulations performed for various defect concentrations, various defect distributions in energy, and various degrees of the layer spatial non-uniformity are discussed. It is shown that both r and r- epsilon hopping transient currents measured in the classical TOF experiment are highly sensitive to the spatial macroscopic scale variations of the total centre concentration. The detailed shape of the transients depends in a complicated way on the system dilution, the energetic centre distribution, and the character of the spatial variations of the total centre density. The existence of the spatial non-uniformity of the layer could be recognized experimentally by observation of the qualitative changes of the current shape with increasing temperature, which leads to lower dispersion, and thus more pronounced characteristic features of the x-dependent total centre density, such as a higher polarity dependence, or the appearance of the current maxima or plateaux just before the effective TOF
A Monte Carlo study of r-e-hopping transient currents in thin dielectric layers with macroscopicallynon-homogeneous spatial distritribution of hopping centres centres
No full textThe authors report the results of the Monte Carlo simulation of the time-of-flight experiment for variable-range hopping transport in thin dielectric layers, which are spatially inhomogeneous on the macroscopic scale (i.e. on a scale comparable with the layer thickness). In particular, the total density of hopping centres (with Gaussian distribution in energy) is assumed to change exponentially as a function of the distance from the contacts. The results of simulations performed for various system dilutions, various widths of the Gaussian distribution in energy, and various degrees of the layer spatial nonuniformity are discussed
Multiple -Trapping Transient Currents in Thin Dielectric Layers with Enhanced Trap Density in the Near-Contact Regions
No full textThe authors present several illustrative multiple-trapping current-time characteristics in thin dielectric layers with enhanced trap concentration in the near-contact regions. Such a spatial trap distribution, for different energy trap distributions, leads to a variety of different non-typical shapes of the characteristics. The results have been obtained with the aid of a Monte Carlo simulation for conditions corresponding to the typical time-of-flight method
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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