Anomalous Dispersive Characteristics

While transit pulses of the preceding form are observed in many crystalline and in some amorphous specimens, the behavior exhibited in other cases differs to a surprising degree. 

The phenomenon, which has come to be termed anomalously dispersive transport, was characterized during this period with respect to the following properties  

These algebraic or power-law characteristics were further suggested to interrelate, in sense that the parameters ai and 02 were identical  

The preceding characterization of anomalously dispersive transit pulses aroused considerable interest, both from a theoretical and an experimental viewpoint. Attention focused on the latter was stimulated by the possibility of using the log-log display technique to identify ft in cases where the dispersion was such as to obscure any change of gradient in conventionally displayed transit pulses. However, it became necessary to question the validity of such measurements of ft under conditions where individual carrier transit times vary over such a wide range. 

The study of the dispersion of photoinjected charge-carrier packets in conventional TOP measurements can provide important information about the electronic and ionic charge transport mechanism in disordered semiconductors [5]. In several materials—among which polysilicon, a-Si H, and amorphous Se films are typical examples—it has been observed that following photoexcitation, the TOP photocurrent reaches the plateau region, within which the photocurrent is constant, and then exhibits considerable spread around the transit time. Because the photocurrent remains constant at times shorter than the transit time and, further, because the drift mobility determined from tt does not depend on the applied electric field, the sample thickness carrier thermalization effects cannot be responsible for the transit time dispersion observed in these experiments. 

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Strictly speaking, A/ depends also on the atomic number of the scattering element, which means that a different correction curve is required for every element. But the variation of A/with Z is not very large, and Fig. 13-8, which is computed for an element of medium atomic number (about 50), can be used with fairly good accuracy as a master correction curve for any element. Figure 13-8 was calculated from data in James [G.7, p. 608]. Anomalous dispersion is also discussed by Guinier [G.21]. Values of A/ as a function of Z at constant 2, for five characteristic wavelengths, can be calculated from data in [G.ll, Vol. 4]. 

With this process the main objective is to produce the same interfacial areas per unit volume on both scales, in order to achieve the same mass transfer. The analysis based on turbulence theory has been confirmed by the knowledge gained in practice in the form of the scale-up criterion P/V = const. This applies to dispersing processes in liquid/liquid and gas/liquid systems. Because of the numerous factors that influence the process (e.g., coalescence properties, physical properties of mixtures, anomalous flow characteristics, static pressure, etc.), substantial... 

An anomalously high degree of dispersion is immediately evident from the fig-nre. There is no flat part, characteristic of idealized current response, to specify a transit time. The transit time is defined by the log / - logt. It is necessary to note that there exist some disadvantages of nsing logarithmic conversion of the transient cnrrent to extract the transit time. In fact, the intersection of power laws and... 

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