Field dependence of conductance

Fig. 11.13. Field dependence of conductance at T= 77K for FedSiCFh-, nanocomposite in metallic state (x m 0.7, curve a) and in the series of dielectric samples (curves b-d) having activation energies, meV (b) 1.0 (c) 1.25 (d) 2.83 [78],...

In oriented metallic conducting polymers, with large anisotropy in conductivity, the anisotropic diffusion coefficient factor should be taken into account in the above model. The robustness of this metallic state can be verified from the field dependence of conductivity at low temperatures. For example, in the case of sample E with oj 2 200 S/cm (see Fig. 3.4), which is just on the metallic side of the M-I transition, a field of 8 T can induce a transition to the insulating state, as shown in Fig. 3.7. The corresponding W vs. T plot (Fig. 3.7a) is consistent with the fact that the system has moved from the metallic to the critical/insulating side. This is a typical example... 

Fig. 6.17. Field dependence of conductivity of TegjGejsSbjSj at various temperatures. Applied field was assumed to be uniform. After Buckley and Holmberg(1972).

In the ion pair (A+, B ) the ions are not yet individually solvated but are partially separated by interpolated solvent. The equilibrium constant Kq is not field dependent. However, high fields naturally favor separation of the ion pair more ions are produced, i.e., is an increasing function of the field. For this reason the field dependence of conductance in weak electrolytes is also known as the dissociation field effect. Arguing from (3.31), the apparent equilibrium constant is... 

Correlated transport studies (including temperature dependent dc conductivity, audio-frequency (lOHz-105 Hz) conductivity and dielectric constant, electric field dependence of conductivity, thermopower, electron spin resonance, microwave conductivity and dielectric constant) have been carried out on oriented emeraldine hydrochloride films [15], the... 

In the above consideration it has been tacitly assumed that the charge carrier mobility docs not depend on the electric field. This is a good approximation for molecular crystals yet not for disordered systems in which transport occurs via hopping. Abkowitz et al. [37] have solved that problem for a field dependence of ft of the form p-po (FIFU) and trap-free SCL conduction. Their treatment predicts... 

Fig 2.9. The field dependence of electric conductivity of a thin sintered film (/) and a pressed ZnO sample (2) [37]... 

Fig. 3.7 Magnetic field dependence of the conductivity of Gd3 xvxS4 at T—300 mK. An estimate of is also indicated. From von Molnar and Penney (1985).

We now consider the magnetic field dependence of the Hall coefficient and conductivity in more detail. By setting jy = 0 in Eq. (A 14b), we can write Eq. (A 14a), with subscript n suppressed, as... 

Fig. 24. Magnetic field dependence of the electronic thermal conductivity at T - 0, normalized to its value at Hc2- Circles are for LuNi2B2C, squares for UPt3 and diamonds for Nb. Note the qualitative difference between the activated thermal conductivity of the s-wave superconductor Nb and the roughly linear growth seen in UPt3, a superconductor with a line of nodes (Boaknin et al. 2001).

Intense research has in recent years been devoted to noncrystalline materials. It was discovered also that the majority of semiconducting boron-rich borides display several properties that resemble those of the noncrystalline solids. Among the amorphous properties are the temperature and field dependencies of electrical conductivity at low temperature, the temperature dependence of thermal conductivity at high temperatures, and the temperature dependence of the magnetic susceptibility. In addition, the boron-rich semiconductors display crystalline properties, for example, the temperature dependence of the thermal condnctivity at low temperatures, the lattice absorption spectra and the possibility to change... 

The origin for this apparent field dependence of rric is not fully understood. However, the nearly perfect two-dimensionality of -(ET)2l3 discussed in Sect. 2.3.3 seems to be the principal reason for the observed strange temperature and field dependence of the magnetic quantum oscillations. It was suggested that in this extremely 2D system quasiparticles with fractional statistics [365] may occur if the cyclotron orbits lie within an individual 2D conducting plane [363]. Since these quasiparticles do not obey Fermi statistics they should not contribute to the quantum oscillations observed. Hence, the effective cyclotron mass determined by the 3D Lifshitz-Kosevich formula could be underestimated. Further experimental verification for this suggestion is lacking. 

The fluids from petroleum wells are collected in prewashed and prerinsed 8 L or 20 L carboys with a bottom spigot. Water and oil require from five minutes to several hours to separate, depending on the temperamre, proportion of water, and the composition of oil and water. Immediately after separation of water from oil, the water is passed through glass wool to remove solids and oil droplets before the samples are collected in separate 125 mL flint-glass bottles with polyseal caps for the field determination of conductance, pH, Eh, alkalinity, and H2S, and for laboratory determination of the carbon isotopes (Lico et al., 1982). 

An example of such a series of transient experiments for a sample of paper without conductive additives (softwood Kraft pulp, 450 CSF, 80 g/m2 basis weight, Sample 1), is shown in Fig. 16. This series of transient currents represents the electric field dependence of charge transport associated with mobile ions within the water associated with the fibrous network of the papor. The initial transient current, labeled (a), corresponds to the first application of an electric field (E = 2.5 x 103 Volts/cm) to the new sample. After reversing the polarity of the power supply an electric field of the same magnitude is applied to the sample which leads to the transient current shown by label (b) in Fig. 16. (N.B. the scale of the ordinate is different for... 

As can be seen in the table, the mobility is reasonably constant as a function of voltage for the undoped paper and the paper doped with NaCl. Some variability can be noted in the conductive base sheet paper however this is more likely to be due to sample variability than intrinsic electric field dependence of mobility, since as previously noted, every electric field measurement in that case was performed on a fresh sample. 

Further information on the transport processes in a-Si H and on the influence of doping can be obtained, e.g., from measurements of the drift mobility (Allan et al., 1977 Moore, 1977), of the photoconductivity (Rehm et al., 1977 Anderson and Spear, 1977), as well as of the magnetic field dependence of the photo- and dark conductivity (Weller et al., 1981). In this chapter, however, we shall confine ourselves mainly to results of conductivity and thermopower measurements. Some results from Hall effect and photoconductivity studies are also discussed. 

However, the Bassler model likewise gives a exp(VF) dependence. The reason for this is the field dependence of the hopping rate (Eq. (8.78)). The overall field and temperature dependence according to this model for the hopping conductivity in disordered materials at high fields (F> 10 V/cm) is given by ... 

By using the point at which the first derivation of raw conductivity data became zero at the end of the reaction, vitrification time of radiation crosslinked UP resins was determined. Vitrification times in the range 293-345 K were approximately constant below upper liquid-liquid transition temperature but exhibited significant temperature dependence above the transition, which can be seen in the Arrhenius plot in Figure 13.3b. In the same manner, electrical field dependence of vitrification was also observed—the stronger the field, the earlier the system reached vitrification. 

This was indeed the effect observed in the thermal conductivity of all of the specimens tested—the thermal conductivity decreased as the magnetic field increased. In very pure materials, where the thermal conduction is essentially all electronic, the effect is large, as shown by the shaded portions of Figs. 3 and 4 for copper and aluminum, respectively. On the other hand, conduction in heavily alloyed materials is primarily due to lattice vibrations which are unaffected by the magnetic field. The resulting small field dependence of thermal conductivity for... 

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