Field-dependent conductivity

The field modulation technique has been successfully applied to a broad variety of systems showing a field dependent conductance. Since a large amount of conductance data Is available for low polar solutions of tetraalkylammonium salts these systems were studied most thoroughly. In the subsequent discussion we will rely mainly on the results obtained for these systems but the general conclusions apply directly to other systems, e.g. fluorenyl-salte In ethereal solvents, metal complexes In benzene, etc. 

The electric-field-dependent conductivity observed in (TMTSF)2 NO3 and (TMTSF)2 PFg is shown in Fig.l. In the normal state, Le. at temperatures above the SDW transition, the conductivity stays constant in the whole field range measured (up to about 0.7V/cm). However, in the SDW state, the conductivity is constant, until a threshold field is reached, above which the conductivity increases. Values of the threshold field, measured at 4.2K are 40 and 7.5 mV/cra for the NO3 and PFg compound, respectively. The sharpness of the threshold field was checked by continuous current measurements (see insert of Fig.l.b.) and by dynamic resistance measurements (Fig.2.). The value of the threshold field is temperature-independent below /2, and, as observed for the PFg compound, increases close to Tc (Fig.l.b.),... 

Finally, the excess current associated with the field-dependent conductivity (eq.(5)), can be expressed as a function of electric field (Fig.4.)... 

Xu, H., Zhang, S.X., Anlage, S.M., Hu, L.B., and Gruner, G. (2008) Frequency-and electric-field-dependent conductivity of single-walled carbon nanotube networks of varying density. Phys. Rev. 

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... 

The infrared spectra suggest bidentate dithiocarbamate coordination, the magnetic susceptibility is markedly field-dependent. Although the compound is monomeric in CHCI3, a molar conductance in nitromethane is found, which increases sharply on dilution, attributed to a weak electrolytic character. Accordingly, the authors formulate this compound as [Nb2(Et2r/tc)sBr2]Br. 

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

Urry, D. W., A molecular theory of ion-conducting channels A field dependent transition between conducting and non-conducting conformations, Proc. Natl. Acad. Sci. USA, 69, 1610 (1972). 

Figure 8.20 TbPc 2 SIM and NEMS based resonator [258]. (a) The magnetization reversal of the Tb3+ ion induced by sweeping the magnetic field parallel to the easy axis of the SMM, resulted in an abrupt increase in the differential conductance through the SW-CNT of NEMS. The switching field depends on the nuclear spin state occupied before magnetization reversal, (b) Elistogram of...

Relaxation dispersion data for water on Cab-O-Sil, which is a monodis-perse silica fine particulate, are shown in Fig. 2 (45). The data are analyzed in terms of the model summarized schematically in Fig. 3. The y process characterizes the high frequency local motions of the liquid in the surface phase and defines the high field relaxation dispersion. There is little field dependence because the local motions are rapid. The p process defines the power-law region of the relaxation dispersion in this model and characterizes the molecular reorientations mediated by translational displacements on the length scale of the order of the monomer size, or the particle size. The a process represents averaging of molecular orientations by translational displacements on the order of the particle cluster size, which is limited to the long time or low frequency end by exchange with bulk or free water. This model has been discussed in a number of contexts and extended studies have been conducted (34,41,43). 

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... 

In a later section (18d) we will find that the mixed-conductivity terms in fact allow a and R to be magnetic field dependent even when is independent of energy. This is because the Hall field Ey can never exactly balance the magnetic field forces for both holes and electrons at the same time. 

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).

Recent field dependent experiments for the sodium in NH8 system by Schettler and Patterson (46) show an unexpectedly large and complex field influence on the conductance of dilute solutions of sodium in liquid ammonia at — 33 ° C. Two distinctive effects are noted. One occurs at very low fields, 2.5 v./cm., and results in an increase, within one microsecond, of the conductance by 2-3% over the observed value on a commercial bridge at 0.2 v./cm. and 1,592 c.p.s. 

The fact that IGFET can be perceived as a field-dependent resistor is seen from the expression for transconductance gm which is defined as the average conductance of the channel for a given Vd. For nonsaturation from (C.27)... 

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