Uncompensated metal

Experimentally the current direction is fixed to a crystal symmetry axis of the sample and the sample is slowly rotated under constant magnetic field which is perpendicular to flie current direction. The presence of open orbits is revealed by (a) spikes against a low background for the uncompensated metal and (b) dips against a large background for the compensated metal. 

Figure 15a shows a multiply connected hole Fermi surface in band 27. A closed orbit centered at the M point on the M-X—F plane is denoted 1 branch c. This branch is equivalent to branch e in the paramagnetic state. Figure 15b shows two eUipsoidal electron Fermi surfaces centered at the R point and a multiply connected electron one centered at die r point in band 28. NdBe in the antiferromagnetic state is thus a compensated metal, as opposed to the uncompensated metal LaBe, or NdBe in the paramagnetic state. Here, the electron Fermi surface of NdBe in die paramagnetic state is essentially a sphere witii a hollow in the center. 

Magnetoresistance experiments (Sellmyer et al. 1975) confirmed that YZn is an uncompensated metal with a multiply connected Fermi surface which favors open orbits, at least, along the (110) direction. These results are consistent widi the results of band calculations mentioned above. 

As mentioned above, LaRu2Si2 is an uncompensated metal, while CeRu2Si2 is expected to be a compensated metal from the results of magnetoresistance. As the primitive cell contains one molecule of LaRu2Si2 or CeRu2Si2, we can conclude that the Ce ion becomes tetravalent, namely implying an itinerant 4f-electron character. Therefore, the situation is similar to the relationship between LaSns and CeSn3. 

In NaxW03-yFy Doumerc (1978) observed a transition that has all the characteristics of an Anderson transition similar phenomena are observed in NaxTayW3 y03. The results are shown in Fig. 7.14. It is unlikely that this transition is generated by the overlap of two Hubbard bands with tails (Chapter 1, Section 4) this could only occur if it took place in an uncompensated alkali-metal impurity band, which seems inconsistent with the comparatively small electron mass. We think rather that in the tungsten (or tungsten-tantalum) 5d-band an Anderson transition caused by the random positions of Na (and F or Ta) atoms occurs. The apparent occurrence of amiD must, as explained elsewhere, indicate that a at the temperature of the experiments. Work below 100 K, to look for quantum interference effects, does not seem to have been carried out. 

The external leads from the potentiostat to the electrodes may also contribute significant resistance and capacitance that must be taken into account if the cell currents are large and if fast response is desired. Most metallic working electrodes will have very low resistance, but a typical diopping-mercury electrode (DME) may have a resistance as large as 100 Q because the mercury-filled lumen of the capillary is so small (— 0.005-cm diameter). This resistance makes a contribution to the total cell resistance and to the uncompensated resistance in a three-electrode circuit. 

The variation in the peak-separation values (AEp) for the cyclic voltammetric data of Table 9.9 may be interpreted in terms of heterogeneous electron-transfer kinetics, but the most reasonable explanation is uncompensated resistance (especially for py and MeCN) and surface reactions (especially for the metal electrodes). 

Uncompensated resistance — A modern 3-electrode potentiostat compensates for the solution resistance between the counter and reference electrodes, but not for the solution resistance between the reference electrode and the working electrode. Such resistance is referred to as the uncompensated resistance (Ru). The resistance of the working electrode itself also contributes to the uncompensated resistance (but is almost always negligible with metal electrodes). The voltage drop across Ru due to a current flow is IRU and results in a potential control error. Factors that influence Ru (and thus IRU) include conductivity of the electrolyte, shape and size of the cell, the location of the reference electrode, the shape of the working electrode, and the size and position of the counter electrode [i,ii]. Ru can be lowered by addition of supporting electrolyte... 

Electrostatic field forms uncompensated charges of mineral surface, whose emergence is associated with broken inter-atom bonds, defects or replacements. Most of such charges form partially dissociated metals (sAl, sFe etc.) or ionogenic groups (first of all, sSi, =0, etc.), which are called active centers, sites or functional groups Sometimes they are also called active sites or simply sites. 

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