Electronic conductivity superconductivity

Note Electronic conductivity, and even superconductivity at ca. 10 K, was observed with a new carbon polymorph, buckminsterfullerene, synthesized in 1990. Electrochemical and photoelectrochemical properties of a semiconducting diamond electrode were studied by Pleskov.)... 

An additional requirement for high-temperature superconductivity is that such hypo-electronic atoms as La, Y, Ba, or Sr can interact with the hyperelectronic Cu atoms. This results in electron transfer from the Cu atoms to the hypoelectronic atoms, which leads to the formation of covalent bonds that resonate among the Y-Y and Y-Cu positions, conferring electronic conductivity on the substance. These two types of resonance caused by the combination of crest and trough metals couple with the phonons to yield superconductivity at relatively high temperatures. 

Electronic Conductivity. Pluorides usually are good insulators owing to the wide band gap of more than 6eV in most compounds. In some cases it is possible, however, to obtain n-type semiconducting properties by doping, for example, in Cdp2(Y). Exceptional metallic conductivity is observed in the metal-rich compound Ag2p, even superconductivity in Hg3 jAsp6 - despite the presence of fluoride. Mixed-valence silver fluorides have been recently discussed as possible candidates for superconductivity. ... 

The most interesting aspect of conduction by electrons, namely superconduction, in which experiment is continually providing us with fresh surprises, scarcely falls within the scope of this lecture, as so far there exists no theory of superconduction to test. Such a theory would have to explain the characteristic conditions under which superconduction occurs, the change in the order of magnitude of the resistance, and the temperature at which the change of resistance takes place. Even empirical relationships for these have still to be discovered. 

A family of materials called cuprates contain Q1O2 planes (Fig. 1) the electron conduction for superconductivity is believed to occur in these planes. 

The search for bi-functionality in molecular materials constitutes a contemporary challenge in materials science. Using the two-network approach described in the previous section it has been possible to construct hybrid crystalline solids formed by a partially oxidized jt-electron donor network that support electronic conductivity or even superconductivity, and transition metal complexes containing magnetic moments. Some... 

The range of electronic conductivity (Fig. 7.6, right-hand side) in ceramics is phenomenal — it varies over 24 orders of magnitude, and that does not even include superconductivity Few, if any, other physical properties vary over such a wide range. In addition to electronic conductivity, some ceramics are known to be ionic conductors (Fig. 7.6, left-hand side). In order to understand the reason behind this phenomenal range and why some ceramics are ionic conductors while others are electronic conductors, it is necessary to delve into the microscopic domain and relate the macro-scopically measurable a to more fundamental parameters, such as carrier mobilities and concentrations. This is carried out in the following subsections. 

The R-based mixed oxides with p-type electronic conductivity as well as the superconducting oxides were found to be inactive for the OCM reaction, and only total combustion of methane was observed. The only exception was Lao Sro MnOj 5, which is probably an n-type conductor under the experimental conditions used. This solid is likely to be n-type under reducing atmosphere due to the stability of the Mn2+ ion. LaftsSro.2Mn03and LaFeo.sNdo.2O3 5, both considered n-type conductors, had low selectivity to C2, whereas La-Sr-Y-O systems, either ionic conductors or insulators, presented a good selectivity to those compounds (table 8). 

Despite the current upsurge of interest about the conduction properties of DNA [105,112], the subject is far from new. Eley and Spivey (1962) were the first to suggest that DNA could act as a conductor [77]. The experimental outcomes about the conductivity of DNA and its mechanisms vary from the wide gap insulating behaviour to the proximity induced superconducting one [95-98, 102,105,113-119]. A clear understanding of the nature of electronic conduction... 

In this chapter the focus is upon electronic conductivity in perovskites. The electrons in perovskites are believed to be strongly correlated that is, they do not behave as a classical electron gas, but are the subject to electron-electron interactions. This leads to considerable modification of the collective electron behaviour of the conduction electrons, resulting in metal-insulator transitions, high-temperature superconductivity, half-metals and colossal magnetoresistance (CMR). The effects of strong correlation are important for the 3d, 4d and4f elements. In many ways the topics described here are thus a continuation of the previous chapter on magnetic perovskites, and in truth the two subject areas cannot be separated in a hard and fast maimer. 

How might the unique anatomy (Fig. 16.7) of the template-synthesized nanostructures affect the mechanism of electronic conduction in thin films prepared from these materials We have been exploring this question in collaboration with Professor H. D. Hochheimer in the Physics Department here at Colorado State University and Dr. P.-H. Hor of the Texas Center for Superconductivity (50,59-63J. These investigations (which are very much ongoing) entail measurements of both the temperature and pressure dependence of conductivity in thin films prepared from our template-synthesized nanostructures. Preliminary results of the effect of temperature on conductivity are briefly reviewed here. 

In metals, the primary mechanisms of both thermal conductivity and electrical conductivity are the same. Accordingly, the mechanisms of electron-lattice interaction described in the previous section for thermal conductivity apply equally well to electrical conductivity. Superconductivity also depends upon the electron-lattice interaction, albeit in a much more subtle way. A brief discussion of superconductivity completes this section. 

---