Modification, effect superconductivity

The Nb sulfides and selenides, moreover, become superconducting (76—78) with different transition temperatures for modifications with different stacking. Substitution of Nb by Ta decreases the transition temperature and Mo has, of course, an even stronger effect. Tellurium substituted for selenium acts in the same way (79). 

First electrochemical studies on structuring and modification of different high superconductor surfaces have recently been started [6.190]. One of the main problems is the instability of oxide ceramic material in neutral and acidic aqueous electrolyte solutions at room temperature [6.196-6.198]. HTSC surfaces corrode, and superconductivity was found to decrease within the topmost layers of IfTSC samples after water contact. This aging effect decreases in alkaline media [6.197]. However, sufficient long term stability of HTSC samples was only found in aprotic solvents such as acetonitrile. Therefore, experiments were carried out in acetonitrile-containing... 

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. 

While, according to Moncton et al. [655], the superlattice remains incommensurate down to 5K, Marezio et al [628] determined at 15 K a commensurate 2ao superlattice (Table 120). A pressure of 1 kbar is sufficient to remove the distortions in NbSe2 [65]. It is noteworthy that the 2Ha-type modification exhibits a superconductive-transition temperature Tc(0) = 7.35K while in the distorted modification Tc(0) = 7.16K [657]. As the structural distortions occur within the layers, sharp discontinuities are observed in the resistivity within the layers but only gradual changes in the much higher resistivity perpendicular to the layers [656]. A consequence of the electronic rearrangement is the reversal of the Hall effect observed in NbSe2 at 26 K [658]. 

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