Superconducting impurity effects

Nodal structure of the superconducting gap and impurity effects As explained before, thermodynamics and transport behaviour points to an extremely anisotropic or nodal gap function in the nonmagnetic borocarbides. A gap function compatible with reported experiments was proposed in Maki et al. (2002). It is a hybrid s - - g wave gap which is fully symmetric (Ai ) under the tetragonal group D4/, and has the form... 

The criterium that the mean free path should be larger than the superconducting coherence length must be met. This is a very strict condition that implies also that the impurity interband scattering rate yab should be very small yah (1/2 )(KB/ft)Tc. Therefore most of the metals are in the dirty limit where the interband impurity scattering mixes the electron wave functions of electrons on different spots on bare Fermi surfaces and it reduces the system to an effective single Fermi surface. 

According to the results obtained already at 3 percent H spikes and dips occur in the density of states curve of the mixed system. At larger concentrations of impurities due to clustering effect of the impurity, a part of the gaps disappear (16). Finally, it should be mentioned that with the increase of the impurity concentration, as the calculations show, the density of states at the Fermi level increases and therefore we would expect an increase of the transition temperature between the superconducting and normal states. (This experiment, as far as we know, has not been performed yet). We can conclude from this CPA calculation that aperiodicity (disorder) has a rather serious effect on the band structure of polymers. 

All of the effects of replacements of each metal site so far described can be understood within a framework of a low dimensional nature of conduction mechanisms. More specifically, superconduction takes place through the O-Cu-O bonds of the interconnected CuOft octahedra or CUO4 squares in the La-Sr-Cu-0 and Y-Ba-Cu-0 systems, respectively. The La or Y sites are less sensitive for the presence of magnetic impurities and seem to be rather away from the superconduction path. This tendency is more significant in the Y-Ba-Cu-0 system, because the conduction path would be along the one dimensional chains of O-Cu-O bonds which are most apart from the Y sites. 

Some chemical aspects such as the effects of comosition and impurities on the manifestation of superconductivity in high-Tc oxides have been described. They are shown to be quite significant in determining superconducting behavior and probably many other physical properties of these materials as well. As the developments of oxide superconductors with higher Tc and better quality continue, it should be essential to take these aspects fully into account. 

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