Über die Ursache der Supraleitung. III / On the cause of superconductivity. III.

Abstract

By consideration of superconducting compounds, whose chemical bond nature is known or can be derived with some certainty, it is shown that superconductivity only occurs, if valency electrons of a band can move in at least one space direction without intersecting any nodal surfaces. This applies both for metallic substances and for semiconductors. The following examples are discussed:

1. Metallic and semiconducting compounds composed of elements of the main groups and having a lattice similar to that of NaCl.

2. Nb- and Ta-chalcogenides with MoS₂-structure.

3. The clathrate-compound [Ag₇O₈]⁺X^-.

4. LaC₂ with CaC₂-structure.

5. Metallic and semiconducting perovskites.

6. Spinels and interstitial compounds with NaCl-structure.

7. Pyrites.

8. Some metals and their alloys.

In the case of s-electrons, this condition is fulfilled for all space directions, in the case of <5-bonds only along the intersecting line of the two nodal planes. A “δ”-bond is often - especially in an NaCl-type lattice - associated with a translation perpendicular to this intersecting line (in the NaCl-type lattice a translation by a/2) and a rotation by 90°. In such a “δ_t”-bond system the four segments of a d_t2g-electron belong to four adjacent canals of quadratic cross-section (in the NaCl-type lattice the side length is a/2) which are not intersected by nodal surfaces. Only electrons fulfilling the condition indicated above can be considered as “free electrons”.