Distinctions Between Classes of Superconductors Involve Their Degree of Structural Connectivity

Abstract

A distinction is made between extreme types, such as high T c cuprates and soft-metal superconductors concerning the extended, relatively exchange frustration-free covalent ring structures (eight-membered) in the former. Phenomenology indicates for cuprates that bond order (BO) effects within a given number of doped bonds can create real space organization of pairs around O super-exchange centers (trijugate position) within a mobile pair kernel. Indications for this postulated new quantum chemical principle have now been found in related electronic crystal behavior by STM. This situation creates small coherence lengths (∼4 atoms) and unusually high pair numbers. For optimal doping, all holes are converted into pairs and these pairs show trends to geometric checkerboard patterns due to characteristic charge-lattice lock-ins, such as configurations with configurations with hop = 0.25 or hop = 0.16. Period of BO and the degree of isolation of the planes determine T c. Doping curves reflect BO events and are roughly predictable from structural data alone. It is suggested that a variety of related materials with large member rings (e.g. C60) follow the relevant BO phenomenology. By contrast, soft metal superconductors (e.g. Pb) are based on 3D close packed structures with small triangular rings in which exchange interactions are frustrated and small. “Normal” BCS isotope effects suggest the operation of phonons in pair formation over extended coherence lengths (>100 atoms). Very small pair numbers are created through phonons.