Superconductivity in Molecular and Oxide Lattices: A Comparison

Abstract

Superconductivity is, perhaps, the most dramatic of the various collective ground states that can be formed by a Fermi gas composed of electrons in the periodic potential of a crystalline lattice. Zero resistance is readily manifested, and the concomitant property of flux exclusion, shown up as perfect diamagnetism, is a necessary observable. In the context of the present Advanced Study Institute, one should recall the simple fact that, to be a superconductor, a material must first be metallic. On the other hand, by no means all metals are superconductors, either because there is competition from other ground states, like charge density wave and spin density wave, or because the electronic band width is too large, and vibronic interaction too weak. The challenge, therefore, is to define, and then optimize, the electronic and structural factors that determine this competition. Structure types providing paradigms for conventional superconductivity have traditionally been three-dimensional continuous lattice in type, from the close-packed structures of elements, such as Hg and Nb, to the more elaborate, but still largely isotropic, arrays of the A15 compounds, like Nb₃Sn. Recent developments, however, have brought superconductivity into contact with molecular and inorganic solid-state chemistry, with an enlargement in the variety of structure types that one would expect, when synthetic chemistry comes to bear on a problem and, in one instance at least, dramatic enhancement of critical temperature.