Superconductors with Superconducting Transition Temperatures Tc = 91K (1999), 120K (1994), 340K (2000), and 371K (1995): Experimental Errors or a Technological Puzzle? Two-Component Nonstoichiometric Compounds and the Insulator--Superconductor--Metal Transition

摘要

One of the reasons for the lack of understanding of both the mechanismsunderlying the HTSC phenomenon and of the instability of materials with Tc >300 K may be the widely accepted but wrong ideas about the types of chemicalbonding in a substance and the radii of the atoms and ions. A revision of theseconcepts started in the beginning of the XX century in connection with theinvestigation of non-stoichiometric compounds (the berthollides) but did notreach a critical level until recently. Most of the HTSC materials, however, areactually non-stoichiometric nanocomposites, whose components "dilute" or"stretch" one another. Each component resides in an "intermediate" state, whichstill remains poorly studied. For instance, in a system of particles having twopaired electrons each, the unbroken electron pairs may start tunneling at acertain "medium" concentration with the system becoming a Bose superconductor(the state between the insulator and the metal with BCS superconductivity). Forunivalent atoms (Na,Ag), however, such possibility realizes neither inintermediate nor in the final state. Univalent metals are not superconductors.In the berthollides, however, a possible Jahn-Teller-Peierls-type instabilitymay give rise to formation of diatomic molecules (Na2, Ag2)with electron pairs,and superconductivity can set in. It is possibly such systems that wereobtained by chance in experiments with univalent components and reported tohave Tc of up to 371 K. Structures of a number of HTSC materials areconsidered.