Unstable and elusive superconductors

Abstract

We briefly review earlier and report original experimental results in the context of metastable or possible superconducting materials. We show that applied electric field induces conducting state in Copper Chloride (CuCl) whose characteristics resemble behavior of sliding charge-density-wave(s) (CDW). We discuss whether the sliding CDW or collective transport of similar ordered charge phase(s) may account for the problem of "high-temperature superconductivity" observed in this and other materials, including Cadmium Sulfide (CdS), metal-ammonia solutions, polymers, amorphous carbon and tungsten oxides. We also discuss a local superconductivity that occurs at the surface of graphite and amorphous carbon under deposition of foreign atoms/molecules.Comment: Invited review article published in a special edition on Superconducting Materials in honor of the 95th birthday year of Ted Geballe, edited by M. B. Maple, J. Hirsch, and F. Marsigli

What T tells

“…A measurement of the superfluid density is needed to obtain direct evidence for a new pairing mechanism. 6 A second possibly very overdoped cuprate is Sr 2 CuO 4−x that crystallizes in the same K 2 NiO 4 structure as the (LaSr) 2 CuO 4 family. It has been reported to be superconducting by Hiroi [62,63,64] and many studies since have reported values of x that correspond to very overdoped superconductors, but uncertainties in x, the presence of superstructures with 4, 5, and incommensurate periods and possible contamination from oxidizing agents have clouded the issues.…”

“…The recent work by Jin et al using SrO 2 as the oxidizing agent has eliminated the latter concern. [65] 6 It is conceivable that only a fraction of the holes introduced by the oxidation become mobile and contribute to the superfluid density and the rest are localized. The Tc is similar to optimally doped cuprates but the band structure must be different.…”

Signatures of Filamentary Superconductivity up to 94 K in Tungsten Oxide WO2.90

“…Among few reported studies concerning superconductivity in WO3-x there was an observation of filamentary superconductivity below Tc =3 K in twin walls of reduced WO3-x single crystals with unusually high second critical field Hc2 above 15T [11]. Also a small drop of resistivity which is suppressed by magnetic field was reported in granular thin films of WO3-x below 7 K [12]. Here we report results of the search for a possible superconducting state in WO3-x in a wide range of oxygen deficiency 0<x<1.…”

Superconducting materials classes: Introduction and overview

“…: To conclude this category, unstable and elusive superconductors are reviewed by Kopelevich et al [34]. These materials show signals of superconductivity, sometimes at very high temperatures, but bulk superconductivity has not been conclusively established.…”