Uncertainties in experiments on strongly-coupled vacuum field modification of superconductivity and ferromagnetism in solids

Abstract

We discuss recent experiments in which fine particles of the organic superconductor Rb${}_3$C${}_{60}$ or the cuprate superconductor YBa${}_2$Cu${}_3$O${}_{6+x}$ are held in a polystyrene film that is spin-coated on to a silicon substrate with or without an intervening gold, or another inert metallic layer. From SQUID magnetisation data for Rb${}_3$C${}_{60}$ there appears to be a striking and completely unexpected increase in the superconducting transition temperature from $30$ to $45$~K, which is ascribed to coupling between the electrons in the superconductor and vacuum fluctuations in the electromagnetic field just above the metallic film. We argue that this could be a non-intrinsic effect associated with the presence of solid oxygen in the Pyrex sample tube. We suggest that the ferromagnetic SQUID signal observed for YBa${}_2$Cu${}_3$O${}_{6+x}$ particles in polystyrene could be attributed to ferromagnetic particles or magnetic clusters of unknown origin.