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

## TL;DR

This paper examines experiments suggesting vacuum field modifications can alter superconductivity and ferromagnetism in solids, but highlights uncertainties and potential extrinsic effects influencing the results.

## Contribution

It presents an analysis of experimental uncertainties in vacuum field modification effects on superconductivity and ferromagnetism in organic and cuprate superconductors.

## Key findings

- Observed increase in superconducting transition temperature in Rb3C60.
- Possible extrinsic effects related to solid oxygen influence.
- Ferromagnetic signals may originate from magnetic impurities.

## 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.