Comment on two papers claiming records for the lowest carrier concentration at which superconductivity has been observed

TL;DR

This paper critiques recent claims of record-low carrier concentrations for superconductivity, highlighting historical evidence of superconductivity at even lower concentrations in SrTiO3 samples from the 1980s.

Contribution

It provides historical context and evidence challenging recent claims, emphasizing earlier observations of superconductivity at lower carrier concentrations.

Findings

Superconductivity observed at lower concentrations in 1980s SrTiO3 samples.

Historical evidence predates recent record claims.

Challenges current record claims based on earlier data.

Abstract

A recent paper and a preprint both claim records for the lowest carrier concentration at which superconductivity has been observed. However colleagues in Australia and I found evidence for superconductivity at concentrations of the order of at least a hundred times lower in the 1980's, in a ceramic sample of SrTiO_3 with 3% of Ti replaced by Zr.