Comment on two papers claiming records for the lowest carrier   concentration at which superconductivity has been observed
  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. 
D.M. Eagles
cond-mat.supr-con