  Strontium titanate has resurfaced as a material prompting vigorous debate
about the origin of its superconductivity in the extremely low carrier
concentration regime. Here, we used simultaneous AC susceptibility and
transport methods to explore the superconducting phase transition region in
this material. We determined that strontium titanate is extremely sensitive to
even small AC fields, which also influence the resistive transition; we suggest
that extreme vortex sizes and mobilities contribute to this large effect. Our
findings will be of importance for accurately determining transition
temperature, informing the debate about the pairing mechanism in strontium
titanate, for which even millikelvin errors may be critical.
