| Summary: | Dilute superconductivity survives in bulk strontium titanate when the Fermi temperature falls well below the Debye temperature. Here, we show that the onset of the superconducting dome is dopant dependent. When mobile electrons are introduced by removing oxygen atoms, the superconducting transition survives down to 2×10^{17}cm^{−3}, but when they are brought by substituting Ti with Nb, the threshold density for superconductivity is an order of magnitude higher. Our study of quantum oscillations reveals a difference in the band dispersion between the dilute metals made by these doping routes and our band calculations demonstrate that the rigid band approximation does not hold when mobile electrons are introduced by oxygen vacancies. We identify the band sculpted by these vacancies as the exclusive locus of superconducting instability in the ultradilute limit.
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