An electronic band sculpted by oxygen vacancies and indispensable for dilute superconductivity

TL;DR

This study reveals that oxygen vacancies uniquely create an electronic band essential for dilute superconductivity in strontium titanate, with the superconducting onset depending on the doping method and band structure modifications.

Contribution

It demonstrates that oxygen vacancies sculpt a specific electronic band critical for superconductivity at ultra-low doping levels, challenging the rigid band approximation.

Findings

Superconductivity persists at extremely low electron densities when doped via oxygen vacancies.

The band structure differs significantly depending on doping method, affecting superconductivity.

Oxygen vacancy-induced bands are the key to superconductivity in ultra-dilute regimes.

Abstract

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 $2 \times 10^{17}$ cm$^{-3}$, but when they are brought by substituting Nb with Ti, the threshold density for superconductivity is an order of magnitude higher. Our study of quantum oscillations reveals a significant 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 theses vacancies as the exclusive locus of superconducting instability in the ultra-dilute limit.