SrTiO₃ is a large-gap insulator, which upon the introduction of n-type carriers undergoes a superconducting transition below 1 K. Discovered in 1964, it has been the first member of a loose family of semiconducting superconductors, which now includes column-IV elements. Recent attention has focused on the interface between SrTiO₃ and other insulators or vacuum, a two-dimensional metal with a superconducting ground state. The origin of superconductivity in the bulk system is a mystery, since the non-monotonous variation of the critical temperature with carrier concentration defies the expectations of the crudest version of the BCS theory.

 

We have found that down to concentrations as low as 5.5 X 10¹⁷ cm^-3, the system has both a sharp Fermi surface and a superconducting ground state. This is by far the most dilute superconductor currently known. Surprisingly, the normal state of this superconductor is a metal whose Fermi energy is as little as 1.1 meV on top of a band gap as large as 3 eV. We argue that the large Bohr radius is the key factor in pulling down the threshold of superconductivity and metallicity in this system. The survival of superconductivity in such a dilute metal with a Fermi temperature much smaller than Debye temperature puts strong constraints for the identification of the pairing mechanism.