Xiao Lin, Zengwei Zhu, Benoit Fauque, Kamran Behnia
SrTiO_3 is a large-gap transparent 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 its interface with 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 most crude version of the BCS theory. Here, employing the Nernst effect, an extremely sensitive probe of tiny bulk Fermi surfaces, we show that down to concentrations as low as 5.5 10^{17}cm^{-3}, the system has both a sharp Fermi surface and a superconducting ground state. The most dilute superconductor currently known has therefore a metallic (and not semiconducting) normal state with a Fermi energy as little as 1.1 meV on top of a band gap as large as 3 eV. The survival of superconductivity at such a low carrier density in presence of a single quasi-spherical Fermi surface puts strong constraints for the identification of the pairing mechanism.
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http://arxiv.org/abs/1211.4761
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