The Fermi surface and the superconducting gap in bulk SrTiO3

X. LIN; A. GOURGOUT; G. BRIDOUX; B. FAUQUE; K. BEHNIA

Hammamet

Conferencia; 8th International Conference on Magnetic and Superconducting Materials-MSM13; 2013

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 find 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. The existence of a well-defined Fermi surface and its various properties are documented by measurements of the Nernst effect, a very sensitive probe of small bulk Fermi surfaces. The most dilute superconductor currently known has therefore a metallic normal state and a Fermi energy as little as 1.1 meV on top of a band gap as large as 3 eV.  The large Bohr radius pulls down the threshold of superconductivity and metallicity in this system. The survival of superconductivity in a metal with such a small single-component Fermi surface puts strong constraints for the identification of the pairing mechanism.