SrTiO₃: From Quantum Paraelectric to Superconducting

Abstract

SrTiO₃ exhibits unusually complex ground states which reach from insulating quantum paraelectric to poor metallic behavior depending on doping and/or oxygen stoichiometry where superconductivity might appear. Upon Nb doping two-gap superconductivity is induced and with this the first two-gap superconductor realized long after its theoretical prediction. Here we concentrate on this state and show that Nb doped SrTiO₃ is the most unconventional multi-band superconductor of known multi-band superconductors since the smaller of the two superconducting gaps deviates substantially from a BCS temperature dependence. We attribute these deviations to two cooperating effects, namely an extreme anisotropy in the frequency dependent effective attractive interactions, involving one very soft mode, and an almost vanishing interband interaction. As a consequence, the superfluid density of Nb doped SrTiO₃ is predicted to exhibit an inflection point close to the superconducting transition temperature T_c and not—as seen in other multi-band superconductors—close to T = 0 K.

Keywords:

• Quantum paraelectrics
• superconductivity
• SrTiO₃

Acknowledgment

It is a pleasure to acknowledge helpful discussions with R. K. Kremer.