Interplay between antiferrodistortive, ferroelectric, and superconducting instabilities in Sr1-x Cax TiO3-δ

SrTiO3 undergoes a cubic-to-tetragonal phase transition at 105 K. This antiferrodistortive transition is believed to be in competition with incipient ferroelectricity. Substituting strontium by isovalent calcium induces a ferroelectric order. Introducing mobile electrons to the system by chemical nonisovalent doping, on the other hand, leads to the emergence of a dilute metal with a superconducting ground state. The link between superconductivity and the other two instabilities is a question gathering momentum in the context of a popular paradigm linking unconventional superconductors and quantum critical points. We present a set of specific-heat, neutron-scattering, dielectric-permittivity, and polarization measurements on Sr1-xCaxTiO3 (0<x<0.009) and a study of lowerature electric conductivity in Sr0.9978Ca0.0022TiO3-δ. Calcium substitution was found to enhance the transition temperature for both antiferrodistortive and ferroelectric transitions. Moreover, we find that Sr0.9978Ca0.0022TiO3-δ has a superconducting ground state. The critical temperature in this rare case of a superconductor with a ferroelectric parent is slightly lower than in SrTiO3-δ of comparable carrier concentration. A three-dimensional phase diagram for Sr1-xCaxTiO3-δ tracking the three transition temperatures as a function of x and δ results from this study, in which ferroelectric and superconducting ground states are not immediate neighbors.